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in  tije  Citp  of  ^eto  l^orfe 

COLLEGE  OF  PHYSICIANS 
AND   SURGEONS 


r 


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Given  by 


ELEMENTS 


OF 


HUMAN    PHYSIOLOGY 


BY 

BENE  ST   H.   STARLING,    M.D.Lond.,   F.K.C.P.,  F.E.S. 

JODRKLL  PROFESSOR  OP  PHYSIOLOGY,  DSIVKRSITY  COLLEGE,  LONDON 


EIGHTH     EDITION 


CHICAGO 
W.    T.    KEENEK   &   CO. 

90   WABASH   AVENUE 
1907 


Printed  in  Great  Britain 


PKEFACE 


In  this  book  I  have  endeavoured  to  present  m  the  shortest 
possible  compass  the  essentials  of  the  science  of  Phj-siology, 
such  as  every  medical  student  should  be  more  or  less 
acquainted  with.  Although  a  few  histological  illustrations 
are  introduced,  the  descriptions  have  been  kept  as  short 
as  possible,  their  object  being  merely  to  remind  the  senior 
student,  or  to  give  a  bare  idea  to  the  junior  student  of 
the  structure  of  the  organs  in  question,  and  they  cannot 
in  any  way  replace  the  study  of  some  special  work  in 
connection  with  a  practical  course  in  Histology.  In  the 
present  edition  only  such  alterations  have  been  introduced 
as  have  been  rendered  necessary  by  recent  advances  in 
Physiology. 

ERNEST   H.    STARLING. 

Ilt07. 


Digitized  by  tine  Internet  Arciiive 

in  2010  witii  funding  from 
Columbia  University  Libraries 


http://www.archive.org/details/elementsofhumanp05star 


CONTENTS 


CHAPTER  I 

PAGE 

Introduction         .  .  .  .         1 


CHAPTEK    II 

The  Material  Basis  of  the  Body 

SECT. 

1.  The  Sources  of  the  Energy  of  the  Body         ....  25 

2.  The  Proteins  .  .  .  .  .  .  .       .  30 

3.  The  Fats  ........  4S 

4.  The  Carbohydrates  .  .  .  .  .  .  .       .  51 

CHAPTER    III 

The  Blood 

1.  The  Formed  Elements  of  the  Blood    .  .  .  .  .57 

2.  The  Coaeulation  of  the  Blood        .  .  .  .  .       .       75 


CHAPTER    IV 

The  Conteactile  Tissues 


1.  General  Characters  of  Muscle  ..... 

2.  Excitation  of  Muscle  ...... 

3.  The  Mechanical  Changes  that  a  Muscle  undergoes  when  it  Contracts 

4.  The  Production  of  Work  and  Heat  by  Voluntary  Muscle     . 

5.  The  Electrical  Changes  in  Muscle  .... 

6.  The  Chemistry  of  Muscle  ..... 

7.  Conditions  modifying   the  Irritability   and  Contraction  of  Muscular 

Tissue       ........ 

8.  Voluntary  Contraction   ...... 

9.  Other  Forms  of  Contractile  Tissue  .... 


87 
95 
102 
113 
119 
131 

140 
145 
149 


vi  PHYSIOLOGY 


CHAPTER  V 

Nerve-fibees  (Conddcting  Tissues) 
sect.  page 

159 
1C2 
167 
177 
182 
187 


1.  General  Properties  of  Nerve-fibres 

2.  Propagation  along  Nerve-fibres 
8.  Excitation  of  Nerves      .... 
i.  The  Conditions  aiifecting  the  Activity  of  Nerve 
5.  Polarisation  Phenomena  in  Nerve 
<i.  The  Nature  of  the  Nervous  Impulse 


CHAPTER   VI 

The  Vascular  Mechanism 

1.  The  Mechanical  Principles  of  the  Circulation              .                           .  189 

2.  The  Changes  occurring  in  the  Heart  at  each  Contraction           .       .  206 

3.  The  Pulse 228 

4.  Cardiac  Ehythm        .             .             .             .             .             .             .       .  233 

5.  Innervation  of  the  Heart            ......  244 

6.  The  Work  of  the  Heart       .             .             .         '   .             .             .       .  252 

7.  Innervation  of  the  Blood-vessels            .....  259 

8.  The  Capillary  Circulation    .             .             .             .             .             .       .  280 

9.  Variations  in  the  Quantity  of  Blood    .....  283 


CHAPTER   VII 

L\MPH  A^•D  Tissue  Fluids  .  .     286 


CHAPTER  VIII 

The  Mechanisms  of  Digestion 

1.  General  Characters  of  the  Processes  of  Digestion  .  .  298 

2.  Salivary  Digestion    .  .  .  .  .  .  -       -  305 

3.  Digestion  in  the  Stomach  ......  316 

4.  Pancreatic  Juice       .  .  .  .  .  ...  328 

5.  The  Bile  .........  338 

6.  Succus  entericus  or  Intestinal  Juice  .  .  .  .       -  347 

7.  Absorption  of  Foodstuffs  ......  349 

8.  Summary  of  the  Changes  undergone  by  the  Food   in  the  Alimentary 

Canal •  .       .  360 

9.  Muscular  Mechanisms  of  Digestion       .....  363 


CONTENTS  Vll 


CHAPTEE   IX 

EESriRATION 

SECT.  PAGE 

1.  The  Respiratory  Movements           .                          .             .  .       .     377 

2.  Chemistry  of  Eespiration           .....  .     386 

3.  The  Regulation  of  Respiration        .             .             .             .  .       .     402 

4.  Effects  of  the  Respiratory  Movements  on  the  Circulation  .             .     422 

5.  Voice  and  Speech    ......  .       .     428 


CHAPTEE   X 

Excretion  —Functions  of  the  Kidneys  and  Skin 

1.  The  Urinary  Constituents  and  their  Origin  in  the  Body      .  .     434 

2.  The  Secretion  of  Urine       .  .  .  .  .  .       .     454 

3.  Micturition  .......  .467 

4.  The  Skin        .  .......     473 


CHAPTEE   XI 
Fate  of  Foodstuffs  in  the  Organism — Metabolism 

1.  Protein  Metabolism        ......  .  478 

2.  Formation  of  Fat     .             .             .             .             .             .  .       .  485 

3.  History  of  Carbohydrates  in  the  Body            ....  489 

4.  The  Source  of  Muscular  Energy     .             .             .             .  .       .  497 

5.  Animal  Heat      .......  .  500 

6.  The  Normal  Diet  of  Man  .             .             .             .             .  .       .  507 


CHAPTEE   XII 

The  Ductless  Glands            .             .  .513 

CHAPTEE   XIII 

Special  Senses 

1.  On  Sensation  in  General     .             .             .             .             •  ■       •     522 

2.  Cutaneous  Sensations    ......  •     526 

3.  Sensations  of  Movement  and  Position       .             .            .  .       .     531 

4.  Taste  and  Smell            ......  .539 

5.  Hearing         .             .             -             .             •             •             •  .       .     544 

6.  Vision     ......••  -     550 


vm  PHYSIOLOGY 


CHAPTEE  XIV 

The  Spixal  Cokd 

SEC'i'.  I'AGE 

1.  Structure  and  Tracts  of  the  Cord       .....     582 

2.  Paths  of  Impulses  in  the  Cord      .  .  .  .  .       .     603 

3.  The  Cord  as  Reflex  Centre      .  .  .  .  .  .606 


CHAPTEE   XV 

The  Bkain 

1.  The  Structure  of  the  Brain  .  .  .  .  .       .  612 

2.  Summary  of  the  Connections  and  Functions  of  the  Cranial  Nerves  634 

3.  Functions  of  the  Cerebral  Axis  .....  642 

4.  Cerebral  Hemispheres  .  .  .  .  .  .       .  653 

n.  The  Vascular  and  Lymphatic  Arrangements  of  the  Central  Nervous 

System  .......  .     6C6 


CHAPTEE   XVI 

The  Visceral  or  Autonomic  System  of  Nerves  .  .     670 

CHAPTEE   XVII 

Eeproduction      ....      685 

APPENDIX 

A    Description     of     some     Electrical     Instruments     used      in 

Physiology  ......  •     694 

INDEX 709 


PHYSIOLOGY 

CHAPTER  I 
INTRODUCTION 

Physiology  is  the  science  of  the  phenomena  of  living  organisms 
and  of  the  laws  regulating  those  phenomena.  In  its  wider 
sense  it  will  thus  include  the  phenomena  of  all  vegetable 
and  animal  life. 

In  this  work  however  our  immediate  object  is  the  phy- 
siology of  man.  Now  in  physiology,  as  m  all  other  sciences, 
the  only  sure  foundation  of  knowledge  is  that  gained  by 
experiment ;  and  since  ethical  considerations  prevent  our 
experimentmg  on  our  fellow-creatures,  we  find  ourselves 
again  and  again  forced  to  judge  of  the  functions  of  men  by 
analogy  with  those  of  lower  animals  on  whom  we  can  experi- 
ment. We  can  however  learn  many  things  from  experiments 
which  we  may  make  on  ourselves,  and  which  do  not  necessitate 
any  mutilation  or  involve  any  danger. 

We  find  means  moreover  of  checkmg  the  results  of  our 
experience  m  lower  animals  by  studymg  the  disorders  of 
function  caused  in  man  by  lesions  of  the  various  parts  of  the 
body,  which  we  may  observe  m  the  wards  and  post-mortem 
room.  Nature  however  rarely  limits  her  experiments  on  our 
bodies  to  one  function  or  organ,  so  that  in  most  diseases  we 
have  such  a  complexity  of  disturbances  that  this  method  of 
investigation  used  by  itself  is  apt  to  lead  to  many  erroneous 
deductions. 

The  phenomena  that  we  commonly  associate  with  the 
possession  of  life  are  those  of  movement  and,  in  the  higher 
animals,  of  warmth. 

Thus,  in  men,  some  part  of  the  body  is  always  in  motion, 
and  even  in   sleep  the  rhythmic  respiratory  movements  still 

1 


2  PHYSIOLOGY 

betoken  to  us  the  presence  of  life.  If  wg  see  a  frog  on  the 
ground,  we  instinctively  poke  it  to  see  if  it  is  alive,  knowing 
that  if  alive  it  will  respond  to  the  stimulus  and  jump  away. 

This  property  of  reaction  to  stimulus,  or  irritability,  is 
fundamental  and  common  to  all  living  beings.  We  shall 
have  to  consider  it  more  in  detail  later  on. 

Then  again  a  living  man  is  warm,  and  in  temperate 
climates  always  warmer  than  the  surroundmg  air.  By  means 
of  the  thermometer  it  is  found  that  a  healthy  man's  tempera- 
ture is  98-4°  F.,  and  is  maintained  constantly  at  this  point. 
The  temperature  of  the  surrounding  medium  being  nearly 
always  below  this  point,  it  is  evident  that  the  body  must  be 
continuously  losing  heat  and  raising  the  temperature  of  sur- 
rounding bodies. 

Thus  we  see  that  a  living  body  is  continuously  losing 
energy,  which  may  appear  as  work  done  on,  or  as  heat 
imparted  to,  some  external  object. 

Yet  we  know  that  an  animal  continues  to  perform  work 
and  to  give  off  heat  durmg  the  whole  of  its  life,  so  that  it 
must  have  some  source  from  which  to  draw  its  energy.  This 
source  is  the  food. 

Common  experience  teaches  us  that  a  man,  to  live,  must 
eat,  drink,  and  breathe.  His  food  consists  of  certam  bodies 
which  we  call  proteins,  carbohydrates  (including  starches 
and  sugars),  and  fats.  Of  these  three  classes,  the  proteins 
(which  exist  in  large  quantities  m  meat)  are  essential  to  the 
maintenance  of  life,  though  life  is  supported  more  advan- 
tageously if  the  other  two  classes  are  also  made  use  of. 

The  proteins  contain  carbon,  hydrogen,  nitrogen,  oxygen, 
and  a  small  proportion  of  sulphur.  Carbohydrates  and  fats 
contain  carbon,  hydrogen,  and  oxygen  only. 

All  these  foodstuffs  are  said  to  possess  potential  energy. 
This  simply  means  that  they  can  combine  with  a  further 
proportion  of  oxygen  to  form  more  stable  compounds,  and 
in  so  doing  set  free  a  certam  amount  of  energy.  This  energy 
may  appear  in  the  form  of  heat,  or  we  may  use  loaves  or 
sugar  or  fat  to  feed  an  engine  furnace  with,  and  so  convert 
the  energy  into  work. 

It  is  the  oxidation  of  the  foodstuffs,  the  burning  of  them 
to  form  CO2  and  water,  that  gives  rise  to  the  energy  which 
appears  in  the  animal  body  either  as  work  or  heat. 


INTRODUCTION  3 

The  oxygen  necessary  for  this  combustion  is  furnished 
by  the  atmosphere.  With  every  breath  we  take  in  oxygen, 
with  every  breath  we  give  out  carbon  dioxide  and  water. 

Thus  we  may  compare  the  animal  body  to  a  heat  engine. 
The  fuel,  the  source  of  energy,  is  represented  by  food.  The 
inlet  for  the  draught  of  air  and  the  outlet  for  the  waste 
gases,  the  products  of  combustion,  are  both  combined  in  one 
organ,  the  lungs,  which  we  use  to  take  in  oxygen  and  give 
out  C0_,  and  water  ;  and  just  as  the  coal  used  in  engines  has 
some  mcombustible  constituents  which  remain  as  ash  and 
have  to  be  raked  out,  so  there  are  parts  of  our  food  which 
the  body  cannot  make  use  of,  and  which  leave  the  body  as 
excrementitious  matter  or  fseces,  having  passed  through  the 
alimentary  canal  without  at  any  time  having  formed  part  of 
the  tissues.  There  are  still  two  constituents  of  foodstuffs 
which  have  to  be  got  rid  of  after  the  elimination  of  CO2  and 
HjO  by  the  lungs — namely,  the  nitrogen  and  sulphur  con- 
tamed  in  the  proteins.  This  f miction  is  served  by  the 
kidneys.  The  nitrogen  is  combined  m  the  body  with  carbon 
and  oxygen  to  form  a  substance  called  urea,  and  in  this  form 
is  excreted  by  the  kidneys,  together  with  salts  and  water,  as 
urme.  The  sulphur  is  oxidised  to  a  sulphate,  and  m  this 
form  also  appears  m  the  urme. 

We  must  mention  here  that  water  and  salts  are  indis- 
pensable and  mvariable  components  of  the  food.  Neither 
of  these  possesses  any  potential  energy,  but  they  are  essential 
constituents  of  all  livmg  substances,  and  must  be  taken  to 
replace  the  loss  of  them  from  the  Imigs,  skin,  and  kidneys, 
the  water  formed  m  the  body  by  the  oxidation  of  the 
hydrogen  of  the  foodstuffs  being  far  too  small  to  compensate 
this  loss. 

The  income  and  output  of  the  body  may  be  arranged  in 
the  form  of  an  equation — 

Food  +  oxygen  taken  up  =  faces  +  nCO.,  +  7iH.O  +  urea  ^CO<^^E;-'\ ; 

and  in   the   same  way  we   may   make   an   equation   of   the 
income  and  output  of  energy — 

Energy  set  free  by  burning  of  food  to  COj,  H.,0,  and  urea 
=  work  done  by  body  +  heat  given  off. 

The   truth   of   these   equations   has   been   proved   by  many 
elaborate  observations  both  on  animals  and  men. 


4  PHYSIOLOGY 

Thus  from  one  point  of  view  ph^^siology  may  be  regarded 
as  the  history  of  all  the  changes  undergone  by  the  food  m 
its  passage  through  the  body,  and  the  mechanisms  by  which 
its  potential  energy  is  transformed  into  the  kinetic  energy  of 
the  various  vital  manifestations. 

But  til  is  analogy  with  the  heat  engine  must  not  ))e  pushed 
too  far.  In  this  the  fuel,  the  source  of  energy,  is  always 
distinct  from  the  machinery  by  which  its  energy  is  converted 
into  work.  In  the  body  it  is  otherwise.  The  food  that  we 
take  in  is  digested,  assimilated,  and  built  up  to  form  part  of 
the  living  framework  of  the  body.  This  living  stuff  at  the 
same  time  takes  up  oxygen,  so  that  a  molecule  is  formed 
containing  all  the  elements  necessary  for  evolution  of  energy. 
Under  certain  conditions  the  foodstuffs  and  the  oxygen  in  the 
living  molecule  combme  together,  the  unstable  living  molecule 
of  high  potential  energy  splitting  up  to  form  stable  compounds 
with  lower  potential  energy,  a  certain  amount  of  energy  in  the 
form  of  heat  or  work  being  rendered  kinetic  or  actual  in  the 
process.  So  the  changes  that  the  foodstuffs  undergo  in 
their  passage  through  the  body  may  be  divided  into  two  main 
stages : 

(1)  Assimilative  or  anabolic  changes. — The  food  that  is 
absorbed  from  the  alimentary  canal,  and  the  oxygen  that  is 
taken  in  through  the  lungs,  are  built  up  into  and  become 
actual  constituents  of  the  livmg  protoplasmic  molecule. 
These  processes,  which  are  spoken  of  under  the  term  ana- 
holisDi,  are  associated  with  evolution  of  very  little  energy ;  or 
it  is  possible  that  energy  may  become  latent  in  the  process,  the 
building  up  being  effected  at  the  expense  of  some  previously 
formed  unstable  compound. 

(2)  Dissimilative  or  hataholic  changes. — These  are  the 
changes  which  are  always  associated  with  activity,  that  is, 
with  the  manifestation  of  some  form  of  energy  (heat,  work, 
electrical  change,  etc.).  The  molecule  of  protoplasm  breaks 
down,  most  of  the  atoms  arranging  themselves  to  form  the 
stable  compounds,  CO9,  water,  and  perhaps  urea,  just  as  a 
molecule  of  nitro-glycerin  when  struck  explodes  with  great 
evolution  of  heat,  and  with  the  formation  of  stable  and  more 
simple  molecules.  But  in  the  living  body  the  explosion  is 
rarely  (under  physiological  conditions)  complete.  There  is 
always  a  remainder  which  does  not  undergo  decomposition, 


INTKODUCTION  5 

and  which  we  endow  with  the  attribute  of  living,  smce  it 
possesses  the  power  of  self-restitution,  and  can  take  up  food- 
stuffs and  oxygen,  and  so  build  itself  up  again  into  the  same 
unstable  molecule  as  before,  ready  to  break  down  in  part  and 
give  rise  to  kmetic  energy. 

These  changes  of  breaking  down  of  a  highly  complex 
livmg  molecule,  with  the  evolution  of  energy,  are  spoken  of 
under  the  general  heading  of  kataholism. 

Before  treatmg  these  processes  any  further,  we  must  pause 
awhile  to  consider  the  manner  in  which  the  body  is  built  up, 
and  what  are  the  essential  morphological  characters  common 
to  all  forms  of  life.  This  miity  in  the  structural  basis  of  all 
living  beings  will  be  seen  better  after  a  study  of  one  of  the 
simplest  forms ;  and  as  a  type  of  these  we  may  take  the  well- 
worn  example  of  the  amoeba. 

This  is  a  mmute  organism  of  variable  size,  found  in  stag- 
nant water  and  in  damp  earth.  If  we  examine  it  under  the 
microscope  we  find  that  it  consists  of  a  small  lump  of  trans- 
parent material,  which  can  be  shown  by  chemical  tests  to 
belong  to  the  protein  group  of  bodies.  But  it  is  distmguished 
from  a  mass  of  inert  protein  by  the  facts  that  it  is  able  to 
ingest  and  digest  food  and  build  it  up  into  its  own  substance, 
that  it  can  move  about  from  place  to  place,  responds  to 
stimulation  by  contractmg  up  into  a  round  ball,  and  has  the 
power  of  reproduction  by  fission — that  is,  one  amceba  divides 
into  two  mdividuals  exactly  similar  to  the  parent  organism 
and  to  one  another ;  m  short,  this  little  lump  is  alive. 

We  find  that  there  is  some  trace  of  differentiation  even  in 
this  primitive  organism.  Thus  towards  the  centre  of  the 
lump  is  a  spherical  or  oval  body  (a  nucleus),  differmg  slightly 
in  its  chemical  characters  from  the  surroundmg  substance, 
which  latter  may  also  be  differentiated  into  an  mner  granular 
or  spongy  portion,  the  endosarc,  and  a  peripheral  hyaline 
material,  the  ectosarc. 

The  stuff'  composing  the  body  of  the  amceba  and  endowed 
with  vital  properties  is  called  protoijlasm. 

The  term  '  protoplasm '  has  been  used  m  two  senses.  By 
histologists  it  is  confined  to  a  substance  fomid  in  livmg  bodies, 
or  bodies  that  were  once  alive,  and  reacting  in  certain  ways 
with  certain  stains  and  reagents.  In  this  book  however, 
we  shall  use  the  term  as  a   convenient   expression    for  the 


6  PHYSIOLOGY 

substance  forming  the  active  living  basis  of  all  our  tissues, 
although  analysis  may  show  considerable  differences  in  its 
chemical  composition  in  various  organs.  Protoplasm  is  in 
fact  living  stuff. 

The  little  mass  of  protoplasm  enclosing  a  nucleus  is  called 
a  cell.     Since  however   there  are  living  organisms  m  which 

Fig.    1. 


Figure  of  amoeboid  corpuscle,  blgbly  magnified,  showing  elongated 
nucleus,  endosarc,  and  ectosarc  (Schafer). 

no  trace  of  a  nucleus  is  to  be  discovered  by  the  most  improved 
methods,  we  do  not  regard  its  presence  as  essential  to  our 
conception  of  a  cell. 

Now  we  find  that  all  the  higher  animals,  includmg  our- 
selves, are  made  up  of  enormous  aggregations  of  similar 
nucleated  masses  of  protoplasm,  and  may  be  regarded  as 
colonies  of  amoebae.  But  just  as  m  a  colony  of  men,  with 
increasmg  growth  of  the  community  there  is  increasing 
differentiation  of  function,  so  in  us  some  cells  are  eminently 
assimilative  and  digestive,  others  respiratory,  others  motile, 
while  some  are  set  apart  for  the  purposes  of  reproduction. 

Hand  in  hand  with  this  physiological  goes  morphological 
differentiation — that  is,  the  structure  of  each  group  of  cells 
becomes  modified  to  fit  it  for  carrying  on  its  own  work  and 
its  own  work  alone. 

Thus  the  motor  cells  do  all  the  external  work  required  by 
the  whole  organism,  both  for  purposes  of  defence  and  offence. 
Under  this  latter  head  we  may  class  the  work  of  getting 
food,  since  this  must  always  be  at  the  expense  of  some  other 
livmg  organism.  In  return  for  this  they  are  supplied  with 
food,  water,  and  oxygen  in  an  assimilable  form  by  the  activity 


INTEODUCTION  7 

of  other  groups  of  cells,  just  as  a  soldier — the  community's 
instrument  of  offence  and  defence — is  clothed,  fed,  and  housed 
at  the  expense  of  the  community  for  which  he  works. 

A  collection  of  cells,  modified  and  built  up  together  for 
some  particular  fmiction,  is  called  an  organ  when  we  are 
considering  its  physiological  import,  or  a  tissue  when  we 
regard  only  its  morphological  aspect. 

The  Assimilation  of  Food 

In  some  of  the  lowest  forms  of  animal  life,  food  can  be 
taken  in  at  any  part  of  the  surface  of  the  body.  In  the 
amoeba  we  can  observe  the  whole  process  with  the  microscope, 
and  we  see  how  the  particle  of  food  that  has  been  taken  in 
undergoes  partial  solution  —  that  is  to  say,  part  of  it  disappears 
and  apparently  becomes  built  up  mto  the  livmg  stuff  of  the 
organism.  The  remnant  that  cannot  be  dissolved  is  again 
turned  out  of  the  body  through  any  part  of  the  surface. 

The  processes  by  which  these  mmute  animals  assimilate 
food  are  very  similar  to  those  taking  place  in  man  and  allied 
forms,  only  m  the  latter  we  find  that  there  is  a  differentiation 
of  fmiction,  a  division  of  labour  in  which  some  cells  of  the 
body  take  up  one  part  of  the  work  of  assimilation,  while  other 
cells  are  told  oft'  to  carry  on  another  part ;  and  we  are  able  to 
study  the  whole  process  much  more  fully  since  we  can  take  it 
bit  by  bit.  In  man  the  work  of  taking  up  food  is  still  per- 
formed by  the  surface  of  the  body,  but  it  is  a  special  part  of 
the  surface,  highly  differentiated,  and  protected  by  its  position 
and  certam  mechanisms  from  coming  in  contact  with  anything 
except  food,  so  that  it  may  devote  its  whole  energies  to  this 
one  function. 

All  the  higher  animals  may  be  considered  as  built  in  the 
form  of  a  tube,  the  external  surface  of  which  is  differentiated 
for  purposes  of  defence,  and  therefore  forms  also  the  organs 
by  which  the  processes  of  the  body  act  in  harmony  with 
changes  in  its  environment. 

The  mternal  surface,  on  the  other  hand,  is  the  special 
alimentary  surface,  and  is  called  the  alimentary  canal. 

Between  these  two  surfaces  the  wall  of  the  tube  contains 
the  supporting  tissues  of  the  body,  the  bones,  etc.,  and  also 
the  organs  for  the  conversion  of  the  potential  energy  of  the 


8  PHYSIOLOGY 

food  into  motion  and  work,  the  muscles.  In  all  the  higher 
animals  cavities  are  developed  between  the  two  surfaces  in 
the  substance  of  the  middle  layer — the  body-cavities,  repre- 
sented in  man  by  the  pleural  and  peritoneal  cavities ;  so  that 
the  alimentary  canal  for  a  considerable  part  of  its  course  is 
connected  with  the  body-wall  only  by  one  side,  and  seems  to 
hang  down  mto  the  peritoneal  cavity. 

The  tube  of  special  alimentary  cells  formmg  the  digestive 
canal  is  surrounded  by  motor  cells  derived  from  the  middle 
layer.  These  serve  to  drive  the  food  from  one  end  to  the 
other,  and  to  expel  the  innutritions  matter. 

In  order  to  get  a  greater  number  of  working  cells,  there 
are  recesses  of  the  surface  lined  with  cells,  which  are  called 

Fig.  2. 


h 
Diagram  showing   relations   of   embryonic   layers,     e.  Epiblastic    or 
outer  layer,     h.  Hypoblastic  or  alimentary  layer,     m.  Mesoblastic 
or    middle    layer,     b.  Body   cavity,     n.  Central    nervous    system. 
al.  Alimentary  canal. 

glands,  and  protuberances  in  the  lumen  of  the  tube,  which 
are  called  villi.  Even  among  the  cells  of  the  alimentary 
surface  there  is  difierentiation  of  function.  Thus  the  cells  of 
the  glands  manufacture  and  pour  out  fluids,  varying  in  com- 
position and  action  at  different  parts,  which  have  the  power 
of  moistening  and  dissolving  the  constituents  of  the  food, 
while  the  cells  covering  the  villi  seem  more  especially  adapted 
for  absorbing  the  food  after  it  has  been  digested  and  rendered 
soluble.  The  glands  may  be  simple  tubular  recesses  in  the 
mucous  membrane,  or  may  branch  to  such  an  extent  as  to 
form  a  bulky  organ.  The  liver  is  a  type  of  such  an  overgrown 
process  of  the  alimentary  epithelium.  This  latter  organ, 
however,  has  othe]'  imjDortant  functions  to  perform  besides  the 


INTRODUCTION  9 

mere  solution  of  foodstuffs.  It  is  to  a  large  extent  concerned 
in  further  elaborating  the  foodstuffs  after  they  have  been 
absorbed  into  the  body,  so  as  to  make  the  function  of  self- 
nutrition  still  easier  for  the  other  servants  of  the  organism. 

We  may  here  run  through  the  various  parts  of  the  alimen- 
tary canal,  with  the  glands  openmg  into  it.  From  the  mouth, 
where  the  food  is  chiefly  broken  up  by  the  teeth  and  moistened 
by  the  saliva,  which  is  secreted  by  the  salivary  glands,  the 

Fig.  3. 


Diagram  of  alimentary  canal,  m.  Mouth  with  salivary  glands,  s, 
opening  into  it.  ce.  (Esophagus,  st.  Stomach,  d.  Duodenum  with 
pancreas,  p,  and  liver,  1,  opening  into  it.  int.  Small  intestine, 
cffi.  Caecum,     c.  Colon,     r.  Eectum.     a.  Anus. 


food  passes  through  the  tubular  oesophagus  or  gullet  into  the 
stomach.  This  is  a  saccular  dilatation  of  the  canal,  situated 
in  the  upper  part  of  the  abdomen.  In  it  the  foodstuffs  are 
acted  upon  by  the  gastric  juice,  secreted  by  small  tubular 
glands,  and  the  dissolved  products  are  partly  taken  up  or 
absorbed. 

After  the  stomach  the  alimentary  canal  becomes  narrowed 
again  to  form  the  small  intestine.     In  man  this  is  divided 


10  PHYSIOLOGY 

into  three  main  divisions — the  duodenum,  aljout  nine  inches 
long ;  and  the  jejunum  and  ileum,  about  twenty  feet  long. 

Into  the  upper  part  of  the  duodenum  two  glands,  the  liver 
and  pancreas,  pour  their  juices,  while  the  whole  internal 
surface  is  taken  up  with  villi  and  tubular  glands  (crypts  of 
Lieberkiihn),  so  that  digestion  and  absorption  go  on  simulta- 
neously. The  ileum  leads  into  the  colon  or  large  intestine. 
This  is  about  twice  as  wide  as  the  small  intestine,  from  which 
it  is  separated  by  a  valve,  the  ileo-colic  valve.  Its  internal 
surface  is  entirely  taken  up  with  tubular  glands,  no  villi  being 
present.  In  this  part  of  the  canal  absorption  probably  pre- 
dominates over  digestion.  The  lower  part  of  the  large  intes- 
tine is  the  rectum,  and  opens  by  an  aperture,  the  anus,  on  the 
surface  of  the  body,  by  which  the  indigestible  residue  of  the 
food,  forming  the  faeces,  is  discharged. 

Circulation  of  the  Blood 

In  order  that  the  foodstuffs  taken  up  by  the  cells  lining 
the  alimentary  canal  should  be  distributed  to  all  parts  of  the 
body,  there  must  be  some  means  of  transporting  the  food. 
This  means  is  furnished  by  the  blood.  All  the  tissues  of  the 
body  are  supplied  with  a  close  meshwork  of  delicate  tubes, 
called  capillaries,  the  walls  of  which  are  formed  by  a  single 
layer  of  cells.  This  layer  is  permeable  to  fluidsj  so  that  the 
surrounding  tissues  are  practically  in  contact  with  the  blood 
in  the  capillary  tubes,  and  can  take  up  nourishment  from  or 
give  ofi'  their  effete  material  to  it.  These  capillaries  com- 
municate with  larger  tubes  which  have  thicker  walls,  and 
these  lead  to  and  from  a  hollow  organ  with  thick  muscular 
walls — the  heart. 

The  heart  is  divided  into  four  cavities,  two  auricles  and 
two  ventricles,  each  auricle  being  separated  from  the  adjoin- 
ing ventricle  by  valves.  These  valves  are  so  arranged  that, 
when  the  heart  contracts  and  diminishes  its  capacity,  the 
blood  can  flow  only  in  one  direction.  We  may  compare  it  to 
an  enema  syringe  in  which  the  compressing  force  is  in  the 
elastic  wall  instead  of  being  supplied  by  the  hand  of  the 
experimenter. 

The  tubes  taking  the  blood  from  the  heart  to  the  tissues 
have  thick  elastic  walls,  and  are  called  arteries  ;  while  those 


INTRODUCTION 


11 


bringing  the  blood  back  from  the  tissues  are  called  veins,  and 
have  thinner  and  more  distensible  walls. 


Fio.  4. 


Diagram  of  capillaries  in  frog's  web. 


Fig.  5  shows  the  course  taken  by  the  blood  in  its  circula- 
tion through  the  body,  as  it  is  impelled  by  the  contracting 
heart. 


Diagram  of  circulation,  l.v.  Left  ventricle,  l.a.  Left  auricle. 
R.v.  Eight  ventricle,  k.a.  Eight  auricle,  ao.  Aorta,  s.c.  Sys- 
temic cai^illaries.  al.  Alimentary  canal,  p.  Portal  vein.  l.  Liver. 
H.v.  Hepatic  vein.     p.c.  Capillaries  of  lungs. 

Starting  from  the  left  ventricle,  the  blood   is   propelled 
through  the  aorta  into  the  systemic  arteries,  and  thence  into 


12  PHYSIOLOGY 

the  capillaries  supplying  the  head,  neck,  body,  limbs,  and 
alimentary  canal. 

From  the  capillaries  of  the  alimentary  canal  the  blood 
flows  into  a  number  of  veins,  which  unite  to  form  a  large 
vessel,  the  portal  vein.  This  then  enters  the  substance  of  the 
liver  and  breaks  up  again  into  a  number  of  capillaries,  which 
ramify  and  anastomose  round  the  hepatic  cells. 

The  blood  from  these  capillaries  is  agam  collected  into 
a  large  vessel,  the  hepatic  vein,  which  flows  mto  the  inferior 
vena  cava.  This  latter  vessel  also  conveys  blood  from  the 
back,  lower  limbs,  and  kidneys. 

The  superior  vena  cava  with  the  blood  from  the  upper 
extremities  and  head  and  neck,  and  the  inferior  vena  cava 
open  into  the  right  auricle. 

From  the  right  auricle  into  the  right  ventricle,  then  along 
the  pulmonary  artery  which  breaks  up  into  mnumerable  capil- 
laries in  the  lungs  (formmg  the  lesser  circidation),  then  from 
the  pulmonary  capillaries  along  the  pulmonary  veins,  the 
blood  reaches  the  left  auricle,  from  which  it  flows  into  the  left 
ventricle,  having  completed  its  whole  circulation. 

Bespiration  and  Excretion 

We  have  hitherto  spoken  of  the  blood  only  as  a  medium 
for  the  distribution  of  food  to  the  various  tissues.  But  it  is 
more  than  this.  Every  cell,  every  protoplasmic  miit  of  the 
body,  to  live,  must  be  supplied  with  oxygen,  and  must  be  able 
to  get  rid  of  the  products  of  its  activity,  namely,  CO^  and  urea, 
or  some  body  allied  to  urea.  In  all  these  functions  the  blood 
acts  as  the  middleman  between  the  cell  hidden  deep  in  the 
body  and  the  cell  on  the  surface  of  the  body. 

Thus  as  the  blood  flows  through  the  lungs,  it  is  separated 
from  the  air  in  the  cavities  (alveoli)  of  this  organ  only  by 
the  thinnest  possible  layer  of  cells.  Here  we  find  that  the 
blood  changes  its  composition,  giving  up  COo  and  taking  in 
oxygen. 

When  the  blood  reaches  the  tissues,  a  process  the  reverse 
of  this  takes  place  ('  internal  respiration '),  the  cells  of  the 
tissues  takuig  up  oxygen  and  giving  up  CO,  to  the  blood. 

The  cells  also  discharge  their  nitrogenous  waste  products 
into  the  blood,  which  immediately  carries  them  to  the  kidneys. 


INTRODUCTION 


13 


In  these  organs  again  we  find  a  single  layer  of  cells  between 
the  blood-capillaries  and  a  cavity  which  is  in  free  communi- 
cation with  the  exterior.     It  is  these  cells  which  take  up  the 


Fig.  6. 


Diagram  of  lung  tissue,  showing — b.  Capillary  blood-vessels  in  walls 
of  alveoli,  e.  Epithelium  lining  the  alveoli,  f.  Cut  edges  of 
alveolar  walls,  consisting  of  connective  tissue  fibres  and  elastic 
tissue. 

Fig.  7. 


Diagram  of  kidney,  av.  Afferent  blood-vessel,  ev.  Efferent  blood- 
vessel, c.  Loop  of  capillaries,  e.  Secreting  epithelium,  u.  Urinary 
tubule  leading  to  bladder  and  exterior. 


waste  products  of  the  other  tissues  from  the  blood  and  dis- 
charge them  mto  the  urinary  tubule,  together  with  water  and 
salts,  as  urme.     From  the  urinary  tubule  the  urine  flows  down 
the  ureter  into  the  bladder,  whence  it  is  voided  periodically. 
Thus  the  blood  is  contmually  taking  up  food   from  the 


14  PHYSIOLOGY 

alimentary  canal  and  oxygen  from  the  lungs,  and  carrying 
them  to  the  tissues.  Here  it  parts  with  them  and  receives  in 
return  the  products  of  tissue  change  (which  are  indeed  only 
the  products  formed  by  the  union  of  the  oxygen  with  the  food- 
stuffs), and  carrying  these  away  discharges  them  on  the 
exterior  of  the  body  by  means  of  the  lungs  and  kidneys. 


Muscula?'  Tissues 

Retaming  our  comparison  of  the  human  body  to  a  heat 
engine,  we  have  still  the  most  important  part  of  the  mecha- 
nism to  consider  —namely,  that  part  in  which  the  heat  produced 
by  the  oxygenation  of  the  food  is  converted  into  motion  and 
so  performs  work. 

This  is  effected  by  specialised  groups  of  cells  united  together 
to  form  the  muscles.  These  are  fleshy  masses  attached  at  two 
ends  to  the  bones  and  other  supporting  tissues  of  the  body. 
They  are  capable  under  certain  circumstances  of  shortening, 
that  is  to  say,  approximating  the  points  to  which  their  two 
ends  are  attached  against  resistance,  so  that  they  do  work. 
Thus  the  biceps  muscle  of  the  arm  is  attached  above  to  the 
shoulder-blade,  and  below  to  the  radius,  one  of  the  bones  of 
the  forearm.  When  it  contracts,  it  thickens  and  shortens, 
and  draws  up  the  forearm  so  as  to  bend  the  elbow-joint. 
Thus  it  may  do  work  in  two  ways.  If  the  shoulder  is  fixed, 
contraction  of  the  biceps  will  raise  a  weight  held  in  the  hand ; 
or  the  hand  may  be  fixed,  as  in  hanging  on  a  horizontal  bar, 
so  that  the  effect  of  contraction  of  the  muscles  is  to  raise  the 
shoulders  and  with  them  the  whole  body. 

We  also  find  muscular  tissue,  though  less  highly  differen- 
tiated, in  the  interior  of  the  body  surrounding  the  heart, 
blood-vessels,  and  alimentary  canal.  It  is  by  the  contraction 
of  these  hollow  muscular  tubes  that  the  blood  is  set  in  motion, 
and  the  food  propelled  from  one  end  of  the  alimentary  canal 
to  the  other. 

Go-ordination  and  Reaction 

Here  our  analogy  of  the  heat  engine  must  cease.  For 
whereas  an  engine  needs  engineers  and  stokers  to  determine 
its  work  and  keep  it  supplied  with  fuel,  a  man's  body  goes 


INTRODUCTION  15 

on  seeking  out  food  and  feeding  itself  and  working  for  sixty 
or  seventy  years. 

According  as  the  external  circumstances  vary,  so  a  man 
must  do  more  or  less  work,  must  take  more  or  less  food. 
Moreover,  in  such  a  complicated  mechanism  as  the  human 
body,  there  must  be  a  delicate  adjustment  of  the  actions  of 
the  various  organs  to  one  another,  so  that  the  part  which  is 
doing  most  work  should  be  best  supplied  with  nutriment  and 
oxygen,  and  no  part  waste  its  stored-up  energies  in  doing 
useless  work. 

Thus  there  is  an  adaptation  of  the  actions  of  the  body  as 
a  whole  to  the  requirements  of  its  environment  ('necessity  '), 
and  also  mutual  adaptation  within  the  body  of  the  actions  of 
the  various  organs  to  one  another.  This  harmonious  working 
of  the  body  and  all  its  parts  is  effected  by  the  governing  and 
directing  power  of  the  central  nervous  system,  the  brain  and 
spinal  cord  of  all  the  higher  animals. 

In  comparing  the  human  body  to  a  tube,  we  alluded  to 
the  outer  layer  of  cells  as  especially  set  apart  for  the  purpose 
of  protection,  and  for  regulating  the  events  of  the  body 
according  to  the  changes  in  the  environment.  Very  early 
in  develoi^ment  however,  we  find  that  a  part  of  this  surface 
becomes  involuted  into  a  groove,  the  walls  of  which  close 
over  so  as  to  form  a  canal — the  primitive  neural  canal. 
The  latter  then  becomes  cut  off  for  most  of  its  extent  from 
the  external  surface  by  an  mgrowth  of  the  middle  layer  or 
mesoblast. 

The  cells  formmg  the  walls  of  the  canal  grow  and  mul- 
tiply, so  that  finally  the  canal  is  extremely  mmute  in  propor- 
tion to  these  walls. 

The  spmal  cord  retains  this  primitive  form  of  a  canal 
with  thick  walls.  At  the  anterior  end  of  the  body  (head 
end)  this  canal  becomes  dilated,  and  sends  out  lateral  and 
mesial  prolongations.  The  walls  of  these  also  become 
thickened  at  some  places  and  thinned  at  others,  so  that  finally 
a  bulky  complicated  organ  is  produced  which  we  call  the 
brain. 

In  figs.  8,  9,  and  10  the  different  parts  of  the  central 
nervous  system  are  shown  diagrammatically,  and  from  these 
the  origin  of  the  whole  brain  and  spinal  cord  from  a  simple 
canal  pinched  off  from  the  epiblast  will  be  evident. 


16 


PHYSIOLOGY 


But  this  tube  of  specially  '  reactive '  or,  as  we  shall  always 
call  them,  nerve-cells,  still  remains  connected  with  the 
periphery  by  strands  of   protoplasm   which   we  call  nerves. 


Fig.  8. 


Diagrams  showing  formation  of  the  nervous  centre  (brain  and  spinal 
cord)  by  a  tuclcing-in  of  the  outer  or  epiblastic  layer,  e.  Epiblast. 
m.  Mesoblast.  h.  Hypoblast,  n.c.  Neural  canal,  n.g.  Neural 
groove. 

These  strands  in  many  cases  end  close  under  the  surface  of 
the  skin  in  various  forms  of  cells,  specially  differentiated  for 
feeling  different  kinds  of  stimuli. 

Fig.  9. 


Diagram  of  the  cerebral  vesicles  of  the  brain  of  a  chick  at  the  second 
day(Cadiat).     1,2,3.  Cerebral  vesicles.     0.  Optic  vesicles. 

By  this  means  the   central  system   may  become   aware 
of  all  the  changes  occurring  at  the  periphery  of  the  body. 


INTRODUCTION 


17 


But  it  is  necessary  that  the  organism  should  be  able  to 
react  to  changes  m  its  surroundings,  and  we  find  that  very 
early  in  the  development  of  the  body  certain  cells  in  the 
wall  of  the  tube  send  out  long  protoplasmic  processes  which 
become  connected  with  the  muscles,  glands,  heart,  and  blood- 
vessels. Through  these  processes  impulses  descend  from  the 
brain  or  spmal  cord  in  response  to  stimuli  which  have  pro- 
ceeded up  the  sensory  nerves  from  the  periphery. 

This  change  in  the  intra -corporeal  events  determined  by  a 
change  in  the  extra-corporeal  through  the  intervention  of  the 
central  nervous  system  is  called  a  reflex  action.     The  meaning 


Longitudinal  section  through  brain  of  chick  of  ten  days  (after 
Mihalkoviez).  olf.  Olfactory  lobes.  h.  Cerebral  hemisphere. 
Iv.  Lateral  ventricle,  pin.  Pineal  gland,  bg.  Corpora  bigemina. 
cbl.  Cerebellum,  o.c.  Optic  comniissure.  pit.  Pituitary  body, 
pv.  Pons  Varolii,  m.o.  Medulla  oblongata,  v^,  v^  Third  and 
fourth  ventricles. 


of  this  term  must  be  carefully  borne  in  mind,  since  we  shall 
meet  with  examples  of  it  in  every  stage  of  our  subject.  In 
fact,  the  whole  of  an  animal's  life  may  be  looked  upon  as  one 
long  series  of  reflex  actions. 

Perhaps  the  idea  will  be  rendered  more  concrete  by  an 
example.  If  we  decapitate  a  frog  and  then  dip  one  of  its  toes 
into  dilute  acid,  the  leg  is  drawn  up. 

This  reaction  may  be  prevented  in  any  one  of  the  follow- 
ing ways : 

1.  Section  of  the  sensory  (afferent)  nerves  from  the  toes 
to  the  spinal  cord. 

2.  Destruction  of  the  spinal  cord. 

3.  Section  of  the  motor  (efferent)  nerves  coming  from  the 
cord  and  running  to  the  muscles. 

4.  Destruction  of  the  muscles  of  the  leg. 


18 


PHYSIOLOGY 


Thus  the  elements  composmg  a  reflex  arc  are — 

(1)  A  sentient  surface  (such  as  the  skin)  connected  by — 

(2)  A  sensory  or  afferent  nerve 

(3)  To  a  cell  or  group  of  cells,  or  of  nerve-tracts  in  the 
central  cerebro-spinal  axis.     This  again  is  connected  by — 

(4)  A  motor  or  efferent  nerve  to 

(5)  A  muscle  or  group  of  muscles. 

For  muscle  in  (5)    we   may  substitute  gland-cell  or  any 
other  cell  in  the  body  capable  of  responding  by  some  change 

Fig.  11. 


Diagram  of  reflex  action,  e.  Sensory  epithelium,  a.n.  Afferent 
nerve-fibre.  s.c.  Sensory  cell.  c.n.ft.  Central  nervous  system. 
n.r.  Branch  of  sensory  cell  in  close  contact  with  processes  of  m.c, 
motor  cell.  e.n.  Efferent  nerve-fibre,  terminating  in  end-plates  on 
the  muscle,  m. 

in  its  condition  to  a  stimulus  reaching  it  from  the  central 
nervous  system. 

To  fire  off  a  reflex  arc,  all  that  is  necessary  is  an  appro- 
priate stimulus  applied  to  the  sentient  surface.  Now  we  find 
that  m  all  animals  almost  anj^  form  of  energy  may  serve  as 
a  stimulus.  Thus  it  may  be  merely  mechanical  as  when  we 
poke  the  frog,  or  chemical,  or  electrical,  or  in  the  form  of 
light,  heat,  or  sound.  In  every  case  where  a  stimulus  is 
applied  there  is  an  expenditure  of  some  energy,  though  the 
amomit  may  be  very  slight — a  conversion  of  one  of  those 
forms  of   motion    (of  masses  or  molecules)  into  some  other 


INTRODUCTION  19 

forms  of  motion  which  cause  or  attend  the  passage  of  an 
impulse  up  the  afferent  nerve. 

It  must  be  noted  however  that  the  work  done  by  the 
stimuhis  is  in  no  way  equivalent  to  the  energy  it  sets  free 
reflexly.  The  slightest  touch  of  a  pin  to  the  skin  causes 
most  powerful  reflex  movements  of  the  thigh  muscles  of  a 
decapitated  frog.  A  stimulus  in  fact  acts  only  by  setting 
free  a  large  amount  of  potential  energy  previously  stored  up 
in  the  muscles,  which  amount  may  depend  on  the  most 
diverse  circumstances :  just  as  the  amount  of  energy  set  free 
in  firing  a  gun  depends  on  the  amount  of  gunpowder  in  the 
charge,  and  not  on  the  size  of  the  fulminating  cap  used  to 
fire  it.  This  simile  however  must  not  be  taken  too  literall}'. 
Under  normal  circumstances  a  stronger  stimulus,  within 
certam  limits,  will  give  rise  to  a  stronger  evolution  of  energy 
(contraction  of  a  muscle,  etc.)  ;  but  in  every  case  the  work 
done  by  the  muscle  far  transcends  in  amount  the  work  done 
m  stimulating  the  muscle. 

In  all  higher  animals  it  is  not  the  whole  sensory  surface 
that  will  respond  to  all  forms  of  stimuli.  Here  agam  there 
is  a  division  of  labour  among  the  sensory  cells,  some  taking 
on  the  function  of  converting  light,  others  heat,  others  sound, 
and  so  on,  into  a  nervous  impulse,  which  may  ascend  to  the 
central  nervous  system  and  there  give  rise  to  some  form  of 
response  as  a  reflex  action. 

Thus  we  have  the  formation  of  organs  of  special  sense 
containing  sensory  cells,  which  under  normal  circumstances 
react  to  one  form  of  stimulus,  and  only  one.  The  eye  receives 
impressions  of  light,  the  ear  of  sounds,  special  cells  in  the 
skin  impressions  of  temperature  (heat  and  cold),  and  of  pres- 
sure and  touch. 

The  higher  sense-organs,  as  they  are  termed— the  eye, 
olfactory  organ,  and  ear — are  connected  with  the  upper  part 
of  the  neural  tube,  the  brain.  The  end-organs  of  temperature 
and  touch  are  distributed  over  the  whole  surface  of  the 
body,  and  are  connected  to  the  neural  tube  through  its  whole 
length. 

The  spinal  cord  has  a  twofold  function.  It  serves  as  a 
conductor  of  impulses  started  by  stimuli  at  the  surface  of  the 
body  to  the  bram,  and  as  a  conductor  of  efferent  impulses 
from  the  brain.     It  may  also  be  regarded  as  a  collection  of 


20  PHYSIOLOGY 

reflex  centres  regulating  all  kinds  of  movements  and  func- 
tions. 

The  nerves,  which  connect  the  cord  with  all  parts  of  the 
body,  are  arranged  symmetrically  on  the  two  sides  of  the 
body,  so  that  there  are  thirty-one  pairs  of  nerves  arising  from 
the  spinal  cord. 

Each  nerve-trunk  at  its  connection  to  the  cord  is  divided 
into  an  anterior  and  a  posterior  root ;  and  we  shall  show 
later  on  that  all  the  impulses  leaving  the  cord  pass  along  the 
anterior  roots,  while  the  posterior  roots  are  composed  exclu- 
sively of  fibres  bearing  afferent  impulses. 

A  short  distance  from  the  cord  these  two  roots  join,  and 
the  mixed  nerve-trunk  thus  formed  divides  again  and  again, 
supplymg  the  sensory  end-organs  and  muscles  of  a  definite 
segment  of  the  body. 

An  important  branch  of  the  mixed  nerve-trunk  (though 
small  anatomically)  is  the  ramus  communicans  of  the  sympa- 
thetic. The  fibres  forming  this  little  nerve  pass  into  a  chain 
of  ganglia  {i.e.  collections  of  nerve-cells)  situated  along  the 
back  of  the  body-cavity,  in  front  of  the  spmal  column,  and 
sending  a  prolongation  up  mto  the  neck. 

Some  of  the  fibres  become  connected  with  these  ganglion 
cells  while  others  simply  pass  through  the  ganglion.  All 
the  fibres  however,  before  passing  to  their  destination,  become 
connected  with  nerve-cells  situated  more  peripherally.  This 
destmation  is  all  the  organs  of  vegetative  life,  the  alimentary 
canal,  heart,  blood-vessels,  glands,  etc. 

The  whole  system  of  visceral  nerves,  formed  from  the 
nerve-fibres  of  the  rami  communicantes,  is  called  the  sym- 
pathetic system. 

The  accompanying  diagram  will  serve  to  illustrate  the 
general  plan  of  distribution  of  a  single  spinal  nerve,  and  its 
connections  with  the  sympathetic  system. 

An  unending  series  of  reflex  actions  is  gomg  on  m  our 
body,  and  of  many  of  these  we  may  be  quite  miaware.  But 
in  very  many  cases  we  learn  that  something  is  going  on  when 
a  stimulus  affects  our  afferent  nerves  by  having  what  we  call 
a>  feeling  or  sensation.  In  order  to  experience  a  sensation  we 
must  be  conscious,  and  therefore  sensations  are  classed  with 
emotions,  ideas,  volitions,  as  states  of  consciousness. 

A  ]5ure  investigation  of  states  of  consciousness,  however, 


INTRODUCTION 


21 


belongs  to  the  province  of  the  psychologist.  We  have  only 
to  deal  with  them  so  far  as  the  function  of  afferent  nerves  is 
concerned,  since  it  is  difficult  to  obtain  an  objective  sign  of 
their  activity  (though,  as  we  shall  see  later,  such  signs  are 
present) . 

We  have  evidence  that  m  man  and  the  higher  animals 
consciousness   is  intimately  bound   up   with  the   outgrowths 

Fig.  1-2. 


Scheme  of  the  nerves  of  a  segment  of  the  sijinal  cord  (Foster).  Gr. 
Grey ;  w,  white  matter  of  the  cord.  a.  Anterior ;  p,  posterior 
root.  G.  GangUon  on  posterior  root.  n.  Whole  nerve  giving  off 
n'  somatic  nerve  to  muscle  m,  or  sensory  cell  s,  and  visceral 
branch  v,  which  passes  through  various  ganglia  of  the  sympa- 
thetic system  before  reaching  the  visceral  muscle  m  or  sensory 
cells,  rv  is  the  grey  ramus  communicans  which  runs  back  from 
the  ganglion  2  to  the  spinal  cord.  It  gives  off  a  branch  vm, 
which  runs  in  connection  with  the  spinal  nerve  to  the  blood-vessels 
of  the  limbs.  Sy.  The  sympathetic  chain  uniting  the  ganglia  of 
the  series  2. 


from  the  front  of  the  neural  tube,  which  we  call  the  cerebral 
hemispheres.  On  the  integrity  of  these  depends  also  the 
carrying  out  of  the  so-called  spontaneous  or  voluntary  move- 


22  PHYSIOLOGY 

ments  —that  is  to  say,  movements  which  are  not,  so  far  as  we 
can  tell,  called  forth  by  any  directly  preceding  stimulus  or 
change  in  the  environment  of  the  animal. 

The  parts  of  the  brain  below  the  hemispheres  seem  built 
up  on  much  the  same  plan  as  the  spinal  cord,  and  like  this 
contain  a  collection  of  reflex  centres  and  nerve-paths.  These 
reflex  mechanisms  however  are  very  important,  since  their 
afferent  channels  are  the  important  nerves  subservmg  the 
functions  of  seeing,  hearmg,  smellmg,  tastmg,  etc. 

The  significance  of  the  cerebral  hemispheres  in  the  normal 
life  of  the  animal  can  be  well  shown  in  the  frog.  If  in  this 
animal  we  remove  the  cerebral  hemispheres,  we  find  that  it 
still  acts  in  all  respects  as  a  normal  frog,  except  that  it  is 
incapable  of  interpreting  its  sensations  or  of  mitiating  volun- 
tary acts.  If  we  put  it  in  water,  it  swims  about  till  it  comes 
to  the  edge  of  the  basin,  when  it  crawls  up  and  sits  on  the 
edge.  Stroke  its  back  and  it  croaks.  Put  it  on  a  horizontal 
board,  it  remains  perfectly  still.  Incline  the  board,  the  frog 
climbs  up  it.  All  these  complicated  movements  are  brought 
about  immediately  by  changes  in  the  environment.  They  are 
examples  of  reflex  action. 

But  if  we  leave  the  brainless  frog  to  itself,  and  protected 
from  disturbing  influences,  it  sits  there  till  it  becomes  a 
mummy.  Having  lost  the  greater  part  of  its  consciousness, 
it  can  no  longer  jeel  hmigry,  it  does  not  hnoio  its  food  when 
it  sees  it,  and  therefore  does  not  loill  to  move  in  order  to 
get  it. 

Beprod'uction  and  Heredity 

The  production  of  new  individuals,  the  crowning  point  of 
an  animal's  existence,  is  carried  out  by  means  of  certain 
special  cells,  the  spermatozoa  in  the  male  and  the  ova  in  the 
female,  which  represent  potentially  all  the  peculiarities  of 
the  parent  organism.  By  the  union  and  fusion  of  parts  of 
a  spermatozoon  and  ovum,  a  single  cell  is  produced,  the  fer- 
tilised ovum,  as  it  is  termed ;  and  from  this  cell  by  division 
and  differentiation  is  formed  the  new  individual,  endowed 
with  structures  and  properties  similar  to  and  derived  from 
both  parents.  The  rest  of  the  cells  of  the  body  afterwards 
die,  havmg  served  their  function  when  they  have  reared  the 
new  family  of  individuals. 


INTEODUCTION  23 

Thus  from  a  broad  standpoint  all  the  complicated  pro- 
cesses that  we  study  in  physiology,  all  the  toil  and  turmoil 
of  human  existence,  are  nothmg  but  '  the  by-play  of  ovum- 
bearing  organisms.'  The  biological  destiny  of  man  is  accom- 
plished with  the  production  and  rearing  of  a  new  mdividual. 

Now  we  have  shown  above  that  in  many  respects  the  body 
may  be  regarded  as  a  mechanism,  controlled  by  external 
circumstances,  and  converting  the  potential  energy  of  food 
into  the  kinetic  energy  of  warmth  and  movement. 

This  comparison  is  further  justified  when  we  find  that 
in  all  processes  of  the  body  there  is  no  creation  of  energy. 
All  energy  possessed  by  the  body  is  derived  from  the  poten- 
tial energy  contained  in  the  food,  which  in  its  turn  repre- 
sents the  stored-up  energy  of  the  sun's  rays. 

On  these  accounts  many  have  thought  that  no  other  fac» 
tors  were  at  work  in  livmg  bodies  than  the  intermolecular 
relations  which  comprise  the  laws  of  physics  and  chemistry, 
and  that  even  the  supreme  facts  of  consciousness  might  be 
explained  in  this  manner.  But  past  experience  warns  us  to 
be  very  careful  before  accepting  purely  physico-chemical  con- 
ceptions of  any  vital  phenomena.  Again  and  again,  as  we 
shall  see  when  discussmg  the  processes  of  absorption,  secre- 
tion, respiration,  etc.,  have  purely  physical  explanations  been 
put  forward,  only  to  be  overthrown  by  further  investiga- 
tions. 

In  fact  every  cell  in  the  body,  like  a  conscious  being 
seems  to  have  a  power  of  selection,  a  power  to  eschew  the 
evil  and  choose  the  good,  the  good  being  that  which  is  neces- 
sary to  its  preservation  as  a  unit  of  the  cell  community.  A 
layer  of  living  protoplasm,  one  twenty-thousandth  of  an  inch 
in  thickness,  is  able  to  take  up  materials  on  one  side  and 
discharge  them  on  the  other,  in  direct  opposition  to  all 
known  physical  laws  of  diffusion  and  osmosis. 

We  may  discover  the  functions  of  a  living  cell  and  the 
conditions  of  its  activity,  and,  m  general  terms,  the  source 
from  which  it  derives  its  energy ;  but  beyond  this  we  have 
been  foiled  in  all  attempts  to  find  out  how  the  cell  uses  the 
energy  of  the  food  for  its  own  aims.  It  does  not  at  present 
seem  likely  that  any  physico-chemical  hypothesis  will  ever 
explain  how  all  the  physical  and  intellectual  peculiarities  may 
be  transmitted  from  father  to  son  through  one  single  minute 


24  PHYSIOLOGY 

cell,  a  spermatozoon,  five  hundred  millions  of  which  would 
hardly  occupy  one  cubic  millimetre. 

We  shall  therefore  m  the  following  pages  confine  our- 
selves to  a  discussion  of  the  functions  of  the  various  organs, 
the  conditions  of  their  activity,  and  the  physical  and  chemical 
changes  which  can  be  demonstrated  to  occur  in  the  organs 
concomitantly  with  their  activity. 

These  objects  of  physiology  are  still  very  imperfectly 
known,  and  probably  need  yet  many  years  of  laborious 
research  for  their  elucidation.  But  when  we  are  fully 
acquamted  with  the  laws  and  conditions  of  the  activities  of 
normal  living  structures,  we  shall  be  able  to  attack  the  pro- 
blems of  disease  with  a  sure  hope  of  success  ;  for,  knowing 
how  the  organism  will  react  to  all  manner  of  circumstances, 
we  shall  be  able  to  put  it  into  an  artificial  environment  which 
will  counteract  the  effects  of  the  previous  abnormal  environ- 
ment, and  so  restore  the  organism  to  a  healthy  condition. 


25 


CHAPTER   II 

THE   MATERIAL   BASIS   OF   THE   BODY 

Section  1 
THE   SOUECES   OF   THE   ENERGY   OF  THE   BODY 

Careful  experiments  have  shown  that  there  is  no  creation  or 
destruction  of  matter  m  the  living  organism,  that  is  to  say, 
the  principle  of  the  conservation  of  matter  applies  withm  as 
without  the  living  body.  Hence  an  ultimate  analysis  of  the 
body  reveals  the  same  elements  as  are  found  in  the  earth's 
crust. 

The  results  of  ultimate  analysis  teach  us  that  twelve 
chemical  elements  enter  into  the  composition  of  all  living 
organisms.  These  are  carbon,  hydrogen,  oxygen,  nitrogen, 
sulphur,  phosphorus,  chlorine,  potassium,  sodium,  calcium, 
magnesium,  and  iron.  They  are  essential  to  the  life  of  the 
animal.  Other  elements,  such  as  silicon,  iodine,'  fluorme, 
manganese,  are  found  occasionally,  but  it  is  not  known  whether 
the  minute  quantities  of  these  substances  which  are  found  are 
merely  accidental  or  necessary  to  life. 

If  the  body  of  an  animal  be  heated  with  free  access  of 
oxygen,  the  carbon  and  oxygen  are  burnt  up  and  escape  as 
carbon  dioxide  and  water ;  the  nitrogen  is  eliminated  as 
ammonia  or  in  the  free  state,  and  the  rest  of  the  elements 
remain  as  the  ash. 

One  hundred  parts  of  the  ash  of  a  yomig  dog  contam 
(Bunge)  — 


K,0       .... 

8-5 

Na^O 

8-2 

CaO 

35-8 

MgO 

1-6 

Fe..03 

0-34 

P.63 

39-8 

CI 

7-3 

1    T-ji; i_    1 ; i-T-i- J i.:-i j. 

'  Iodine  seems  to  be  an  invariable  and  essential  constituent  of  the  active 
principle  of  the  thyroid  gland  (vide  Chap.  XII.). 


26  PHYSIOLOGY 

Most  of  these  constituents  of  the  ash  exist  in  the  tissues  sm 
salts,  either  free  or  in  a  very  loose  state  of  combination  with 
other  substances. 

Nearly  all  the  iron,  sulphur,  and  a  considerable  amount  of 
the  phosphorus  however  are  found  in  the  body,  not  as  salts, 
but  as  complicated  organic  compounds  with  proteins  and  allied 
bodies. 

The  same  elements  are  found  in  plants,  although  here 
certain  of  the  non-essential  elements  may  occur  in  greater 
quantities  than  they  do  in  the  case  of  animals.  Thus  silica 
forms  a  large  proportion  of  the  ash  of  certain  grasses,  although 
a  normal  growth  can  be  obtained  in  the  almost  entire  absence 
of  this  substance. 

All  these  elements  are  derived  by  plants  from  the  atmo- 
sphere or  from  the  earth's  crust.  Here  however  the  substances, 
which  present  themselves  as  foodstuffs  to  the  plant,  are  in  a 
state  of  maximum  oxidation,  and  are  devoid  of  potential  energy, 
the  mineral  matters  occurring  as  salts,  the  carbon  as  the  CO.^ 
of  the  atmosphere,  and  the  nitrogen  as  minute  traces  of 
ammonia  or  nitrites.  In  the  plant  these  elements  are  built 
up  into  substances  which  can  develop  energy  by  combustion, 
and  therefore  are  possessed  of  potential  energy.  This  energy 
is  put  into  the  plant  by  the  sun's  rays.  The  green  parts  of 
plants  are  able  to  utilise  the  radiant  energy  of  certain  parts 
of  the  spectrum  in  the  splitting  up  of  CO2,  giving  off'  the 
oxygen  to  the  atmosphere,  and  building  up  the  carbon  into 
complex  compounds,  of  which  the  earliest  appear  to  be  sugar 
and  starch.  It  is  supposed  that  the  first  change  which  occurs 
is  a  formation  of  formaldehyde,  CO.,  +  H.p  =  CHjO  +  0.,,  and 
that  this  formaldehyde  by  simple  condensation  yields  a  carbo- 
hydrate of  the  formula  C,;H,20,;.  The  plant  thus  obtains  a 
store  of  potential  energy,  which  it  can  utilise  for  the  formation 
of  still  higher  products  or  for  the  synthesis  of  such  bodies 
as  fats  and  proteins.  All  three  classes  of  substances,  proteins, 
fats,  and  carbohydrates,  are  built  up  together  with  salts  and 
water  to  form  the  unstable  material  which,  as  the  chemical 
basis  of  life,  is  known  as  protoplasm. 

In  animals  this  power  of  utilising  the  sun's  rays  for  the 
synthesis  of  complex  organic  substances  from  C0._,  and  nitrites 
or  ammonia  is  wanting.  They  are  therefore  dependent  for 
their  life  on  the  co-existence  of  plants.     On  these  they  feed, 


THE   MATEKIAL  BASIS   OF  THE   BODY  27 

taking  iii  their  food  in  the  form  of  proteins,  fats,  and  carbo- 
hydrates. In  the  animal  body  these  substances  are  oxidised, 
setting  free  once  more  the  energy  which  had  been  snatched  by 
the  plant  from  the  smi's  rays,  and  are  excreted  in  their  original 
form  as  CO,,,  and  ammonia  or  allied  bodies  such  as  urea. 

A  marked  distinction  thus  seems  to  obtam  between  the 
chemical  processes  in  plants  and  animals.  In  the  former  the 
processes  are  chiefly  synthetic,  while  in  the  latter  they  are 
chiefly  oxidative.  No  hard  and  fast  line  however  exists  between 
the  two  classes  of  living  bemgs.  The  power  of  splittmg  up 
CO.,  is  confined  to  the  chlorophyll  granules  of  plants.  In  all 
other  parts  of  plants,  and  perhaps  in  these  granules  themselves, 
a  process  of  respiration  is  also  going  on,  in  which  oxygen  is 
absorbed  and  CO.^  is  given  ofl' ;  and  in  the  entire  absence  of 
oxygen  every  plant,  except  certain  anaerobic  bacteria,  dies. 
Respiration  therefore  is  a  common  function  of  all  protoplasm, 
and  is  merely  hidden  in  green  plants  exposed  to  smilight  by 
the  more  energetic  assimilative  function  of  the  chloroph3dl 
granules. 

Although  the  animal  body  cannot  build  up  proteins,  fats, 
and  carbohydrates  from  ammonia  and  COg,  yet,  given  these 
three  classes  of  foodstufls,  it  can  from  these  carry  out  many 
complicated  synthetic  processes.  Thus  it  was  long  ago  shown 
by  Wohler  that  benzoic  acid  administered  to  an  animal  was 
excreted  in  the  urine  as  hippuric  acid.  In  this  case  a  synthesis 
of  benzoic  acid  with  glycine  (amino-acetic  acid)  had  taken  place. 

C«H,.COOH  +  CH.,NH,COOH  =  C,H3.C0.CHNH,C00H  +  H,0. 

We  shall  have  occasion  to  discuss  many  other  examples 
of  synthetic  processes  occurring  in  the  animal  body.  The 
best  known  are  perhaps  the  formation  of  fats  from  carbo- 
hydrates, and  the  formation  of  the  complex  conjugated  pro- 
teins, nucleo-proteins,  etc.,  from  the  simple  proteins,  fats,  and 
carbohydrates  of  the  food. 

Although  therefore  we  must  allow  to  all  protoplasm,  whether 
vegetable  or  animal,  synthetic  as  well  as  disintegrative  powers, 
the  fact  remains  that  the  one  source  of  the  energy  of  the 
animal  body  is  the  disintegration  and  oxidation  of  the  com- 
pounds presented  to  it  in  its  food.  Now  all  substances  can  be 
divided  into  two  classes :  those  which  undergo  spontaneous 
combination  with  oxygen,  such  as  phosphorus  or  an  alkaline 


28  PHYSIOLOGY 

solution  of  pyrogallic  acid,  which  may  be  termed  auto-oxidis- 
able,  and  those  which  are  unaffected  at  ordinary  temperatures 
by  oxygen,  which  may  be  termed  dysoxidisable.  To  the  latter 
class  belong  the  foodstuffs,  and  the  question  arises,  by  what 
means  their  oxidation  is  effected  in  the  living  body.  Outside 
the  body  we  can  effect  a  combination  of  these  substances  with 
oxygen  by  raising  their  temperature  to  a  red  heat,  but  this 
method  is  obviously  inapplicable  within  the  body.  The  diffi- 
culty is  that,  at  ordinary  temperatures,  molecular  oxygen,  Og, 
is  relatively  inert,  the  combinmg  affinities  of  each  of  its  atoms 
being  satisfied  within  the  molecule,  0  =  0.  Oxidation  how- 
ever can  at  once  be  effected  if  by  any  means  we  can  break  up 
this  molecular  compound,  so  as  to  leave  the  combining  affinities 
of  the  two  compounds  unsaturated.  Many  theories  have  been 
put  forward  as  to  the  manner  in  which  this  activation  of  the 
oxygen  is  carried  out  in  the  body.  Thus  it  has  been  suggested 
that  the  total  oxidation  occurs,  so  to  speak,  m  two  stages.  By 
the  ordinary  disintegrative  processes,  reducing  (auto-oxidisable) 
substances  are  formed,  and  these  combining  with  oxygen  split 
up  the  oxygen  molecule,  leaving  one  half  of  the  molecule 
free  to  attack  less  readily  oxidisable  substances.  Many  such 
instances  are  known  of  the  simultaneous  progression  of  re- 
ducing with  oxidative  processes.  Thus  ozone  is  formed  in 
the  slow  oxidation  of  phosphorus  by  the  air.  The  oxidation  of 
pyrogallol  in  ammoniacal  solution  is  associated  with  the  con- 
version of  a  certain  amomit  of  the  ammonia  into  ammonium 
nitrite.  Of  the  existence  of  reducing  substances  in  the  body 
we  have  ample  evidence.  Thus  methylene  blue  injected  into 
the  living  animal  undergoes  reduction  in  the  tissues,  forming 
a  colourless  compound  which  is  re-oxidised  to  methylene  blue  on 
exposure  to  the  atmosphere.  It  is  difficult  however  to  decide 
whether  these  reducing  substances  are  formed  as  by-products 
in  the  cell  metabolism,  or  whether  they  are  an  integral  part 
of  the  protoplasmic  molecule,  as  was  imagined  by  Pffiiger. 

Closely  allied  to  this  process  is  the  oxidation  which  is 
associated  with  the  splitting  up  of  a  molecule  of  water.  Thus 
if  benzaldehyde  is  shaken  up  with  water  and  air,  it  becomes 
oxidised  to  benzoic  acid,  and  at  the  same  time  can  effect  oxida- 
tion of  other  dysoxidisable  substances.  In  this  case  a  water 
molecule  is  split  up  into  H  and  OH,  the  OH  group  taking  the 
place  of   the  H  in  the  aldehyde  group  (COH)  of  the   benz- 


THE   MATEEIAL   BASIS   OF   THE   BODY  29 

aldehyde,  wliile  the  two  hydrogen  atoms  set  free  conibine  with 
an  atom  of  oxygen  to  form  water,  thus  releasing  an  atom  of 
oxygen  in  the  active  state  and  therefore  ready  to  effect  other 
oxidations. 

According  to  other  authorities,  the  mechanism  of  oxidation 
m  the  body  is  similar  to  that  employed  m  the  manufacture  of 
sulphuric  acid.  In  this  case  sulphur  is  burnt  to  form  SO2, 
and  then  nitric  oxide,  NO,  is  added  together  with  air.  The 
NO  combines  with  the  oxygen  of  the  air  to  form  NO.;,  and  this 
reacts  with  the  sulphurous  acid  to  form  SO.j  and  NO.  The  NO 
thus  acts  as  a  carrier  of  oxygen  between  the  air  and  the  SO.,. 
In  the  same  way  an  ammoniacal  solution  of  cupric  hydrate, 
which  is  reduced  on  boiling  with  dextrose,  becomes  at  once 
blue  again  on  exposure  to  the  air,  and  can  be  used  to  oxidise 
an  indefinitely  large  quantity  of  the  dextrose.  In  these  cases 
we  can  make  precise  chemical  equations  for  the  various  steps 
of  the  oxidation.  In  other  cases  this  is  not  possible.  Thus 
spongy  platinum  will  effect  a  rapid  union  of  hydrogen  and 
oxygen  to  form  water.  Here  there  is  no  evidence  of  the  for- 
mation of  an  oxide  of  platmum  as  an  intermediate  stage  of  the 
process,  and  we  say  that  the  substance  acts  katalytically.  In 
all  cases  however  it  is  difficult  to  explain  why  the  presence  of  a 
third  substance  should  render  the  oxidation  possible.  We  have 
here  an  example  of  a  whole  series  of  phenomena  m  chemistry, 
where  the  presence  of  a  factor,  which  adds  nothing  to  the  total 
energy  of  the  reaction,  yet  materially  alters  the  velocity  of 
the  reaction,  often  rendering  a  reaction  possible  which  with- 
out it  would  take  an  mfinity  of  time  for  its  accomplishment. 

It  has  been  definitely  shown  that  substances  can  be  ex 
tracted  from  the  blood  and  tissues  of  animals  and  from  the 
substance  of  plants,  which  possess  the  property  of  effectmg 
certain  oxidations,  and  have  therefore  been  termed  oxj^gen 
carriers  or  oxygen  ferments  {oxydases).  The  exact  chemical 
character  of  these  substances  is  undetermined,  though  in  some 
cases  they  seem  to  belong  to  the  class  of  nucleo-albumens. 

It  is  of  course  possible  that  all  these  methods  are  made 
use  of  in  the  livmg  cell  for  the  oxidation  of  foodstuffs.  It 
is  certain  that  the  oxidation  is  not  accomplished  at  one 
stroke,  but  that  it  proceeds  by  stages.  It  will  be  our  office 
later  on  to  try  to  trace  out  some  of  these  stages,  and  so 
obtain  some  idea  of  the  history  of  the  foodstuffs  in  the  body. 


30  PHYSIOLOGY 


Section    2 
THE   PEOTEINS 

Since  our  chief  foods  are  derived  from  other  living 
organisms,  a  consideration  of  the  proximate  constituents  of 
our  bodies  covers  that  of  the  proximate  constituents  of  the 
food.  These  can  be  divided  into  three  main  classes,  proteins 
which  contain  nitrogen  and  sulphur,  fats,  and  carbohydrates. 
Of  these  the  proteins  are  the  most  important,  since  they  repre- 
sent the  only  irreplaceable  portions  of  the  food.  Many  animals 
can  live  on  a  pure  diet  of  proteins,  and  even  in  man  fats  and 
carbohydrates  are  largely  interchangeable  ;  but  in  no  case 
can  the  place  of  proteins  be  taken  by  any  other  substances. 

Proteins  are  found  in  all  protoplasm,  and  are  more  abun- 
dant in  those  tissues  where  growth  is  actively  going  on.  They 
are,  in  the  condition  in  which  we  generally  come  across  them, 
amorphous,  indiffusible,  and  varying  in  their  solubilities. 
They  are  inert  bodies,  tasteless,  and  presenting  no  distinct 
acid  or  basic  characters.  Although  it  is  possible  to  obtain 
certain  proteins  in  a  crystalline  condition,  they  all  belong  to 
the  class  of  bodies  known  as  colloids. 

The  term  '  colloid  '  was  applied  by  Graham  to  distinguish  certain  substances 
such  as  egg-albumen  or  gum  or  gelatin,  from  easily  crystallisable  substances 
such  as  salts  and  sugars.  Whereas  the  latter  are  easily  diffusible  through 
animal  membranes,  colloids  are  absolutely  indiffusible.  All  colloids  appear  to 
possess  very  large  molecules,  and  it  is  possible  that  their  indiffusibility  is  con- 
nected with  this  fact.  It  is  difficult  however  to  be  certain  that  the  apparent 
solutions  of  colloids  in  water  are  really  strictly  comparable  with  such  a  solution 
as  that  of  sodium  chloride.  In  many  cases  the  apparent  solution  is  really 
only  a  suspension  of  fine  particles,  and  very  simple  means  will  suffice  to  bring 
most  colloidal  solutions  into  such  a  condition  of  suspension.  All  colloidal  solu- 
tions may,  under  favourable  conditions,  present  the  phenomenon  of  coagula- 
tion, a  phenomenon  which  depends  on  an  alteration  of  the  relations  between 
suspended  particle  and  solvent,  and  on  an  aggregation  of  the  smaller  into 
larger  particles,  which  may  fuse  to  form  networks.  This  coagulated  condition 
may  be  brought  about  by  various  means  :  by  the  application  of  heat,  by  simple 
mechanical  shaking,  or  in  some  cases  by  the  mere  addition  of  neutral  salts. 
In  some  cases  the  reaction  is  reversible.  Thus  gelatin  and  water  form  a  solid 
jelly  when  cold,  but  become  fluid  when  the  temperature  is  raised.  An  alkaline 
solution  of  casein,  to  which  calcium  chloride  has  been  added,  foi-ms  a  solid  clot 
when  warmed,  but  becomes  fluid  again  at  a  normal  temperature  ;  and  a  similar 
phenomenon  is  observed  in  the  early  stages  of  the  action  of  pancreatic  juice 
on  milk.     In  most  cases  however  the  action  is  irreversible.     Silicic  acid,  which 


THE   MATERIAL   BASIS   OF   THE    BODY  31 

has  been  coagulated  by  the  addition  of  neutral  salt,  cannot  be  dissolved  again 
on  removing  the  salt  by  dialysis.  The  coagulum  which  is  formed  in  a  solution 
of  egg  albumen  by  heating  is  permanent,  and  insoluble  in  weak  acids  or  alkalies 
or  in  salt  solutions.  A  colloidal  membrane  permits  as  a  rule  the  free  passage  of 
water  and  salts,  but  is  impermeable  for  dissolved  colloids,  so  that  on  filtering 
a  colloidal  solution  under  pressure  through  a  colloidal  membrane,  the  filtrate 
consists  simply  of  water  with  salts,  all  the  colloid  remaining  behind  on  the  filter. 

All  proteins  contain  oxygen,  hydrogen,  nitrogen,  carbon, 
and  sulphur,  and  their  compositions  vary  round  the  following 
numbers. 

C  .  .  .  .  50*6  — 54"5  per  cent. 

H  .  .  .  .       6-5  —  7-3       „ 

N  .  .  .  .  15-0  —17-6       „ 

S  .  .  .  .      0-3  —  2-2       „ 

P  .  .  .  .       0-42—  0-8.5     „ 

0  .  .  .  .  21 -50 -23 -.50    „ 

Of  the  sulphur  a  certain  portion  is  oxidised,  while  the  rest 
exists  in  the  unoxidised  condition,  and  can  be  split  off  as  a 
sulphide  by  boiling  with  caustic  potash  or  soda.  Hence  we 
must  assume  the  presence  of  at  least  two  atoms  of  sulphur  in 
the  protein  molecule,  and  this  assumption  enables  us  to  arrive 
at  the  smallest  possible  empirical  formuhie  of  many  proteins. 
Some  of  these  formulfe  may  be  given  here,  smce  they  serve  to 
indicate  the  enormous  size  and  complexity  of  the  molecules  of 
this  class  of  substances. 

Egg  albumen  ....  C.,„jH3.,2N.20e6S- 

Protein  in  hemoglobin  (from  horse)  .  CssoHh]!isN2io024iS.,. 

(from  dog)  .  .  C;,,H„;',NJ,„02„SJ. 

Crystallised  globulin  (from  pumpkin-seeds)  C.„,2H„,N,,„Os3S.^. 

Crystallisation  of  pi-oteins. — It  has  long  been  known  that  proteins  occur  in 
the  crystalline  form  in  the  seeds  of  certain  plants,  as  in  hemp-seeds,  paranut, 
and  pumpkin  and  castor-oil  seeds.  These  crystals,  which  are  known  as  aleuron 
grains,  consist  of  proteins  belonging  to  the  class  of  globulins.  By  mechanical 
means  they  can  be  separated  from  the  surrounding  tissues,  and  after  washing 
dissolved  in  a  solution  of  magnesia.  On  dialysing  this  solution  against  alcohol, 
the  fluid  is  gradually  concentrated,  and  crystalline  granules  of  the  magnesia 
compound  of  the  protein  separate  out.  These  crystals  contain  1-4  per  cent. 
MgO,  pointing  to  a  molecular  weight  of  the  protein  of  about  2800.      (If  X  be 

X      100-1-4 
the  molecular  weight,  ^q  =  — ^^ >  therefore  X  =  2817.) 

It  is  also  easy  to  crystallise  egg  albumen  and  serum  albumen.  White  of 
egg  is  treated  with  an  equal  bulk  of  saturated  solution  of  ammonium  sulphate 
to  precipitate  the  globulins,  and  filtered.  The  filtrate  is  rendered  slightly  acid 
with  dilute  acetic  acid,  which  is  added  until  a  slight  precipitate  is  formed. 
The  mixture  is  put  aside  for  twenty-four  hours,  at  the  end  of  which  time  the 
greater  portion  of  the  albumen  has  been  precipitated  as  fine  needle-shaped 
crystals.     These  may  be  freed  from  ammonium  sulphate  by  washing  with  a 


32  PHYSIOLOGY 

saturated  acidified  solution  of  sodium  chloride.  A  similar  method  is  used  in 
the  case  of  serum-albumen.  By  repeated  crystallisation  a  product  may  be 
obtained  which  is  absolutely  constant  in  both  its  physical  and  chemical 
characters.     (Hopkins.) 

The  Structure  of  the  Protein  Molecule 
We  can  only  arrive  at  some  idea  of  the  manner  in  which 
the  protein  molecule  is  built  up,  by  breaking  it  down  bit  by 
bit,  employmg  methods  which  will  not  act  too  forcibly  in 
changing  the  whole  arrangement  of  the  constituent  parts  of 
the  molecule.  In  the  body  almost  all  the  protein  is  con- 
verted into  urea  or  carbamide  (^^<Cnh!)'  "^  which  form  it  is 
excreted,  but  we  have  evidence  that  the  urea  is  formed  by 
a  synthetic  process  from  ammonium  carbonate  or  carbamate, 
which  must  therefore  be  regarded  as  the  last  stage  in  the 
physiological  dismtegration  of  protein.  In  the  body  many 
substances  are  found  which  are  presumed  to  be  intermediate 
stages  in  the  conversion  of  protein  into  urea.  Owing  how- 
ever to  the  comcidence  of  synthetic  with  disintegrative 
changes  in  the  body,  it  is  impossible  to  assign  these  sub- 
stances their  exact  place  in  the  protein  metabolism  until  we 
have  thrown  more  light  on  protein  disintegration  by  chemical 
experiments.  In  order  to  study  this  question  therefore  we 
split  up  the  protein  molecule  by  simple  hydrolysis.  For  this 
purpose  we  may  heat  the  protein  in  sealed  tubes  with  baryta 
water,  or  treat  it  with  very  active  hydrolytic  ferments,  such 
as  the  trypsin  of  the  pancreatic  juice.  The  most  convenient 
method  however  is  to  heat  the  protein  for  twenty-four 
hours  with  a  mixture  of  hydrochloric  acid  and  stannous 
chloride.  (The  use  of  the  latter  is  to  prevent  any  coincident 
oxidative  changes,  which  would  tend  to  destroy  the  primary 
products  of  hydrolysis.) 

We  obtain  in  this  way  an  acid  fluid  containing  an 
extremely  complex  mixture  of  various  substances,  which  may 
be  regarded  as  the  proximate  constituents  of  the  protein 
molecule.     They  may  be  classified  as  follows  : — 

(a)  Moii-amino- acids  of  fatty  scries.      \    (6)    Mon-amino-ackls     of     aromatic 
Glycine  (amino-acetic  acid)         |  sc7-ies. 


Alanine  (amino-propionic  acid) 

Serine 

Leucine  (amino-caproic  acid) 

Aspartic  acid 


Tyrosine  (oxyphenyl-alanin) 

Phenyl-alanine 

Tryptophane 

Proline  (pyrrolidine-carboxylic 


Glutamic  acid  acid) 


THE   MATERIAL  BASIS   OF   THE   BODY 


33 


(c)  Di-amino  acids.  (d)  Bases, 

Ornithine  (di-amino-valerianic  Ammonia 

acid)  Arginine 

Lysine  (di-amino-caproic  acid)  Histidine 

(f)  Sulphur-containing  body. 
Cystine 

The  amino-  or  amido-acids  are  derived  from  the  fatty  acids 
by  the  replacement  of  one  atom  of  hydrogen  in  the  radicle 
by  the  group  amidogen,  NH.,. 

CH, 

Thus  from  acetic  acid   I        ,  we  get  amino-acetic  acid  or 

COOH 
CH,NH2 

glycine    |  ;  from  propionic  acid,  amino-propionic  acid  or 

COOH 
CH, 

I 

! 

alanine  CH.NH,.     Leucine  is  amino-caproic  acid,  and  has  the 

COOH 

formula 

CH.CH, 

\/ 

CH 

I 
CH,. 

I 
CH.NH, 


COOH 

^spar^ic  acicZ  is  amino-succinic  acid,  C^H3(NH.,)  (COOH).,, 
and  glutamic  acid  is  the  next  higher  homologue  in  the  same 
series,  C3H,(NH.,)  (COOH),. 

In  all  these  bodies,  the  acid  characters  of  the  group  COOH 
are  almost  neutralised  by  the  presence  of  the  basic  group 
NH,,  so  that  the  amino-acids  as  a  whole  are  inert  substances 
with  very  feeble  acid  qualities.  They  can  indeed  act  as  bases, 
giving  crystalline  compounds  with  the  mineral  acids. 

Tyrosine  is  the  chief  aromatic  amino-acid  which  occurs  in 
acid  protein  digests.  It  is  formed  by  the  coml)ination  of  a  fatty 
amino-acid  with  an  aromatic  group,  being  para-oxyphenyl- 
amino-propionic  acid 

CH.,.CHNH...COOH. 

/\' 

OH 

Both  leucine  and  tyrosine  may  be  prepared  by  evaporating 
down  a  solution  of    proteins  which   have  been  acted  on  for 

3 


34  PHYSIOLOGY 

twenty-four  hours  with  pancreatic  juice.  When  the  Hquid 
is  allowed  to  cool,  crystals  of  tyrosine  separate  out.  These 
crystals  form  slender  needles  arranged  in  sheaves  or 
radiating  from  a  centre.  The  mother-liquor  is  evaporated 
to  a  syrupy  consistence,  extracted  with  alcohol,  and  the 
extract  allowed  to  stand.  As  the  alcohol  evaporates, 
yellowish-brown  spheres,  consisting  of  masses  of  ill-formed 
needle-shaped  crystals  of  leucine,  separate  out. 

A  solution  of  tyrosine  with  Millon's  reagent  gives  a  red 
colour,  the  tmt  of  which  deepens  on  heatmg. 

When  proteins  are  hydrolysed  by  means  of  trypsine,  the 
proteolytic  ferment  of  pancreatic  juice,  the  digest  at  a  certain 
period  gives  a  rose  colour  on  acidification  and  addition  of 
bromine  water.  This  reaction  is  due  to  the  presence  of 
another  aromatic  body,  called  tryptophane,  which  is  a 
derivative  of  indol,  being  indol  amino-propionic  acid. 

C.CH.,.CHNH  ,.COOH 
C,H,        CH 

\  y 

NH 

Acted  on  by  bacteria  of  putrefaction  this  body  gives  rise  to 
indol  and  skatol,  substances  with  a  pronounced  faecal  odour 
and  constantly  produced  in  the  putrefaction  of  proteins. 

Fig.  13.  Fig.  14. 


Leucine  '  cones  '  (imperfect  crystals)  Tyrosine  crystal?  (Frey). 

(Frey). 

After  separating  all  the  monamino-acids,  a  considerable 
amount  of  the  original  protein  remains  unaccounted  for.  Of 
this  remainder  a  certain  amount  is  present  in  the  form  of  the 
hexone  bases,  so  called  from  the  fact  that  they  all  contain  six 


THE   MATEEIAL  BASIS   OF   THE   BODY  35 

atoms  of  carbon  in  their  molecules.  These  bases  can  be 
thrown  down  from  the  mother-liquor  after  the  separation  of 
the  amino-acids,  or  from  the  original  acid  decoction  by  the 
addition  of  phosphotungstic  acid.  Three  of  these  bases  have 
been  isolated — lysine,  arginine,  and  histidine. 

Lysine  has  the  formula  CgHi^N^O.^,  and  is  a  diamino- 
caproic  acid,  the  insertion  of  another  NH.^  group  into  leucine 
conferring  on  this  body  strongly  basic  properties. 

Arginine  (C,;H,jN^02)  was  first  prepared  from  the  seedlings 

of  certain  plants.     Its  chief  interest  lies  in  the  fact  that  on 

boiling  with  baryta  water  it  is  split  up,  giving  as  one  of  its 

decomposition -products    urea.      We   thus  see   that  a  certain 

proportion  (not  more  than  one-ninth)  of   the  urea  which  is 

excreted  by  the  body  may  be  derived  by  a  process  of  simple 

disintegration    of    the   proteins    of    the    food,   without    any 

accompanying  oxidation.     Arginine  belongs  to  the  same  class 

of  bodies  as  creatine  — an  important  constituent  of  all  muscular 

tissues.     Creatine    or   methyl-guanidin-acetic   acid    has    the 

formula 

NHx     ; 

^C  -N(CH3).CH.,C00H. 
NH,/     j 

On  boiling  creatine  with  baryta  water,  it  takes  up  a  molecule 
of  water  and  splits  into  two  halves  in  the  situation  of  the 
dotted  line  in  the  formula,  giving 

NH,v 

>C0  (urea)  and  NH(CH3)CH,C00H. 
NH/ 

This  latter  substance  is  known  as  sarcosine,  and  is  derived 
from  glycine  by  the  replacement  of  one  atom  of  hydrogen  by 
a  methyl  group,  CH3. 

Arginine  has  a  similar  formula.  On  the  left-hand  side  of 
the  dotted  line  the  formula  would  be  identical  with  that  of 
creatine.  On  the  right-hand  side  however,  the  sarcosine 
group  is  replaced  by  a  diamino-acid  of  the  fatty  series, 
diamino-valerianic  acid  or  ornithin. 

Histidine  has  the  formula  CgHgNgO.^.  It  contains  the 
alanine  group,  being  imidazol  amino-propionic  acid.  It  does 
not  yield  urea  on  boiling  with  baryta  water. 

Nearly  the  whole  of  the  sulphur  in  the  protein  mole- 
cule exists  or  can  be  separated  in  the  form  of  cystine.     This 


36 


PHYSIOLOGY 


substance  is  a  crystalline  solid,  which  under  some  con- 
ditions may  occur  in  considerable  quantities  in  the  urine 
(cystinuria),  and  in  such  cases  may  give  rise  to  a  urinary 
calculus.  It  can  be  regarded  as  a  sulphur  derivative  of 
lactic  acid  CH3.CHOH.COOH,  or  of  amino- propionic  acid 
CHg.CHNH^.COOH,  and  has  the  formula 

CH.,— S— S-CH.. 


CH.NH.. 


COOH 


CH.NH, 


COOH. 


Other  amino-acids  and  allied  substances,  besides  those 
mentioned,  occur  in  various  tissues  and  fluids  of  the  body. 
Since  however  they  do  not  lie  on  the  direct  line  of  protein 
katabolism,  M'e  shall  consider  their  chemical  properties  in 
connection  with  the  organs  in  which  they  occur. 

A  comparison  of  the  relative  amounts  of  these  substances  to  be  obtained 
by  the  decomposition  of  different  proteins  shows  marked  variations,  pointing 
to  fundamental  differences  in  their  constitution.  This  is  well  seen  in  the 
following  table,  in  which  is  given  the  relative  i^roportions  of  the  more  important 
decomposition  products :  — 


Casein  (of  milk) 

Globiii  (from  lisemo- 

Edestin  (a  vegetable  pro- 

gloDin) 

tein  from  hemp-seeds) 

Glycine 

0 

0 

3-8 

Alanine 

0-9 

4-2 

3-6 

Leucine 

10-5 

290 

20-9 

Glutamic  acid 

10-7 

1-7 

6-3 

Aspartic  acid 

1-2 

4-4 

4-5 

Tyrosine 

4-5 

1-3 

21 

Phenylalanine 

3-2 

4-2 

2-4 

Proline . 

31 

2-3 

1-7 

Lysine  . 

5-8 

4-2 

20 

Arginine 

4-8 

5-4 

11-7 

Histidine 

2-6 

10-9 

10 

Cystine 

006 

0-3 

0-25 

Tryptophane 

1-5 

present 

present 

A  class  of  bodies  known  as  protamines  occurs  in  the  heads  of  spermatozoa 
of  salmon  and  some  other  fishes  in  combination  with  nucleic  acid.  The 
protamines  react  in  many  ways  like  proteins,  giving  the  biuret^  reaction. 
When  they  are  hydrolysed  by  strong  hydrochloric  acid,  they  give  off  nearly 
all  their  nitrogen  in  the  form  of  the  '  hexone  bases,'  arginine,  lysine,  and 
histidine.  They  give  also  small  amounts  of  monamino-acids.  It  has  been 
suggested  by  Kossel  that  the  typical  protein  molecule  may  be  regarded  as  a 
nucleus  composed  from  the  hexone  bases,  to  which  is  tacked  on  a  number 
of  monamino-acid  groups,  the  relative  proportion  of  the  bases  in  the  nucleus, 
as  well  as  of  the  subsidiary  amino-acids,  differing  according  to  the  nature 
of  the  protein  in  question.  According  to  this  view,  the  protamines  would 
lepresent  the  simplest  type  of  protein. 


THE   MATERIAL   BASIS   OF   THE    BODY  37 

Most  nuclei,  including  the  spermatozoa  of  most  animals,  contain  another 
class  of  bodies,  called  liistones,  in  combination  with  the  nucleic  acid.  Histones 
give  the  ordinary  protein  reactions,  their  chief  characteristic  being  the  fact 
that  they  are  precipitated  by  ammonia.  Their  disintegration  products  are 
such  as  would  be  obtained  from  a  mixture  or  combination  of  ordinary  protein 
with  a  protamine.  Thus,  roughly  speaking,  on  hydrolysis  by  means  of  acids, 
the  ordinary  proteins  of  our  food  and  tissues  give  about  10  per  cent,  of  hexone 
bases,  histones  about  20  to  25  per  cent.,  while  protamines  yield  about  90  per 
cent,  of  lysine,  arginine,  and  histidine. 

Polypeptides. — E.  Fischer  has  succeeded  in  uniting  two  or 
more  molecules  of  various  amino-acids  to  form  large  molecules 
which  give  the  biuret  reaction,  and  might  be  regarded  as 
elementary  proteins  or  parts  of  proteins.  Some  of  these 
bodies  may  be  hydrolysed  by  trypsine  (the  proteolytic  ferment 
of  pancreatic  juice)  into  their  constituent  amino-acids.  To 
these  synthesised  substances  Fischer  has  given  the  name 
polypeptide.  Examples  of  such  bodies  are  glycyl-glycine, 
leucyl-tyrosine,  glycyl-leucyl-tyrosine. 

A  similar  body — namely,  glycyl-alanine — has  been  obtained 
directly  by  the  hydrolysis  of  silk,  which  suggests  that  these 
combinations  actually  occur  as  part  of  the  constituent  frame- 
work of  proteins.  As  a  type  of  the  manner  in  which  these 
substances  are  built  up,  we  may  give  the  formula  of  glycyl- 
glycine  :  — 

H-N-CH.,CO.OH 

I 

CO  -  CH,.NH,. 

General  Tests  for  Proteins 

1.  On  boiling  proteins  in  a  very  slightly  acid  solution, 
they  are  coagulated,  and  form  an  insoluble  white  precipitate. 

The  best  method  of  carrying  out  this  test  is  to  boil  the  solution  in  neutral 
or  slightly  alkaline  solution,  and  then  while  in  active  ebullition  to  drop  in 
dilute  (1  or  2  per  cent.)  acetic  acid  until  the  reaction  is  slightly  acid.  If  the 
solution  is  poor  in  salts,  1  or  2  per  cent,  of  NaCl  should  be  added  at  the  com- 
mencement. By  this  means  a  nearly  perfect  separation  of  all  the  coagulable 
proteins  may  be  effected. 

2.  On  pouring  a  solution  of  protein  carefully  down  the 
side  of  a  test-tube  containing  strong  nitric  acid,  so  as  to  form 
a  layer  on  the  top,  a  white  layer  of  coagulated  protein  is  pro- 
duced at  the  junction  of  the  two  fluids. 

3.  Acetic  acid  and  potassium  ferrocyanide  give  a  white 
precipitate. 


38  PHYSIOLOGY 

4.  Saturation  of  protein  solutions  with  annnonium  sul- 
phate causes  complete  precipitation  of  all  proteuis  present. 

5.  All  coagulahle  proteins  are  completely  precipitated  by 
adding  to  their  solutions  an  equal  bulk  of  10  per  cent,  tri- 
chloracetic acid,  or  of  a  5  per  cent,  solution  of  metaphosphoric 
or  of  salicyl-sulphonic  acid. 

6.  All  proteins  are  precipitated  and  coagulated  by  the 
addition  of  solutions  of  picric  acid,  mercuric  chloride,  copper 
sulphate,  tannic  acid,  or  of  strong  alcohol. 

7.  Excess  of  caustic  potash  or  soda  with  a  drop  of  dilute 
copper  sulphate  gives  a  violet  colour  (Piotrowski's  reaction). 

8.  On  adding  strong  nitric  acid  to  a  protein  solution  and 
boiling,  a  yellow  colour  is  produced  which  turns  to  deep 
orange  when  ammonia  is  added  (xanthoproteic  reaction). 

9.  Millon's  reaction.  An  acid  solution  of  nitrate  of 
mercury  gives  a  white  precipitate  which  turns  a  brick-red 
on  boiling. 

10.  On  addmg  some  weak  glyoxylic  acid  (prepared  by  the 
action  of  sodium  amalgam  on  oxalic  acid)  to  a  protein  solution 
and  then  some  strong  sulphuric  acid,  a  deep  red-purple  colora- 
tion is  produced  (Adamkiewicz-Hopkins  reaction). 

Meaning  of  the  Protein  Tests. — The  last  four  of  these  tests  are  of  interest 
as  throwing  some  hght  on  _the  constituent  parts  of  the  protein  molecule. 
Thus   Piotrowski's  reaction  (often  called  the  biuret  reaction)  depends  on  the 

CH,-NH, 
presence  in  the  protein  molecule  of   groups  having  the   formula    |  , 

CO-NH 
and  is  given  by  a  number  of  substances  in  which  two  CO.NHo  or  CH._>.NH.> 
groups  are  coupled  together  either  directly,  or  by  a  NH  or  CH.  group.     Since 
the  terminal  NH„  group  is  destroyed  by  nitrous  acid,  this  acid  at  once  destroys 
the  power  of  proteins  to  give  the  biuret  reaction. 

The  xanthoproteic  reaction  depends  on  the  presence  in  the  protein 
molecule  of  an  aromatic  group  with  the  benzene-nucleus: — 


H 

c       c 


hI^      % 

V 


Millon's  reaction  depends  on  the  presence  of  a  hydroxy-derivative  of 
benzene,  and  is  mainly  conditioned  in  the  protein  by  the  tyrosine  group,  a 
compound  of  hydroxyphenyl  with  alanine  (amino-propionic  acid). 

The  Hopkins-Adamkiewicz  reaction  is  determined  by  the  tryptophane 
group  in  the  protein  molecule. 


THE   MATERIAL  BASIS   OF   THE   BODY  39 

Classification  of  Peoteins 

The  only  satisfactory  classification  of  proteins  would  be 
one  founded  on  an  exact  study  of  their  disintegration  products. 
Since  however  we  do  not  know  all  these  products  in  the  case 
of  even  a  single  protein,  it  is  obviously  impossible  to  attempt 
any  classification  on  such  a  basis,  and  we  have  therefore  to 
make  use  temporarily  of  a  purely  artificial  classification  based 
on  the  solubilities  of  the  various  proteins  in  water  and  salt 
solutions.  The  fact  that  simple  mechanical  shaking  may 
convert  a  globulin  into  a  coagulated  protein  will  serve  to 
illustrate  how  entirely  artificial  is  our  present  mode  of  classi- 
fication. Adopting  the  solubilities  as  a  basis,  we  may  divide 
the  proteins  into  the  following  classes  : 

1.  Native  Albumens. — These  are  soluble  in  pare  water  and 
are  precipitated  by  saturation  with  sodio-magnesium  sulphate 
or  with  ammonium  sulphate. 

Egg  albumen  forms  the  greater  part  of  the  white  of  egg. 
It  gives  the  ordinary  protein  tests,  and  is  precipitated  if 
shaken  up  with  a  drop  of  dilute  sulphuric  acid  and  excess  of 
ether.  It  rotates  the  plane  of  polarised  light  to  the  left  35-5°. 
If  injected  into  the  circulation  it  gives  rise  to  albuminuria  and 
is  partially  excreted  by  the  kidneys.' 

Serum  albumen  occurs  m  large  quantities  in  the  blood- 
plasma  and  serum,  and  m  small  quantities  in  most  tissues  of 
the  body.  It  coagulates  at  75°  C,  and  is  distmguished  from 
egg  albumen  by  its  greater  specific  rotatory  power  (56°),  and 
by  the  fact  that  it  is  not  precipitated  by  ether  and  sulphuric 
acid,  and  if  mjected  into  the  circulation  does  not  reappear  in 
the  urine. 

2.  Globulins. — These  bodies  are  insoluble  in  pure  water 
and  require  a  certain  amount  of  neutral  salt  present  to  dis- 
solve them.  They  are  the  most  mterestmg  of  all  protein 
groups,  playing  an  important  part  in  nearly  all  vital  processes. 

'  This  statement  has  given  rise  to  the  idea  that  the  cells  of  the  kidney  are 
permeable  to  egg  albumen  but  not  to  serum  albumen.  This  idea  is  not 
warranted.  The  fact  is  that  egg  albumen  is,  or  contains  an  ingredient  which 
is,  poisonous  and  injures  the  kidney  cells.  These  therefore  become  permeable 
to  proteins,  and  albuminuria  results.  But  the  proteins  of  the  urine  are  those 
which  normally  occur  in  albuminuria,  viz.  serum  albumen  and  serum  globulin, 
containing  however  a  certain  amount  of  tlie  foreign  protein  circulating  in  the 
blood,  namely  egg  albumen. 


40  THYSIOLOGY 

But  it  must  be  remembered  that  we  are  not  justified  in  speak- 
ing of  the  globulins  whicli  we  extract  from  the  tissues  and 
treat  by  precipitation  and  washing  till  they  are  no  longer 
altered  by  these  processes,  as  the  active  agents  in  the  com- 
plex protein  interactions  which  make  up  the  sum  of  vital 
phenomena.  Our  purified  protein  is  a  wreck,  and  represents 
merely  the  framework  on  which  the  living  protoplasmic 
molecule  was  built  up. 

All  the  globulins  are  precipitated  from  their  solutions  by 
saturation  with  magnesium  sulphate  and  partially  by  satura- 
tion with  sodium  chloride.  The  chief  members  of  this  class 
are — 

Crystallin,  ol)tained  from  the  crystalline  lens  by  passing 
a  stream  of  CO^  through  an  aqueous  extract  of  this  body. 

Paraglohulin. 

Fibrinogen. 

Myosin  or  2)ciramyosinogeii. 

These  three  bodies  will  be  considered  in  the  chapters  on 
blood  and  muscle. 

3,  Derived  Albumens. — These  may  be  regarded  as  compounds 
of  proteins  with  acids  or  alkalies. 

Acid  albumen  is  formed  by  the  action  of  warm  dilute 
acids  or  by  strong  acids  in  the  cold  on  any  of  the  preceding 
bodies.  If  an  alkaline  solution  be  added  so  as  to  nearly 
neutralise  the  solution  of  acid  albumen,  this  latter  is  precipi- 
tated. If  the  precipitate  be  suspended  in  water  and  heated,  it 
is  coagulated  and  becomes  insoluble  in  dilute  acids  or  alkalies. 

Alkali  alhuineoi  is  formed  by  the  action  of  strong  caustic 
potash  on  white  of  egg  or  on  any  other  protein,  or  by  adding 
alkali  in  excess  to  a  solution  of  acid  albumen.  It  is  pre- 
cipitated on  neutralisation  of  its  solution. 

In  close  association  with  this  group  may  be  included  the  proteins  as  they 
occur  in  combination  with  the  metallic  salts,  such  as  copper  sulphate.  On 
splitting  off  the  copper  moiety  from  these  compounds,  the  protein  left  is 
practically  free  from  ash,  and  behaves  in  many  respects  like  an  albuminate, 
being  insoluble  in  absolutely  pure  water,  but  easily  dissolved  by  the  addition  of 
a  trace  of  free  acid  or  alkali. 

An  inoeresiing  group  of  protein  derivatives  has  been  described  by  Hopkins, 
produced  by  the  action  of  the  free  halogens  on  protein  solutions.  We  get 
in  this  way  two  definite  classes  of  compounds.  One  class,  which  contains 
the  largest  iiercentage  of  halogens,  is  obtained  by  treating  a  protein  solution 
with  chlorine,  bromine,  or  iodine,  dissolving  up  the  resultant  precipitate  in 
alcohol  and  pouring   the   alcoholic   solution   into   ether,   when   the   halogen 


THE  MATERIAL  BASIS  OF  THE  BODY         41 

compound  is  thrown  down  as  a  fine  wliite  precipitate.  By  dissolving  up  this 
precipitate  in  weak  soda  and  precipitating  with  acid,  we  obtain  a  series  of  com- 
pounds containing  only  about  one-third  as  much  of  the  halogen  as  is  contained 
in  the  first  precipitate,  suggesting  that  the  halogen  forms  both  substitution 
and  additive  compounds  with  the  protein  molecule. 

4.  Fibrins. — The  chief  of  these  is  blood-fibrin,  a  .stringy 
protein  formed  in  the  clotting  of  the  blood  and  giving 
solidity  to  the  clot.  It  is  insoluble  in  water  and  salt  solutions. 
In  dilute  hydrochloric  acid  it  swells  up,  and  if  kept  at  40^  C. 
it  dissolves  with  the  formation  of  acid  albumen.  If  suspended 
in  water  and  heated  it  is  coagulated,  and  is  no  longer  capable 
of  swelling  up  in  dilute  hydrochloric  acid.  Similar  bodies, 
myosin-  and  myogen-fibrin,  are  formed  in  the  coagulation  or 
rigor  of  voluntary  muscle. 

5.  Coagulated  Proteins. — Any  member  of  the  preceding 
classes,  when  heated  in  a  neutral  or  slightly  acid  solution, 
is  converted  into  coagulated  protein.  In  this  condition  it 
is  insoluble  in  water,  saline  solutions,  or  weak  acids.  It  is 
dissolved  by  strong  acids  or  alkalies  or  by  the  digestiv 
ferments  such  as  the  gastric  and  pancreatic  juices. 

Hydrated  Proteins 

When  proteins  are  subjected  to  the  action  of  superheated 
water  or  steam,  or  are  heated  with  acids,  or  acted  on  at  the 
body  temperature  by  certain  ferments  (trypsine  or  papain), 
they  undergo  a  change  which  is  supposed  to  be  attended 
with  the  addition  of  one  or  more  molecules  of  water  to  the 
protein  molecule  (hydrolysis).  The  final  result  of  this  action 
is  the  group  of  nitrogenous  bodies,  chiefly  belonging  to  the 
amino-acids,  which  we  have  already  dealt  with.  As  inter- 
mediate products  in  this  series  of  hydrolytic  changes,  we  find 
a  group  of  bodies,  still  presenting  many  of  the  protein  re- 
actions, which  are  classed  as  hydrated  proteins,  and  include 
the  proteoses  or  albumoses  and  the  peptones. 

1.  Albumoses. —  These  are  all  precipitated  from  their  solu- 
tions by  saturation  with  ammonium  sulphate.  On  addition 
of  nitric  acid  they  give  a  precipitate  in  the  cold,  which  is 
dissolved  on  heating  but  reappears  on  cooling.  On  adding 
an  excess  of  caustic  potash  and  a  drop  of  verij  dilute  copper 
sulphate  to  a  solution  of  albumoses,  a  pink  colour  is  pro- 
duced (biuret  reaction).     If  more  copper  sulphate  be  added, 


42  PHYSIOLOGY 

the  pink  colour  is  changed  to  violet  similar  to  that  produced 
in  a  solution  of  proteins. 

According  to  their  solubilities  three  varieties  of  albumoses 
may  be  distinguished  : 

a.  Proto-albumoses. Soluble  in  pure  water;  precipitated 
by  saturation  with  sodium  chloride  or  ammonium  sulphate. 
With  acetic  acid  and  potassium  ferrocyanide  it  gives  a  white 
precipitate. 

b.  Hetero-alhumose. — Insoluble  in  pure  water  ;  soluble  in 
weak  saline  solutions  or  dilute  acids ;  precipitated  by  satura- 
tion with  salt,  or  by  acetic  acid  and  potassium  ferrocyanide. 

c.  Deutero-albmnoses. — Soluble  in  pure  water ;  not  pre- 
cipitated by  saturation  with  common  salt,  except  after  addi- 
tion of  a  little  strong  acetic  acid  ;  entirely  precipitated  by 
saturation  with  ammonium  sulphate.  They  give  no  precipitate 
with  acetic  acid  and  potassium  ferrocyanide. 

2.  Peptones. — Soluble  in  pure  water ;  diffusible  through 
animal  membranes  ;  with  caustic  potash  and  copper  sulphate 
they  give  the  same  reaction  as  albumoses.  From  the  latter 
class  peptones  are  distinguished  by  the  fact  that  they  are 
not  precipitated  at  all  by  saturation  with  ammonium  sulphate 
or  with  any  neutral  salt. 

Albumoses  and  peptones  give  the  xanthoproteic  and 
Millon's  reactions  common  to  all  proteins ;  and  like  these 
are  precipitated  by  tannic  acid,  mercuric  chloride,  and 
potassio-mercuric  iodide. 

Conjugated  Proteins 

This  name  may  be  applied  to  various  complicated  bodies, 
which  resemble  one  another  only  in  the  fact  that  in  each  of 
them  a  protein  radical  is  combined  with  some  other  body. 

1.  Haemoglobin,  the  red  colouring  matter  of  the  blood,  is 
readily  crystallisable.  On  boiling  an  aqueous  solution  it 
splits  up  into  coagulated  protein  (globin)  and  an  iron -contain- 
ing body  named  haematin  (Cy.^Hg^N^O^Fe).  Its  properties  will 
be  described  in  the  chapters  on  blood  and  respiration. 

2.  Nucleo-proteins. — These  are  a  group  of  bodies  occur- 
ring in  large  quantities  in  the  protoplasm  of  cells.  They 
are  also  found  in  the  chyle,  in  lymph,  and  in  blood-plasma. 
They  consist  of  protein  combined  with  a   nitrogenous   body 


THE  MATERIAL  BASIS  OF  THE  BODY         43 

rich  ill  phosphorus  called  iiuclein.  As  they  occur  in  the 
cells,  they  are  in  most  cases  soluble  in  water  or  in  salt 
solutions,  but  after  separation  they  need  the  presence  of 
free  alkali  for  their  solution.  When  subjected  to  gastric 
digestion,  they  are  split  up,  the  protein  half  being  converted 
into  albumoses  and  peptones,  while  the  nuclein  is  pre- 
cipitated. The  nuclein  thus  obtained  is  a  white  amorphous 
powder,  insoluble  in  water,  salt  solutions,  or  acids,  but  soluble 
in  strong  alkalies.  It  forms  the  chief  constituent  of  cell 
nuclei,  and  consists  of  a  combination  of  protein,  or  pro- 
tamine, or  histone,  with  a  complex  body  containing  nitrogen 
and  phosphorus  known  as  nucleic  acid. 

When  the  latter  is  heated  with  strong  acids,  it  breaks  up 
into  phosphoric  acid  and  a  number  of  other  substances, 
amongst  which  the  most  important  are  bases  allied  to  uric 
acid  and  belonging  to  the  xanthin  or  purin  series  (xanthin, 
hyj)oxanthin,  adenin). 

In  many  cases  a  carbohydrate  is  found  among  the 
products  of  disintegration  of  nuclein.  As  first  extracted 
from  the  animal  cell,  the  nucleo-proteins  are  associated 
with  a  considerable  proportion  of  lecithin,  and  in  this  fresh 
labile  condition  form  the  tissue-librinogens  of  Wooldridge. 
To  prepare  these  substances,  an  organ  rich  in  cells  such  as 
the  thymus  is  minced  and  extracted  with  water  or  normal  salt 
solution.  After  separating  the  cells  by  means  of  the  centri- 
fugal machine,  the  clear  fluid  is  decanted  off  and  acidified 
with  acetic  acid.  A  precipitate  is  produced,  consisting  of 
tissue-fibrinogen.  This  substance  is  soluble  in  excess  of  acid 
and  is  easily  soluble  in  alkalies.  All  the  tissue-fibrinogens 
are  highly  unstable  bodies  and  undergo  changes  m  the  mere 
act  of  precipitation  and  resolution.  When  injected  mto  the 
blood,  they  cause  intravascular  clotting.  On  digestion  with 
gastric  juice,  they  yield  a  precipitate  of  nuclein,  and  this 
precipitate  contains  a  large  proportion  of  the  lecithin  present 
in  the  original  substance. 

These  complex  bodies  form  the  greater  part  of  the  proto- 
plasm of  a  living  cell.  The  chromatic  part  of  the  nuclei 
appears  to  be  formed  in  most  cases  of  nuclein,  but  the  purest 
nuclear  material  at  our  disposal,  the  heads  of  spermatozoa, 
is  found  to  consist  almost  entirely  of  a  definite  compound  of 
nucleic  acid  and  either  protamines  or  histones,  so  that  the 


44 


PHYSIOLOGY 


essential  chemical  constituents  of  a  living  cell  are  here  reduced 
to  their  simplest  possible  expression. 

The  chemical  relationships  of  the  iiucleo-proteins  may  be 
rendered  clearer  by  tlie  following  table. 

Cell  protoplasm 
(extracted,  treated  with  acetic  acid). 


NxwJeo-protein  (precipitate) 
(on  digestion). 


Histone,  etc.  (in  solution) 


Nuclein  (precipitate) 

(dissolved  in  strong  alkali,  and  precipitated  with  HCl). 


Peptone  (in  solution). 


Nucleic  acid  (precipitate) 
(heated  in  sealed  tube  with  HCI). 


Acid  albumen  (in  solution) 
or  Protamine. 


Pliosphoric  odd.       Purine  bases.     Rediicing  stigar  ?  Pyrimidine  bases  and 

Xanthine,  CjH^N.iO.^.  other  less  known  sub- 

Hypoxanthine,  C^HiNiO.  stances. 
Adenine,  C-H^N,. 

TJie  caseinogen  of  milk  is  often  grouped  with  the  nucleo- 
proteins,  since  like  them  it  gives  a  phosphorus-containing 
precipitate  on  digestion  with  gastric  juice.  Like  them  too 
it  is  precipitated  by  the  addition  of  excess  of  acid  and  is 
soluble  in  alkalies.  The  precijDitate  however  which  is  yielded 
by  gastric  digestion  is  not  a  true  nuclein,  since  it  does  not 
give  the  xanthin  or  purin  bases  on  heating  with  hydrochloric 
acid.  It  seems  in  fact  to  be  little  more  than  an  insoluble 
compound  of  phosphoric  acid  with  protein,  and  has  been 
termed  pseudo-nuclein  or  paranuclein.  The  precipitate  is 
stated  to  be  absent  in  the  case  of  caseinogen  of  human  milk. 
With  rennet  ferment  caseinogen  undergoes  coagulation,  form- 
ing casein  or  cheese  (y.  Chap.  VIII.).  Caseinogen,  with  certain 
other  proteins  presenting  similar  properties,  are  often  put  in 
a  distinct  class  as  the  phospho-proteins. 


THE    MATERIAL   BASIS   OF   THE    BODY  45 

3.  Glyco-proteins. — This  class  of  bodies  is  of  considerable 
interest,  since  in  many  cases  its  members  may  represent 
stages  in  the  upbuilding  of  carbohydrates  in  the  protoplasmic 
molecule.  They  have  been  regarded  as  glucosides  of  the 
proteins,  but  in  all  cases  where  the  carbohydrate  moiety  has 
been  isolated  in  a  state  approaching  purity,  it  has  been 
found  to  contain  nitrogen.  Probably  therefore  in  most  in- 
stances the  conjugation  is  not  between  glucose  and  protein, 
but  between  a  body  of  the  composition  of  glucosamme  and 
protein. 

Glucosamine  was  first  procured  by  boiling  the  chitinous 
shells  of  Crustacea  with  hydrochloric  acid.  It  is  formed  from 
a  molecule  of  grape-sugar  or  other  liexose  by  the  replacement 
of  an  OH  group  by  NH^.  In  chitin  this  substance  occurs  in 
combination  with  acetic  acid  or  some  derivative  of  acetic  acid. 
A  somewhat  similar  substance  is  found  in  the  cartilage  of 
vertebrates.  The  cartilage  matrix  contains  two  chief  sub- 
stances, chondro-mucoid  and  chondrin.  By  long  extraction 
with  weak  alkali,  a  paired  acid,  chondroitin-suljjhuric  acid, 
can  be  extracted  from  both  these  bodies,  leaving  behind  in 
the  case  of  chondro-mucoid  a  substance  of  the  protein  class, 
in  the  case  of  chondrin  pure  gelatin.  On  boiling  the  chon- 
droitin- sulphuric  acid  with  weak  acid  it  is  decomposed,  with 
the  formation  of  a  body,  chondrosin,  which  reduces  Fehling's 
solution  and  appears  to  be  compounded  of  two  derivatives 
of  glucose,  viz.  glycuronic  acid  (C,.H|,|0-)  and  glucosamine 
(C,H„0,.NH,). 

It  is  possible  that  many  so-called  simple  proteins  contain 
a  small  amomit  of  a  carbohj^drate  residue  wrapped  up  in  their 
molecule.  Thus  from  egg  albumen,  even  crystallised,  it  is  easy 
to  prepare  a  reducing  substance  which  gives  a  definite  osazone. 
We  have  seen  moreover  that  a  carbohydrate  may  occur  in 
the  decomposition  of  nucleic  acid.  In  the  typical  glyco-pro- 
teins however,  the  carbohydrate  moiety  forms  a  much  greater 
part  of  the  molecule  than  in  the  last  two  cases.  They  may 
be  classified  as  follows. 

1.  Mucins.  Mucin  occurs  in  the  saliva,  and  as  a  product 
of  secretion  of  the  mucous  glands  throughout  the  alimentary 
tract,  and  as  a  skin-secretion  in  many  lower  animals.  It  also 
forms  an  important  constituent  of  the  ground  substance  of 
connective  tissue,  from  which  it  may  be  extracted  by  treat- 


46  PHYSIOLOCtY 

ment  with  lime  or  baryta  water.  On  boiling  with  dilute 
mineral  acids,  it  is  converted  into  an  acid-albumen  and  a 
reducing  substance  containing  nitrogen  which  appears  to  be 
isomeric  with  glucosamine,  and  has  been  called  mucosamine. 
Owing  to  its  content  of  protein  it  of  course  gives  the  ordinary 
protein  tests.  It  swells  up  in  water,  forming  a  viscid  slimy 
mass.  It  is  precipitated  by  acetic  acid,  and  is  insoluble  in 
excess  of  this  reagent.     It  is  soluble  in  dilute  alkalies. 

2.  The  mucoids  include  a  number  of  substances,  such  as 
colloid,  ovo-mucoid,  pseudo-mucin,  which  are  obtained  from 
ovarian  cysts  and  other  pathological  formations.  They  also 
appear  to  be  glucoside-like  combinations  of  a  protein  with 
some  body  allied  to  glucosamine,  or  the  anhydride  of  such 
a  body. 

3.  The  chondro-proteiiis  include  substances  which  are 
compounded  of  chondroitin-sulphuric  acid  and  protein.  One 
example  of  this  class  we  have  already  mentioned  mider  the 
name  of  chondro-mucoid.  Another  important  substance  is 
the  amyloid  substance  or  lardacein  which  is  found  in  the 
middle  coats  of  the  blood-vessels,  in  the  liver,  and  other  organs 
under  certain  pathological  conditions.  It  is  insoluble  in 
water,  alkalies,  acids,  or  gastric  juice.  It  gives  a  red-brown 
colour  with  iodine,  which,  on  the  addition  of  strong  sulphuric 
acid,  turns  to  a  dirty-blue  colour. 

Bodies  allied  to  Proteins,  or  Albuminoids 

Under  this  heading  we  may  group  a  number  of  diverse 
bodies. 

1.  Gelatin  may  be  extracted  from  all  connective  tissues, 
especially  bone  and  white  fibrous  tissue,  by  prolonged  boiling 
with  water.  It  forms  a  solution  in  water,  which  is  liquid  at 
high  temperatures  but  sets  into  a  jelly  when  cold.  It  is  pre- 
cipitated by  tannic  acid,  but  not  by  acetic  acid.  No  tyrosine 
can  be  formed  from  it  by  boiling  with  dilute  sulphuric  acid,  and 
on  this  account  pure  gelatin  does  not  give  a  red  colour  when 
boiled  with  Millon's  reagent.  Gelatin  is  not  present  as  such 
in  the  tissues,  but  is  formed  from  a  precursor  {collagen)  by 
prolonged  boiling  with  water. 

2.  Ghondrin  may  be  extracted  by  boiling  cartilage.  Its 
solutions  are  precipitated  by  acetic  acid,  and  form  a  jelly 
when  cold.     On  boiling  with  dilute  acids  it  is  split  up,  with 


THE   MATERIAL   BASIS   OF   THE   BODY  47 

the  formation  of  a  body  possessing  the  power  of  reducing 
Fehling's  solution.  It  has  been  shown  that  chondrin  is  a 
compound  of  gelatin  with  a  sulpho-acid  (chondroitin- sulphuric 
acid,  q.v.,  p.  45). 

3.  Elastin,  the  substance  of  which  the  yellow  fibres  of 
connective  tissue  are  composed,  is  insoluble  in  water  and  dilute 
acids  or  alkalies.  It  is  very  slowly  dissolved  by  gastric 
juice. 

4.  Keratin  forms  the  main  part  of  the  horny  layer  of  the 
skin,  nails,  hair,  hoofs,  etc.  It  is  very  insoluble.  It  presents 
the  same  elementary  composition  as  the  proteins,  but  is 
distinguished  from  them  by  the  very  large  quantity  of  sulphur 
present,  which  may  amount  to  5  per  cent.  A  very  similar 
substance,  neuro-keratin,  can  be  obtained  from  the  supporting 
framework  (neuroglia)  of  nervous  tissues. 

Both  elastin  and  keratin  give  the  colour-reactions  of  pro- 
teins, especially  the  xanthoproteic  and  Millon's  reactions. 


48 


PHYSIOLOGY 


Section   3 
THE   PATS 

These  bodies  consist  of  the  elements  carbon,  hydrogen, 
and  oxygen.  They  occur  to  some  extent  in  most  tissues  and 
form  the  greater  part  of  adipose  tissue. 

The  lower  acids  of  the  fatty  series  are  represented  by 
formic,  acetic,  propionic,  butyric  acid,  etc.,  and  have  the 
general  formula  C„H^„0.j.  The  following  formuhe  will  serve 
to  show  the  manner  in  which  they  are  built  up  : 


>rmic  acid. 

H 

1 

Acetic  acid. 
CH3 

rroploiiic  acid. 

CH3 

1 

Butyric 

CH, 

1 

acid. 

Valerianic  acid. 
CH3 

1 

Caproic  acid 
(normal). 

CH3 

1 

COOH 

COOH 

i 
CH.. 

1 

1 
CH,. 

1 

CH, 

1 
CH, 

1 

COOH 

1 
CH,, 

1 

CH., 

1 

1 
CH, 

I 

COOH 

1 
CH., 

1 

CH, 

1 

COOH 

1 
CH.. 

i 
COOH 

Thus  by  adding  the  group  CH^  a  whole  series  of  bodies 
can  be  formed  increasing  in  molecular  weight.  The  16-carbon 
acid  is  palmitic  acid,  and  the  18-carbon  acid  is  known  as 
stearic  acid. 

Besides  these  we  have  another  group  of  the  general 
formula  C,^.^,,^^^..,  in  which  two  of  the  CH,  groups  are 
replaced  by  two  CH  groups    linked    by    double    bonds,  thus 

II     .     To  this  group  belongs  the  other  chief  acid  of  the  fats, 

CH 

viz.  oleic  acid,  CjgHg^O.^. 

The  fat  of  adipose  tissue  consists  of  a  mixture  of  olein, 
palmitin,  and  stearin,  the  first  being  liquid  and  the  two  latter 
solid  at  ordinary  temperatures. 

Fats  may  be  considered  as  compomids  of  the  triatomic 
alcohol,  glycerin  (C3H^^(OH)3),  with  oleic,  stearic,  or  pal- 
mitic acid,  water  being  eliminated  in  the  act  of  combination. 
Thus  : 

C3H,(OH)3  +  8(C.,H330.0H)  =  C3H,(O.C,,H330),  +  3H.,0. 

Glycerin.  Oleic  acid.  Olein,  Water. 


THE   MATERIAL   BASIS   OF   THE   BODY  49 

Fats  are  insoluble  in  water,  but  soluble  in  ether  and  in 
hot  alcohol.  When  boiled  with  alkaline  solutions,  they  are 
split  up  with  the  formation  of  glycerin  and  a  compound  of 
the  fatty  acid  with  the  alkali,  which  is  called  a  soap.  The 
alkaline  soaps  are  soluble  in  water. 

In  the  fats  of  milk  (butter)  we  hnd  lower  acids  of  the 
fatty  series,  such  as  butyric,  caijrylic,  and  caproic  acids, 
combined  with  glycerin.  Acetic  acid  is  also  a  member  of  the 
fatty  acid  series.  It  occurs  in  the  body  as  an  amino-acid, 
glycine. 

Lecithin. — This  substance  is  a  wax-like  body  which  is 
universally  distributed  in  the  organism,  and  is  found  in  espe- 
cially large  quantities  in  the  white  matter  of  nerves  and  of 
the  spinal  cord.  It  may  be  regarded  as  a  compound  of  a 
molecule  of  glycerin  with  two  of  stearic  acid,  one  of  phos- 
phoric acid,  and  a  molecule  of  a  nitrogenous  base,  cholin.  Its 
composition  is  represented  by  the  following  formula  : 

((C.sH3,0,). 

[^     ^"^  (0-aH,— N(CH3)30H. 

Lecithin  is  miscible  in  all  proportions  in  ether,  alcohol, 
and  fats.  It  swells  up  in  water,  of  which  it  can  imbibe  a 
large  quantity. 

Cholesterin  may  be  considered  here  although  it  does  not 
belong  to  the  group  of  fats.  Like  lecithin  it  is  found  wherever 
protoplasm   is   present   and    seems   to   be  an   essential  con- 

Fiu.    l.j. 


Cholesterin  crystals. 

stituent  of  every  living  cell.  It  is  a  monatomic  alcohol 
(C2,;H,3.0H).  It  is  easily  soluble  in  ether  or  hot  alcohol. 
From  the  latter  it  is  deposited  on  cooling  in  typical  plate-like 

4 


50  PHYSIOLOGY 

crystals,  each  of  which  has  a  corner  knocked  out.  It  is 
insohible  in  water  but  sHghtly  sohible  in  a  sokition  of  bile- 
salts.  Its  history  and  use  in  the  body  are  absolutely  un- 
known. 

Tests  for  cholesterin.~{\)  On  dissolving  cholesterin  in  chlorofoi-ni  and  then 
adding  an  equal  volume  of  concentrated  sulphuric  acid,  the  cholesterin  solution 
becomes  first  blood-red  and  then  a  more  violet-red,  while  the  sulphuric  acid 
appears  dark  red  with  green  fluorescence. 

(2)  On  moistening  cholesterin  crystals  with  sulphuric  acid  diluted  with  one- 
fifth  of  its  bulk  of  water,  the  edges  of  the  crystals  become  reddish  and  then 
violet.  If  a  little  iodine  solution  be  now  added,  the  crystals  become  by  degrees 
violet,  bluish-green,  and  then  blue. 


THE   MATEEIAL  BASIS   OF  THE   BODY  51 


Section    4 
THE   CAEBOHYDEATES 

These  substances  occur  in  large  quantities  in  plants,  and 
therefore  are  important  constituents  of  the  food.  Only 
small  amounts  of  carbohydrates  however  are  at  any  given 
time  present  in  the  body,  where  they  may  occur  as  sugar  or 
animal  starch,  or  built  up  with  proteins  and  allied  bodies 
into  more  complex  compounds.  They  all  consist  of  carbon, 
hydrogen,  and  oxygen,  both  the  latter  substances  being 
present  in  the  same  proportions  as  they  exist  in  water.  Their 
general  formula  is  therefore  C^H.^^O,,.  They  may  be  divided 
into  three  classes :  mono- saccharides,  di- saccharides,  and 
poly-saccharides. 

1.  The  Mono-saccharides 

These  bodies  are  all  ketone  or  aldehyde  derivatives  of 
polyatomic  alcohols.  Thus  from  alcohols  of  the  formula 
CgH,^Og  may  be  derived  by  oxidation  either  the  aldehyde, 
d-glucose  or  grape-sugar,  CH.,OH(CHOH)^COH,  or  the  ketone, 
d-fructose  or  Ifevulose,  CH,0H(CH0H)3C0.CH,0H. 

The  sugars  (of  which  grape-sugar  is  a  type)  containing  the  aldehyde  group 
— COH,  are  spoken  of  as  aldoses.  Those  wlrich,  like  Itevulose,  contain  the 
ketone  group — CO — are  called  kctoses. 

The  di-  and  poly-saccharides  are  derived  from  the  mono- 
saccharides by  a  process  of  condensation  attended  with  the 
elimination  of  water.  The  mono-  as  well  as  the  di-saccharides 
are  all  distinguished  by  names  ending  in  — ose.  Thus 
according  to  the  number  of  carbon  atoms  present,  we  may 
distinguish  trioses,  tetroses,  pentoses,  hexoses,  etc.  Of  the 
sugars  only  those  with  six  carbon  atoms,  viz.  the  hexoses, 
which  are  of  physiological  importance,  need  concern  us  here. 

All  the  sugars  resemble  the  ordinary  aldehydes  in  being 
possessed  of  strongly  reducing  properties.  Thus  they  reduce 
cupric  to  cuprous  hydrate  on  boiling,  and  ammoniacal  solu- 
tions of  silver  to  the  metallic  condition.  From  a  practical 
standpoint  one  of  their  most  important  reactions  is  that  with 


52  PHYSIOLOGY 

phenylhydrazine.  On  boiling  an  aqueous  solution  of  sugar 
with  phenylhydrazine  and  acetic  acid,  two  sets  of  bodies  are 
formed,  first  hydrazones  and  then  osazones.  The  reaction 
which  goes  on  is  represented  by  the  following  equations  : 

CH20H(CHOH)3CHOH.CHO  +  H2N.NH.C,H5  = 

Glucose.  Phenylhydrazine. 

CH,0H(CH0H)3CH0H.CH  :  N.NH.C.H.  +  H^O. 

Phenylglucosehydrazone. 

The  hydrazone  then  reacts  with  another  molecule  of 
phenylhydrazine  to  produce  an  osazone. 

CH20H(CHOH)3CHOHCH  :  N.NH.C,H,  +  H2N.NH.C,H,= 
CH,0H(CH0H)3C.CH :  N.NH.C.H, 

II  +H2O  +  H,. 

N.NH.CfiHs 

The  hydrogen  however  is  not  given  off  in  the  free  state 
but  acts  on  a  third  molecule  of  phenylhydrazine,  splitting  it 
into  aniline  (C,.Hg  NH2)  and  ammonia. 

The  osazones  are  yellow  crystalline  compounds,  which 
have  definite  solubilities  and  melting-points,  varying  accord- 
ing to  the  sugars  from  which  they  have  been  derived.  Hence 
these  compounds  are  of  great  value  in  the  separation  and 
identification  of  sugars  in  a  complex  mixture.  By  acting  on 
the  osazones  with  reducing  agents,  such  as  zinc  and  acetic 
acid,  an  osamine  is  formed,  e.g.  glucosamine,  and  from  this 
a  sugar  can  be  re-obtained  by  the  action  of  nitrous  acid.  In 
this  case  however  the  sugar  is  always  a  ketose,  so  that  we 
may  in  this  way  convert  glucose  into  fructose  or  laevulose. 

A  very  large  number  of  hexoses  are  known,  the  differ- 
ences depending  partly  on  the  variations  in  the  structural 
formula,  partly  on  the  varying  position  of  the  constituent 
atoms  of  the  molecule.  Owing  to  the  asymmetric  structure 
of  the  molecules,  there  are  three  sugars  corresponding 
to  each  structural  formula.  Two  of  these  are  optically 
active  and  rotate  the  plane  of  polarised  light  either  to  the 
right  or  to  the  left,  while  a  third  modification  is  inactive 
and  can  be  regarded  as  consisting  of  an  equal  number 
of  molecules  of  the  two  active  varieties.  It  is  customary 
to  classify  the  glucoses   into   d-  (dextro-rotatory),   1-    (laevo- 


THE   MATEKIAL   BASIS   OF   THE   BODY  53 

rotatory),  and  i-  (inactive)  groups,  and  to  distinguish  the  other 
sugars  by  the  letters  of  the  ghicose  with  which  they  are 
most  nearly  connected,  without  any  reference  to  their  optical 
characters.  Thus  fructose  or  laevulose,  although  it  is  la3vo- 
rotatory,  is  distinguished  as  d-fructose  owing  to  its  relation- 
ships with  grape-sugar,  d-glucose. 

The  different  varieties  of  sugars  are  however  easily  con- 
vertible one  into  the  other.  Thus  by  the  action  of  weak 
alkalies,  glucose  is  converted  into  fructose  and  mannose  and 
two  other  sugars. 

A  number  of  the  mono- saccharides  are  acted  upon  by  yeast 
with  the  formation  of  alcohol,  CO2  being  evolved  in  the  pro- 
cess. This  action  however  is  confined  to  the  mono-saccha- 
rides  with  three,  six,  or  nine  carbon  atoms.  Moreover 
certain  of  the  artificially  prepared  hexoses,  as  well  as  some 
occurring  m  combination  with  proteins  in  the  glyco-proteins, 
are  insusceptible  to  the  action  of  yeast. 

Of  the  numerous  hexoses  known,  we  Dieed  only  describe 
three  which  are  of  physiological  importance,  i.e.  grape-sugar, 
galactose,  and  fructose  or  Itevulose,  the  two  first  being  aldoses 
while  the  latter  is  a  ketose. 

Gra])e-siigar  (d-glucose)  or  dextrose  is  found  in  small 
quantities  in  the  blood  and  in  numerous  tissues  of  the  body, 
and  is  the  form  to  which  all  carbohydrates  are  converted 
before  they  reach  the  circulation.  It  occurs  m  large  quanti- 
ties in  grapes  and,  mixed  with  hievulose,  m  honey  and  various 
fruits.  When  pure  it  forms  colourless  crystals  which  are 
easily  soluble  in  water.  Its  solutions  rotate  the  plane  of 
polarised  light  to  the  right. 

Tests  for  dextrose. — 1.  Moore's  test.  On  warming  a  solu- 
tion of  dextrose  with  caustic  potash  or  soda,  the  solution 
turns  first  yellow  and  then  brown. 

2.  Trommer's  test.  On  adding  caustic  potash  or  soda  to 
a  solution  of  dextrose  and  then  a  few  drops  of  copper  sul- 
phate, the  cupric  hydrate  produced  is  dissolved  to  form  a 
deep-blue  fluid  solution.  On  heating  to  boiling,  the  cupric 
is  reduced  to  cuprous  hydrate,  which  is  thrown  down  as 
a  yellow  or  red  precipitate.  Fehlmg's  reaction  is  merely 
a  modification  of  this  test.  An  alkaline  solution  of  cupric 
hydrate  is  prepared  by  adding  Eochelle  salt  and  potash  to 
a  solution  of  copper  sulphate,  the  tartrate  serving  to  keep 


54  PHYSIOLOGY 

the  cupric  hydrate  in  yolution.  On  boihng  some  of  this 
'  FehHng's  sohition '  with  dextrose,  it  is  reduced  with  the 
production  of  the  red  cuprous  hydrate.  The  proportions  of 
the  ingredients  are  so  adjusted  that  1  c.c.  of  FehHng's  solu- 
tion is  exactly  reduced  by  0-005  gramme  dextrose. 

3.  The  phenylhydrazme  test  for  sugar  has  already  been 
described.  The  phenylglucosazone  forms  masses  of  yellow 
]ieedle-shaped  crystals  which,  when  purified  by  recrystallisa- 
tion  from  alcohol,  melt  at  204°  C. 

Lixvulose,  fruit-sugar  or  d-fructose,  occurs  in  fruits  and 
honey  in  association  with  dextrose.  It  is  also  produced  by 
the  hydrolysis  of  a  species  of  starch  called  inulin,  and 
together  with  dextrose  by  the  inversion  of  cane-sugar.  It 
gives  the  same  tests  as  dextrose,  but  rotates  the  plane  of 
polarised  light  to  the  left. 

Galactose  is  obtained  together  with  dextrose  by  the  inver- 
sion of  milk-sugar,  and  is  also  present  in  the  glucoside, 
cerebrin,  which  is  found  in  the  brain.  It  rotates  the  plane 
of  polarised  light  to  the  right. 

2.  The  Di-saccharides 

The  di-saccharides  can  be  regarded  as  anhydrides  formed 
by  the  combination  of  two  mono-saccharides  with  the  elimina- 
tion of  one  molecule  of  water.  Hence  their  general  formula 
is  Ci2H220,i.  Under  the  action  of  hydrolytic  agents  they 
take  up  a  molecule  of  water  and  split  into  two  molecules  of 
hexobe.     Thus 

Cane-sugar  +  H^O  =  dextrose  -1-  laevulose. 
Lactose  -I-  H20  =  dextrose  +  galactose. 
Maltose  +  H^O  =  dextrose  +  dextrose. 

Cane-sugar  is  the  most  important  member  of  this  group, 
and  takes  a  prominent  place  in  our  dietary.  It  is  crystalline 
and  easily  soluble  in  water.  On  boiling  with  dilute  mineral 
acids  or  under  the  action  of  certain  ferments,  it  undergoes 
inversion,  taking  up  one  molecule  of  water  and  splitting  into 
dextrose  and  Itevulose. 

C^H^^O^ -f  H,0  =  C,3H.  A  +  CeHi  A- 

Cane-sugar.  Dextrose.       Lasvulose. 


THE   MATERIAL   BASIS   OF   THE   BODY  55 

Under  the  action  of  the  yeast  fungus,  cane-sugar  is  first 
inverted,  and  the  invert-sugar  is  then  converted  into  alcohol 
with  the  ebullition  of  CO.^. 

C,H,A  =  2C2H,0-f2CO,. 

Alcohol. 

Cane-sugar  does  not  reduce  alkaline  solutions  of  cupric 
hydrate  such  as  Fehling's  solution.  When  warmed  with 
sulphuric  acid  it  turns  black.  On  addmg  concentrated 
hydrochloric  acid  to  a  strong  solution  of  cane-sugar  and 
heating,  the  fluid  turns  a  deep-red  colour.  The  same  reac- 
tion is  given  by  Isevulose,  but  not  by  dextrose. 

Lactose  or  milk-sugar  occurs  in  milk.  It  is  much  less 
soluble  in  water  than  cane-sugar  and  is  only  faintly  sweet. 
It  reduces  Fehling's  solution.  On  boiling  with  dilute  acids 
it  takes  up  water  and  is  converted  into  dextrose  and  the 
isomeric  sugar  known  as  galactose. 

C,,H,,0„  4- H,0  =  C.H.  A  +  C,3H,,0,. 

Lactose.  Galactose.       Dextrose. 

With  the  lactic  acid  organism  it  is  converted  into  lactic  acid. 
To  this  conversion  of  lactose  into  lactic  acid  is  due  the  sour- 
ing of  milk. 

C„H,,>0^,-i-H.,0  =  4C3HA- 

Lactose.  Lactic  acid. 

Maltose  is  the  end-product  of  the  action  of  diastase, 
salivary  or  pancreatic  ferments  on  starch.  It  reduces 
Fehling's  solution  and  is  converted  on  boiling  with  dilute 
mineral  acids  into  dextrose. 

3.  The  Poly-saccharides 

This  group  includes  a  large  number  of  highly  complex 
bodies  which  all  occur  in  the  amorphous  condition  and 
belong  to  the  class  of  colloids.  Their  general  formula  is 
(CyH,||0^)x,  where  x  may  be  anything  from  twenty  up  to  a 
hundred  or  more.  By  hydrolysis  they  are  all  converted 
finally  into  mono-saccharides.  They  may  be  divided  into 
three  groups,  starches,  dextrins  and  gums,  and  celluloses  ; 
but  we  need  concern  ourselves  here  only  with  those  members 
which  are  of  physiological  importance. 

Starch  does  not  occur  in  the  living  body,  but  constitutes 
an  important  foodstuff,  being  present  in    large  quantities  in 


56  PHYSIOLOGY 

nearly  all  vegetable  food.  It  is  a  white  powder  consisting 
of  microscopic  grains  with  concentric  rings.  It  is  insoluble 
in  cold  water.  When  boiled  with  water,  it  swells  up  to  form 
an  opalescent  semi-solution.  This  solution  gives  an  intense 
blue  colour  with  iodine.  On  boiling  with  dilute  acids,  starch 
is  converted  first  into  dextrin  and  then  into  dextrose.  With 
various  ferments,  such  as  diastase  (malt  ferment),  salivary 
or  j)ancreatic  ferments,  it  undergoes  hydrolysis  and  is  con- 
verted into  dextrin  and  maltose.  The  change  that  occurs  on 
boiling  with  acids  may  be  thus  represented  : 

C,H„0,  +  H,0  =  C,H,,0,. 

starch.  Water.        Dextrose. 

Glycogen  or  animal  starch  is  found  in  the  liver,  muscles, 
and  other  tissues  of  the  body,  and  occurs  in  especially  large 
quantities  in  all  fcetal  tissues.  It  is  a  white  powder  soluble 
in  cold  water,  forming  an  opalescent  solution.  With  iodine 
it  gives  a  mahogany-red  colour.  It  is  affected  by  acids  and 
ferments  in  the  same  way  as  starch.  It  is  precipitated  from 
its  solution  on  the  addition  of  alcohol  to  60  per  cent.  It 
is  moreover  precipitated  by  saturation  of  its  solutions  with 
ammonium  sulphate. 

Dextrin.  Two  varieties  of  this  body  may  be  distinguished 
according  to  their  reaction  with  iodine — erytJtrodextrin which 
gives  a  red  colour  with  iodine  ;  and  achroo dextrin  which 
gives  no  colour. 

It  is  said  to  occur  m  small  quantities  in  blood,  muscle, 
and  liver,  but  it  is  chiefly  of  importance  as  being  an  inter- 
mediate product  in  the  digestion  of  starch.  It  is  gummy  and 
amorphous,  readily  soluble  in  water  but  insoluble  in  alcohol 
and  ether.  On  boiling  with  acids  it  is  converted  into  dex- 
trose. It  is  not  thrown  do^vn  on  saturation  with  ammonium 
sulphate. 

Cellulose  is  the  colourless  material  which  composes  the 
cell-walls  and  woody  fibre  of  plants,  and  so  occurs  to  a  large 
extent  in  our  food.  In  man  however  it  does  not  undergo 
digestion  and  therefore  need  not  be  further  considered  here. 

In  herbivorous  animals  cellulose  undergoes  digestive  changes,  and  forms 
an  important  constituent  of  their  food.  In  this  case  the  digestion  is  mainly 
effected  through  the  intermediation  of  micro-organisms,  and  results  in  the 
formation  of  acetic  and  butyric  acids  and  marsh  gas,  besides  other  substances 
In  certain  invertebrata  a  true  cellulose-digesting  ferment  [r.ytase)  is  secreted  by 
the  walls  of  the  alimentary  canal. 


57 


CHATTER   III 

THE   BLOOD 

Section  1 
THE   FORMED   ELEMENTS   OF   THE   BLOOD 

The  blood,  which  circulates  through  all  parts  of  the  living 
body  and  comes  into  close  relationship  with  all  the  tissues, 
acts  as  a  medium  of  communication  between  the  cells  in 
the  interior  of  the  bod}'  and  those  on  the  surface.  Thus  it 
carries  the  absorbed  foodstuffs,  which  have  been  taken  up  by  the 
cells  lining  the  alimentary  canal,  to  all  the  other  cells  of  the 
body,  and  from  these  receives  in  exchange  their  waste  products, 
CO.^  and  urea,  or  some  precursor  of  these  substances,  to  dis- 
charge them  through  the  intermediation  of  excretory  cells  on 
the  surface  or  lining  involutions  of  the  outer  surface  of  the 
body,  such  as  the  cells  of  the  skin,  kidney,  and  lungs. 

It  is  evident  that  the  composition  of  the  blood  must  be 
always  varying,  according  to  the  nature  of  the  tissues  it  has 
just  traversed,  and  these  variations  will  be  more  fitly  con- 
sidered when  we  come  to  the  discussion  of  the  activities  of  the 
various  tissues.  But  we  find  that  the  blood  has  a  certain 
power  of  regulating  its  composition,  or  perhaps  this  function 
must  be  ascribed  to  the  various  tissues  through  which  the 
blood  passes.  However  this  may  be,  the  fact  remains  that 
the  blood  has  an  average  composition  which  it  is  our  duty 
in  this  chapter  to  describe,  and  round  which  its  composition 
varies  only  to  a  certain  (definite)  extent. 

The  blood  of  man  and  most  vertebrates  is  a  red  opaque 
liquid,  rather  viscous,  and  to  the  naked  eye  homogeneous. 
Arterial  blood,  i.e.  the  blood  in  the  pulmonary  veins,  left  side 
of  the  heart,  and  the  arteries  generally,  is  bright  scarlet ; 
while  venous  blood,  i.e.  blood  in  the  systemic  vems,  right 
heart,  and  pulmonary  artery,  is  of  a  bluish-red  hue. 

Shaking  up  venous  l)lood  with  air  or  oxygen  changes  it  to 


58  PHYSIOLOGY 

arterial  blood,  and  we  shall  see  later  that  the  bright  colour 
is  due  to  the  formation  of  a  loose  combination  of  one  of  the 
constituents  of  the  blood,  haemoglobin,  with  oxygen.  This 
combination  is  normally  formed  in  the  lungs,  and  is  robbed 
of  its  oxygen  in  the  tissues. 

On  microscopic  examination  the  blood  is  found  to  consist 
of  a  nearly  colourless  fluid,  the  liquor  sanguinis  or  hlood- 
plasma,  holding  in  suspension  an  enormous  number  of  solid 
bodies,  the  red  and  white  blood-corpuscles. 

The  colour  of  the  blood  is  entirely  due  to  the  red  corpuscles. 
These  are,  in  man,  non-nucleated  biconcave  discs  about  g-gV o 
of  an  inch  in  diameter,  and  a  third  of  this  in  thickness.     The 

Fig.   16. 


Non-nucleated  red  blood-discs  of  human  blood.     On  the  right  of  the 
figure  the  corpuscles  are  seen  on  edge.     (Swale  Vincent.) 

colour  of  a  single  corpuscle  is  yellow,  the  red  colour  being 
apparent  only  when  large  numbers  of  them  are  seen  together. 
They  form  about  40  per  cent,  of  the  total  mass  of  the  blood, 
there  being  about  five  million  red  corpuscles  in  every  cubic 
millimetre  of  blood. 

They  are  soft,  flexible,  and  elastic,  so  that  they  can  readily 
squeeze  through  apertures  and  canals  narrower  than  them- 
selves without  being  permanently  distorted.  Each  red  cor- 
puscle consists  of  a  framework  or  stroma  composed  chiefly  of 
protein  material,  containing  in  its  meshes  or  in  a  state  of 
loose  chemical  combination  with  it,  a  red  colouring  matter, 
haemoglobin,  to  which  is  due  the  colour  of  the  corpuscles  and 
of  the  blood  itself. 

By  treating  the  blood  with  weak  solutions  of  tannic  or 
boracic  acid,  a  separation  occurs  between  the  haemoglobin 
and  the  stroma.     The  haemoglobin  appears  as  a  small  ball 


THE   BLOOD  59 

near  the  centre  of  a  colourless  blood-disc,  or  it  may  be  ex- 
truded and  lie  just  outside  the  stroma.  If  the  plasma  be  made 
denser  by  evaporation  or  l)y  addition  of  salts  to  it,  water 
diffuses  from  the  corpuscle  into  the  plasma,  and  the  corpuscle 
shrinks  and  becomes  wrinkled  or  crenated.  If  on  the  other 
hand  the  plasma  be  diluted,  water  diffuses  from  the  surround- 
ing medium  into  the  corpuscle,  which  swells  up  and  becomes 
spherical. 

It  is  found  that  in  a  0*9  per  cent,  solution  of  sodium  chloride 
the  red  corpuscle  of  mammalian  blood  neither  gains  nor  loses 
in  volume.  In  solutions  of  less  concentration  the  corpuscle 
swells,  while  in  those  of  greater  concentration  the  corpuscle 
shrinks  and  becomes  crenated.  These  facts  show  that  the 
external  limiting  layer  of  the  corpuscle  is  impermeable  to 
sodium  chloride,  but  permits  the  easy  passage  of  water  to 
equalise  the  concentration  within  and  without  the  corpuscle. 

This  movement  of  water  is  due  to  the  fact  that  dissolved 
substances,  like  gases,  are  always  tending  to  expand  within 
their  solvents,  a  tendency  which  gives  rise  to  diffusion  from 
places  of  higher  to  those  of  lower  concentration,  or  to  a 
pressure  (oswo^ic  pressure)  on  any  membrane  or  wall  which 
is  permeated  by  the  solvent  but  which  prevents  the  passage 
of  the  dissolved  substances.  When  the  corpuscles  are  im- 
mersed in  a  weak  salt  solution,  the  osmotic  pressure  within  the 
corpuscle  is  greater  than  that  outside,  so  that  the  corpuscle 
expands,  and  may  indeed  burst  if  the  difference  of  concentra- 
tion is  sufficiently  great.  In  solutions  of  higher  concentra- 
tion, the  external  osmotic  pressure  is  greater  than  that  in  the 
corpuscle,  and  water  is  squeezed  out  through  the  limiting 
layer,  so  that  the  corpuscle  shrinks. 

It  is  evident  that  these  changes  of  form  will  occur  only 
when  the  corpuscles  are  immersed  in  a  solution  of  some  sub- 
stances which  cannot  penetrate  the  corpuscle.  Most  neutral 
salts  {e.g.  NaCl,  Na.SO^,  KNO3  &c.)  belong  to  this  category. 
A  solution  of  urea,  on  the  other  hand,  behaves  towards  the 
corpuscles  like  distilled  water.  If  some  red  corpuscles  be 
added  to  a  1  per  cent,  solution  of  urea  in  normal  saline  solu- 
tion (0-9  per  cent.  NaCl),  they  neither  shrink  nor  swell ;  and  if 
the  mixture  be  centrifuged,  and  the  corpuscles  and  super- 
natant fluid  analysed  separately,  the  percentage  of  urea  in  the 
two  cases  will  be  found  identical — though  there  will  be  a  great 


60  PHYSIOLOGY 

preponderance  of  sodium  chloride  in  the  supernatant  fluid. 
There  are  a  large  number  of  substances  besides  water  to  which 
the  corpuscles  are  permeable  (and  in  this  respect  they  resemble 
most  other  vegetable  and  animal  cells),  e.g.  alcohol,  chloro- 
form, ether,  etc.  Speaking  generally,  we  may  say  that  animal 
cells  are  freely  permeable  to  all  those  substances  which  are 
soluble  in  fats  and  the  allied  lipoid  substances,  cholesterin, 
lecithin,  and  protagon,  which  are  invariable  constituents  of  all 
living  cells.  If,  for  instance,  we  wish  to  stain  a  living  cell, 
we  must  choose  some  dye-stuff  which  is  soluble  in  this  class 
of  bodies. 

In  birds,  amphibia,  and  fishes,  the  red  corpuscles  differ  from  those  of 
mammals  in  being  nucleated.  Those  of  the  frog  for  instance  are  oval  struc- 
tures, each  containing  an  oval  nucleus  with  a  well-marked  nuclear  network. 
The  hajmoglobin  is  diffused  through  the  protoplasm  of  the  cell-body,  and  does 
not  extend  to  the  nucleus.  Mammals  during  the  early  part  of  their  foetal  life 
also  possess  nucleated  red  corpuscles.  These  however  soon  disappear  entirely, 
to  make  way  for  the  ordinary  non-nucleated  red  discs.  In  the  camel  the  red 
corpuscles  are  oval  in  shape  like  those  of  the  frog,  but  possess  no  nucleus. 
They  are  also  much  smaller  than  those  of  the  frog. 

The  colourless  corpuscles,  or  leucocytes,  are  rather  larger 
than  the  red  (^-gVo  ii^ch  in  diameter),  and  much  fewer  in 
number,  there  being  only  one  white  corpuscle  to  about  300  to 
600  red. 

They  are  colourless  nucleated  masses  of  protoplasm  very 
similar  to  the  simple  organism  described  in  the  Introduction 
as  the  amoeba.  Like  this  they  have  the  power  of  moving 
from  place  to  place,  of  taking  up  food  particles,  and  probably 
of  reproduction  by  fission. 

Several  varieties  of  leucocytes  exist  in  the  blood.  The 
most  numerous  variety,  the  '  polymorphonuclear '  leucocyte, 
presents  a  nucleus  which  is  lobed  or  composed  of  several  parts 
united  by  fine  threads.  The  protoplasm  contains  some  very 
fine  granules  which  have  only  a  faint  affinity  for  acid  dyes 
such  as  eosin. 

More  sparse  are  the  leucocytes  known  as  '  hyaline.'  These 
possess  only  a  single  round  or  oval  nucleus,  and  their  proto- 
plasm is  free  from  granules. 

About  5  per  cent,  of  the  leucocytes  present  a  mass  of 
coarse  highly  refracting  granules  in  their  protoplasm.  These 
granules  stain  intensely  with  eosin  and  other  acid  dyes,  and 
are  therefore  designated  eosinophile.  The  nucleus  is  lobed  or 
reniform. 


THE    BLOOD 


61 


The  fourth  variety  represented  m  the  figure  is  the  hjmijho- 
cyte.  This  consists  of  a  large  round  nucleus  surrounded  by 
a  thin  layer  of  hyaline  protoplasm.  It  is  derived  from  the 
lymphatic  glands,  and  probably  represents  an  immature  form 
of  the  hyaline  leucoc^'te. 

Fig.  17. 


Various  forms  of  leucocytes,  a.  Eosinophile  corpuscle,  h.  Ordinary 
polynuclear  leucocyte  (' neutrophile ').  c.  Hyaline  corpuscle. 
d.  Lymphocyte. 

Very  rarely  we  find  a  fifth  form  of  corpuscle  contain- 
ing granules  which  stain  deeply  with  basic  dyes,  such  as 
hsematoxylin  or  methylene  1)1  ue,  and  are  therefore  called 
hasophile. 

Besides  the  red  and  white  corpuscles,  a  third  formed 
element  has  been  described  under  the  name  of  blood -platelets. 
These  are  small  bodies,  disc-shaped  or  irregular,  about  one 
quarter  the  diameter  of  a  red  corpuscle,  and  are  always  to  be 

Fig.  18. 


Blood-corpuscles  and  blood-platelets,  within  a  small  vein. 
(Schafer,  after  Osier.) 


observed  on  examining  the  blood  immediately  after  it  has  left 
the  body.  They  have  also  been  called  haematoblasts,  on  the 
assumption  that  they  were  precursors  of  the  red  blood - 
corpuscles.  These  platelets  have  been  observed  m  normal 
circulating  blood,  but  it  seems  probable  that  they  may  be 
added  to  by  a  process  analogous  to  one  of  precipitation  in 
the  plasma  as  it  commences  to  die  or  to  cool  down. 


62 


PHYSIOLOGY 


Chemistry  of  the  Bed  Blood-corpuscles 

We  have  already  mentioned  that  these  can  be  regarded  as 
consisting  of  two  parts,  the  haemoglobin  and  the  stroma,  pro- 
bably in  a  state  of  loose  chemical  combination. 

By  various  means  it  is  possible  to  destroy  this  combina- 
tion and  to  dissolve  out  the  haemoglobin,  leavmg  the  colour- 
less, swollen-up  stroma  floating  in  the  plasma.  The  effect  of 
this  is  to  make  the  blood  darker  but  more  transparent.  In 
this  condition  it  is  spoken  of  as  '  laky  blood.' 

Blood  may  be  made  laky  by  the  following  means  : 

(a)  Addition  of  a  small  amount  of  ether. 

(6)  Free  dilution  with  water. 

(c)  Alternate  freezing  and  thawing. 

{d)  Addition  of  bile  salts  ;  and  various  other  ways. 

If  this  blood  be  allowed  to  stand  in  a  cool  place  for  an 
hour  or  two,  a  mass  of  crystals  is  deposited,  which  consist  of 


Fio.  19. 


^1 


^:k   ^-^ 


Crystals  of  oxyheemoglobin.     1.  Fioin  rat. 
3.  From  squirrel. 


'2.  From  guinea-pig. 


oxyliasmoglobin.  This  crystallisation  occurs  very  readily  in 
the  blood  of  some  animals,  such  as  the  rat,  guinea-pig,  and 
dog  ;  much  less  so  in  that  of  others,  such  as  man,  rabbit,  and 
sheep. 

Oxyhmnoglohin  thus  obtained  and  purified  by  recrystal- 
lisation  forms  rhombic  prisms  or  tablets  of  a  'dark-red  colour 
(Fig.  19).  It  is  a  compound  of  a  protein  with  an  iron- 
containing  residue  (haematin),  and  is  distinguished  from  all 
other  proteins  by  the  ease  with  which  it  crystallises.  Its 
percentage  composition  probably  varies  slightly  in  different 
animals. 


THE   BLOOD  63 

Elementary  analysis  of  haemoglobin  crystals  from  the 
blood  of  the  horse  gave  the  following  results : 

per  cent. 

Carbon 51-15 

Hydrogen 6*76 

Nitrogen 17-94 

Sulphur 0-389 

Iron 0  336 

Oxygen ........  23-425 

The  empirical  formula  for  haemoglobin  calculated  from 
this  would  be — 

The  most  important  property  of  haemoglobin  is  its  power 
of  uniting  with  a  definite  proportion  of  oxygen  to  form  an 
easily  dissociable  compound,  oxyhaemoglobin.  One  gram  of 
haemoglobin  will  combine  with  about  1-5  c.c.  of  oxygen 
(measured  at  O''  C.  and  760  mm.  pressure).  This  compomid 
can  be  dissociated  again  by  various  agencies,  such  as  heat  or 
simple  exposure  to  a  vacuum,  and  we  shall  see,  when  talking 
of  respiration,  how  very  valuable  to  the  organism  is  this  easy 
dissociability  of  the  oxyhaemoglobin  molecule. 

If  carbon  monoxide  gas  be  led  through  a  solution  of 
oxyhaemoglobin,  the  oxygen  is  replaced  by  an  equivalent 
proportion  of  CO,  so  that  a  more  stable  compound,  CO- 
haemoglobin,  is  formed  ;  and  this  in  its  turn  can  be  split 
up  by  nitric  oxide  gas  with  the  formation  of  NO-haemo- 
globin.  Thus  the  order  of  stability  of  these  three  compounds 
would  be — 

NO-haemoglobin. 
CO-haemoglobin. 
O.j-hsemoglobin. 

The  poisonous  properties  of  CO  gas  are  due  to  this  power 
it  has  of  turning  out  the  oxygen  from  the  oxyhemoglobin, 
thus  depriving  the  tissues  of  the  oxygen  which  is  normally 
carried  to  them  by  the  red  corpuscles. 

Oxyhaemoglobin  is  a  brighter  red  and  slightly  less  soluble 
than  haemoglobin.  Solutions  of  the  latter  are  dichroic, 
appearing  green  by  reflected  and  bluish-red  by  transmitted 
light. 

The  reduction  of  oxyhaemoglobin  to  haemoglobin  is  easily 
effected  by  various  reducing  agents,  such  as  ammonium  sul- 


64  PHYSIOLOGY 

phide  or  Stokes'  fluid  (an  alkaline  solution  of  ferrous  tartrate). 
This  change  in  the  colour  of  the  compound  is  accompanied 
by  a  very  evident  change  in  its  absorption  spectrum. 

Dilute  solutions  of  oxyhsemoglobin  placed  in  front  of  the 
slit  of  a  spectroscope  give  a  very  pronounced  absorption 
spectrum,  showing  two  black  l)and8  between  Framihofer's 
lines  D  and  E.  On  adding  a  few  drops  of  Stokes'  fluid  to  the 
solution  and  warming  gently,  these  two  bands  disappear,  and 
are  replaced  by  a  single  band,  rather  fainter  and  broader  than 
the  0„Hb  bands,  and  situated  between  them.  On  shaking 
the  solution  up  with  air,  the  bands  of  O^Hb  return,  only  to 
disappear  again  when  it  is  allowed  to  stand. 

Carhoxylimmoglohin  has  a  very  similar  spectrum  to  oxy- 
hsemoglobin,  but  is  a  much  brighter  red.  This  difference  in 
colour  is  best  observed  on  diluting  the  blood,  when  oxyhaemo- 
globin  acquires  a  yellowish  tint,  while  the  pink  colour  of  CO- 
hsemoglobin  is  retained  so  long  as  any  colour  is  visible.  The 
most  important  distinction  between  the  two  kinds  of  blood 
lies,  however,  in  the  fact  that  CO-htemoglobin  is  unaltered 
by  the  addition  of  ammonium  sulphide  or  Stokes'  fluid. 

Another  compomid  of  hajmoglobin  with  oxygen  is  methce- 
moglohin.  This  substance,  although  not  of  normal  occurrence 
in  the  body,  is  found  in  urine  and  in  blood  whenever  there 
is  a  sudden  breaking  down  of  red  corpuscles  with  the  setting 
free  of  excess  of  htemoglobin  hi  the  blood  plasma.  It  may 
be  prepared  by  the  addition  of  potassium  ferricyanide,  per- 
manganate, or  other  oxidising  agent  to  the  blood,  or  to  a 
solution  of  haemoglobin.  It  is  a  chocolate-brown  substance, 
crystallisable,  and  gives  a  distinct  absorption  band  in  the  red 
between  the  Fraunhofer  lines  C  and  D.  On  treatment  with 
a  few  drops  of  ammonium  sulphide,  the  methaemoglobin  is 
converted  into  haemoglobin,  and  this  on  shaking  with  air 
will  re-form  oxyhaemoglobin.  This  reduction  cannot  be 
effected  by  simple  physical  means,  such  as  exposure  to  a 
vacuum,  and  it  seems  probable  that  methaemoglobin  contains 
exactly  the  same  relative  amount  of  oxygen  as  oxyhaemo- 
globin, but  in  a  different  state  of  combination,  thus^ 

,0  0 

Hb(    I  Hbf 

\0  ^0 

Oxyha?moglobin.  Metlifemoglobin, 


THE   BLOOD  65 

It  is  interesting  to  note  that,  on  treating  oxyhaemoglobin 
with  potassium  ferricyanide,  the  whole  of  its  oxygen  is  given 
off  in  the  free  state,  the  ferricyanide  only  then  attacking  the 
reduced  hasmoglobin  and  oxidising  it  again  (but  in  a  different 
way)  to  methaemoglobin. 

This  fact  has  beeu  utilised  by  Haldane  in  devising  an  easy  metliod  for 
determining  the  oxygen  capacity  of  any  given  sample  of  blood  {i.e.  the  total 
amount  of  oxygen  which  the  blood  can  hold  in  combination). 

A  measured  quantity  of  arterial  blood  is  placed  in  a  bottle  and  laked  by  the 
addition  of  some  weak  ammonia.  A  small  tube  containing  potassium  ferri- 
cyanide is  then  placed  in  the  bottle,  which  id  then  closed  and  connected  by  a 
rubber  tubing  with  a  graduated  gas  burette  containing  water.  The  bottle  is 
then  inverted  so  as  to  mix  the  ferricyanide  solution  with  the  blood.  All  the 
combined  oxygen  of  the  oxyhasmoglobin  is  given  off,  and  its  volume  determined 
by  reading  off  the  amount  of  water  displaced  in  the  gas  burette.  Care,  of 
course,  must  be  taken  to  keep  the  temperature  of  the  whole  apparatus  constant 
during  the  operation. 

Hsemoglobin  is  very  easily  destroyed  by  various  means 
(heat,  alcohol,  weak  acids,  and  strong  alkalies),  being  split 
up  into  an  iron-containing  pigment,  haematin,  and  a  protein 
residue,  which  is  called  glohin.  The  latter  can  be  prepared 
by  treating  the  pure  haemoglobin  with  weak  hydrochloric 
acid,  which  dissolves  and  splits  up  the  haemoglobin.  The 
globin  can  then  be  precipitated  by  the  addition  of  ammonia. 
Although  coagulable  by  heat  it  presents  many  analogies  with 
the  group  of  proteins  which  have  been  described  as  Jiis tones, 
and  can  therefore  be  included  for  the  present  in  this  class. 

Hcematin  (C32H3|)N^0.3Fe),  when  dried  and  purified,  forms 
a  bluish-black  crystalline  mass,  insoluble  in  water  and  alcohol, 
but  easily  soluble  in  acids  or  alkalies  in  alcoholic  or  watery 
solutions.  It  forms  compounds  with  acids  and  alkalies  which 
are  known  as  acid  and  alkaline  haematin,  each  of  which  gives 
a  characteristic  absorption  spectrum. 

With  hydrochloric  acid  it  forms  a  crystalline  hydrochloride 
known  as  hcemin.  This  compomid  is  prepared  with  extreme 
ease,  and  this  fact,  combined  with  the  very  definite  appearance 
of  its  crystals,  renders  it  a  very  delicate  test  for  blood. 

To  prepare  hamin  crystals  a  little  dried  blood,  haemoglo- 
bin, or  haematin  is  heated  with  a  minute  crystal  of  common 
salt  and  glacial  acetic  acid,  and  then  allowed  to  cool.  Haemin 
crystallises  out,  and  can  be  recognised  on  examination  under 
the   microscope.      The  crystals  are  dark   l)rown,  sometimes 

5 


66 


PHYSIOLOGY 


nearly  blaclc,  and  present  the  form  of  vliombic  plates,  often 
arranged  in  radiating  bundles. 

Alkaline  haematin  is  interesting  from  the  fact  that  it  may 
be  reduced  by  ammonium  sulphide  and  re- oxidised  on  shaking 
with  air,  just  like  a  solution  of  O.Hb.  The  spectrum  of  oxy- 
alkaline  hsematin  shows  one  very  indefinite  absorption  band 

Fig.  20. 


Absorption  spectra  of  haemoglobin  and  its  derivatives.  1.  Oxy- 
hsemoglobin.  2.  lleduced  haemoglobin.  3.  Methipmoglobin.  4.  Al- 
kaline metliffimoglobin.  5.  Acid  hsematin  in  ether.  6.  Alkaline 
haematin  in  rectified  spirit.  7.  Eeduced  haematin.  8.  Acid  hfema- 
toporphyrin.     9.  Alkaline  htematoporphyrin.     (From  MacMunn.) 

close  to  D.  Eeduced  alkaline  haematin  gives  two  sharp 
absorption  bands,  similar  to  those  of  oxyhaemoglobin,  but 
rather  nearer  the  blue  end  of  the  spectrum  (Fig.  20). 

A  body  similar  to,  if  not  identical  with,  reduced  alkaline 
haematin,  Juemochromogen,  is  formed  when  haemoglobin  is 
warmed  with  caustic  potash  in  a  vessel  from  which  all  air  has 


THE   BLOOD  67 

been  driven  out  by  the  passage  of  a  stream  of  hydrogen  or 
other  neutral  gas. 

If  pure  haemin  be  treated  with  ammonium  sulphide,  it 
is  reduced  to  haemochromogen.  If  to  this  solution  globin 
(prepared  from  haemoglobin)  be  added,  and  the  mixture 
allowed  to  stand  for  some  days,  it  is  found  that  reduced 
haemoglobin  is  formed.  It  is  interesting  to  note  that  egg- 
white  may  be  used  in  this  experiment  instead  of  globin, 
showing  that  other  proteins  can  take  the  place  of  globin  in 
the  haemoglobin  molecule. 

Other  derivatives  of  haemoglobin  are — 

a.  Hcematoporphyrin  (C,,;H,gN20;3),  or  iron-free  hiematin, 
is  easily  prepared  by  the  action  of  strong  sulphuric  acid  on 
blood,  oxy haemoglobin,  or  haematin,  or  by  the  action  of  10 
per  cent,  hydrochloric  acid  on  reduced  haemoglobin,  the 
separation  of  the  iron  from  the  rest  of  the  molecule  taking 

Fig.  21. 


Hffiiain  crystals. 

place  with  greater  readiness  in  the  absence  of  oxygen.  It 
forms  a  deep  purple  solution  with  characteristic  spectrum  in 
the  acid,  from  which  it  is  precipitated  as  a  black  powder  on 
free  dilution  with  distilled  water.  It  is  isomeric  with 
bilirubin.  With  alkalies  it  forms  alkaline  hcematoporphyrin, 
with  spectrum  differing  somewhat  from  the  acid  compound. 
Htematoporphyrin  is  found  in  minute  traces  in  normal  urine, 
but  may  occur  in  large  amount  in  urine  after  poisoning  by 
sulphonal.  Hsmatin  may  be  regarded  as  consisting  of  two 
haematoporphyrin  groups  linked  together  by  means  of  one 
atom  of  iron,  which  also  attaches  an  oxygen  atom,  as  is 
represented  by  the  formula 

h.  Hydrohilirubin  (C-jaH^^N^O-)  is  produced  by  the  action 
of  tin  and  sulphuric  acid  on  an  alcoholic  solution  of  haematin. 


68  PHYSIOLOGY 

c.  HcBmatuidi)i  (probably  identical  with  bilirubin)  occurs 
as  orange-red  rhombic  tables  in  old  blood-clots  in  the  body. 

The  STROMA  may  be  obtained  from  laky  blood  by  the 
addition  of  dilute  sulphuric  acid  or  acid  sodium  sulphate,  by 
which  the  swollen -up  stromata  are  shrivelled  up  ;  they  may  be 
collected  by  allowing  the  liquid  to  stand,  or  by  means  of  the 
centrifuge.  On  chemical  analysis  they  are  found  to  consist 
chiefly  of  nucleo-proteins.  They  are  soluble  in  weak  alkalies, 
and  on  prolonged  gastric  digestion  give  an  insoluble  residue, 
rich  in  phosphorus  (nuclein).  On  extraction  with  alcohol 
and  ether  they  yield  a  certain  amount  of  lecithin,  cholesterin, 
and  fats. 

Important  constituents  of  the  red  corpuscles  are  the  salts 
and  water.  The  salts  are  chiefly  potassium  and  phosphoric 
acid  compounds,  there  being  very  little  chlorides  present,  and 
little  or  no  sodium  (cf.  the  serum). 

The  corpuscles  contain  about  two-thirds  of  ilieii"  total 
weight  of  water. 

Life-history  of  the  Bed  Blood-corpuscles 

There  can  be  no  doubt  that  a  continual  destruction  of  red 
blood-corpuscles  is  going  on  in  the  body.  Thus  an  animal 
secretes  every  day  by  the  agency  of  the  liver  a  considerable 
amount  of  bile,  containing  a  pigment,  bilirubin.  This  pig- 
ment can  be  shown  to  be  derived  from  the  haemoglobin  of 
the  red  blood -corpuscles.  In  cases  where  an  effusion  of  blood 
has  taken  place  mto  the  brain  or  the  connective  tissues,  we 
often  find  some  months  after  the  lesion  that  the  corpuscles 
and  red  pigment  have  disappeared,  and  that  the  connective 
tissue  in  the  vicinity  contains  a  number  of  yellowish-brown 
crystals,  known  as  haematoidin  crystals.  These  crystals  are 
identical  in  form,  composition,  and  reactions  with  bilirubin, 
the  colouring  matter  of  the  bile. 

Under  normal  circumstances  however,  the  conversion  of 
haemoglobin  into  bile-pigment,  as  we  shall  see  later  on,  takes 
place  exclusively  in  the  liver.  It  is  found  that  if,  by  the 
injection  of  poisons  such  as  pyrogallol  or  toluylene  diamine,  a 
number  of  red  corpuscles  are  broken  up  and  destroyed,  setting 
free  haemoglobin  in  the  blood-plasma,  there  is  marked  increase 
in  the  amount  of  ))ile-pigmcnt  formed  by  the  liver  ;  and  a 
similar    increase  may  be  brought  about  by  the  injection  of 


THE   BLOOD  69 

solutions  of  pure  haemoglobin  into  the  blood.     What  is  the 
chemical  change  involved  in  this  conversion  ? 

From  a  comparison  of  the  formula  of  hfematin  (C.j.,H3„N_, 
OgFe)  with  that  of  bilirubin  (C,,;H|^N203),  we  see  that  the 
change  is  associated  with  a  loss  of  iron  ;  and  we  find,  as  a 
matter  of  fact,  that  in  all  cases  in  which  there  is  an  increased 
iKcmafoJijfiifi  (destruction  of  red  corpuscles),  there  is  at  the 
same  time  an  accumulation  of  iron  in  the  liver.  This  ac- 
cumulation is  especially  well  marked  in  cases  of  pernicious 
anfemia.  It  seems  prol)able  that,  under  normal  circumstances, 
the  ha3moglobin  is  broken  up  in  the  liver,  part  of  the  hsematin 
molecule  being  transformed  into  bilirubin  and  turned  out  of 
the  body  with  the  faeces,  while  the  iron  is  stored  up  in  the 
liver  to  assist  in  the  formation  of  fresh  htiemoglobin  or  new 
red  blood-corpuscles. 

Tlie  presence  of  iron  in  the  liver  cells  may  be  easily  demonstrated  in  fresh 
or  alcohol-hardened  specimens  bj-  treating  a  cut  sm'face  or  section  succes- 
sively with  potassium  ferroeyanide  and  dilute  hydrochloric  acid.  After 
severe  blood  destruction  the  whole  liver  may  present  on  this  treatment  a 
deep-blue  colour,  due  to  the  fomnation  of  iron  ferroeyanide. 

We  have  not  yet  been  able  in  the  laboratory  to  convert 
hfematin  directly  into  bilirubin.  Iron-free  hsematin  or 
hsematoporphyrin  is  however  very  nearly  allied  to  bilirubin, 
the  empirical  formulpe  of  these  two  bodies  being  identical. 
We  can  moreover  by  the  action  of  reducing  agents  o1)tain 
identical  products  from  haematin  and  bilirubin.  Thus  by 
treating  haematin  with  tin  and  hydrochloric  acid  or  by  acting 
on  bilirubin  with  sodium  amalgam,  a  body,  hydrobilirubin,  is 
produced,  which  apparently  belongs  to  the  same  class  of 
bodies  as  urobilin,  a  pigment  occurring  in  small  quantities  in 
the  urine.  There  is  no  doubt  that  this  latter  pigment  can  be 
derived  from  bilirubin  under  the  influence  of  putrefactive 
organisms,  and  it  is  in  this  way  that  the  urobilin  of  the  faeces 
(the  so-called  stercobilin)  is  produced. 

It  is  evident  then  that  the  pigments  excreted  from  the 
body  in  the  urine  and  f feces  are  derived  from  haematin,  and 
therefore  that  a  disintegration  of  the  red  blood -corpuscles 
must  be  continually  taking  place. 

Since,  in  a  healthy  animal,  the  amount  of  corpuscles  in 
the  blood  remains  approximately  constant,  a  continual  pro- 
duction of  new  corpuscles  must  go  on  to  take  the  place  of 


70 


PHYSIOLOGY 


those  destroyed  and  discharged  to  form  l)ilo  and  urinary 
pigments.  There  is  considerable  doubt  as  to  the  exact  mode 
and  place  in  which  this  regeneration  occurs,  and  the  process 
seems  to  be  quite  different  according  as  we  consider  the 
foetal  or  adult  condition.  We  may  therefore  consider  first 
the  formation  of  red  corpuscles  in  the  eml)ryo  and  new-l)orn 
animal. 

The  red  corpuscles  at  an  early  stage  of  fontal  life  are 
nucleated,  like  those  of  the  frog  or  l)ird.  Li  the  vascular 
area  of  the  chick,  nests    of   nuclei   are  found  embedded  in 

Fig.  22. 


a    -" ,„ 


Section  of  red  marrow  of  mammal  (Bohm  and  Davidoff).  a,  e,  ery- 
throblasts  ;  b,  reticulum  ;  c,  mj'eloplax ;  c7,  g,  marrow  cells ;  /,  a 
marrow  cell  dividing  ;  li,  a  space  which  was  occupied  by  fat. 


colourless  masses  of  non-difierentiated  protoplasm.  A  little 
later  it  is  seen  that  these  nuclei  are  all  surrounded  with 
a  differentiated  portion  of  the  protoplasm,  which  now  con- 
tains hemoglobin,  the  intervening  undifferentiated  portions 
having  become  more  fluid  and  representing  the  future 
blood-plasma.  Very  soon  the  masses  of  protoplasm  become 
channelled  and  connected  with  one  another  and  with  the 
large  vessels  coming  from  the  heart,  and  the  fully  formed 
blood  moves  on  into  the  general  circulation  in  response  to 
the  heart-beat. 

According  to  Schafer,  non-nucleated  blood-discs  may  be 


THE   BLOOD 


71 


produced  in  a  veiy  similar  fashion.  Towards  the  end  of 
foetal  life  we  find  in  the  developing  connective  tissue  branched 
masses  of  protoplasm  containing  nuclei.  These  nuclei  how- 
ever are  not  wasted  on  mere  oxygen  carriers  but  are  entirely 
used  to  form  the  endothelium  of  the  capillary  wall ;  and 
non-nucleated  red  corpuscles  are  developed  by  a  simple 
differentiation  of  the  central  part  of  the  protoplasmic  mass, 
the  parts  of  the  protoplasm  between  the  corpuscles  again 
appearing  to  furnish  the  material  for  the  fluid  plasma. 

This   process   however   seems  to  come  to  an  end  within 
a  few  days  after  birth.     In  order  to  find  the  seat  of  blood 

Fir,.  23. 


Section  of  red  marrow  of  pigeon  (Denys).  Ic,  eosinophile  leuco- 
cytes ;  eg,  fat  cells ;  e,  nucleus  of  endothelial  cell  of  blood-vessel ; 
ca,  blood  capillary  ;  er,  erythroblasts  lying  within  vascular  endo- 
thelium ;  gir,  fully  formed  red  corpuscles. 


production  after  this  period,  the} best  method  is  to  increase 
the  activity  of  the  process  b}^  inducing  an  abnormally  large 
destruction  of  corpuscles,  either  by  the  injection  of  poisons  or 
by  the  actual  removal  of  blood.  If  the  animal  be  killed  when 
by  such  means  its  blood-forming  powers  have  been  called 
upon  to  the  utmost,  the  only  organ  i  or  tissue  of  the  body 
which  exhibits  any  sign  of  hypertrophy* is  the  red  marrow  of 
the  bones.  Ordinarily  in  the  adult  animal  this  red  marrow  is 
found  occupying  only  the  spaces  of  the  cancellous  tissue  of 
the  ribs  and  vertebrre,  and  at  the  ends  of  the  long  bones. 
But  after  bleeding  it  may  be  found   occupying  a  large  extent 


7'2  PHYSIOLOGY 

of  the  shafts  of  the  bones.  On  microscopic  examination  it  is 
found  that,  of  the  numerous  cells  which  compose  this  tissue, 
myelocytes,  osteoblasts,  marrow  and  fat  cells,  the  only 
elements  present  in  excess  are  nucleated  cells  containing 
haemoglobin  in  their  protoplasm.  Many  of  these  cells  are  in 
a  state  of  active  division,  and  all  intermediate  stages  can  be 
found  l»etween  the  daughter-cells,  containing  well-formed 
nuclei  and  stained  with  haemoglobin,  and  the  ordmary  non- 
nucleated  red  blood-disc,  the  nuclei  apparently  degenerating 
and  undergoing  solution  or  extrusion  from  the  cell.  The 
capillaries  in  the  red  marrow  are  bounded  by  walls  which 
may  be  imperfect,  thus  admitting  the  entry  of  the  newly 
formed  corpuscles  into  the  circulating  blood.  According  to 
some  authorities,  the  large  red  cells  are  themselves  derived 
from  colourless  nucleated  cells,  the  so-called  erythroblasts. 
Among  the  complex  of  cells  forming  the  red  marrow  it  is 
very  difficult  to  make  certain  of  this  fact,  but  there  seems  to 
be  little  doubt  that  such  is  the  case  in  the  bird.  If  we 
examine  the  red  marrow  of  the  pigeon  we  find  that  the 
capillary  walls  are  perfect,  but  are  lined  internally  by  a  layer 
of  blood-forming  cells  (Fig.  23).  In  this  layer  all  stages  can 
be  found  between  the  colourless  erythroblast  and  the  mature 
oval  nucleated  red  blood-corpuscle,  which  is  characteristic  of 
this  class  of  animals. 

The  occasional  occurrence  of  nucleated  red  blood-corpuscles 
in  the  spleen,  especially  after  great  loss  of  blood,  has  been 
regarded  by  some  physiologists  as  a  proof  that  this  organ  also 
takes  part  in  blood  formation.  It  seems  more  probable  that 
the  chief  function  of  the  spleen  in  this  regard  consists  in  the 
destruction  by  its  cells  of  inefficient  or  weakened  corpuscles,  and 
that  it  is  rather  a  blood-destroying  than  a  blood-forming  organ. 

We  need  at  present  only  mention  the  view  put  forward  by  Hayem  and 
others  that  the  red  blood-discs  are  produced  in  the  circulating  blood  by  a 
gradual  modification  of  the  blood-platelets  or  liDematoblasts. 

We  have  no  evidence  to  tell  us  how  long  a  red  corpuscle 
lives,  or  how  long  it  can  carry  on  its  functions  before  it  is 
broken  up  in  the  liver  or  spleen  and  cast  out  of  the  body. 
Experiments,  such  as  injection  of  the  blood  of  a  bird  into  a 
mammal  when  the  introduced  blood-corpuscle  can  be  always 
identified,  naturall_y  give  us  no  idea  of  the  duration  of  activity 
of  a  normal  corpuscle. 


THE   BLOOD  78 

Since  iron  is  an  essential  constituent  of  hfemoglobin,  it  is 
evident  that  our  food  must  contain  enough  iron  to  restore  the 
loss  of  it  in  the  corpuscles.  The  amount  however  need  be 
only  very  small  if  it  were  all  assimilated,  for  the  whole  blood 
of  an  average-sized  man  does  not  contain  more  than  about 
2-5  grms.  Fe. 

Inorganic  forms  of  iron,  such  as  the  iron  salts,  are  how- 
ever absorbed  only  in  very  small  quantities,  and  there  is 
some  probability  that  a  good  deal  of  the  htemoglobin  is  re- 
integrated from  an  organic  form  of  iron  contained  in  the 
food,  called  hsematogen — a  protein  belonging  to  the  group  of 
nucleo-proteins  and  containing  iron  in  a  state  of  intimate 
chemical  combination.  After  administration  of  organic  iron 
salts,  this  metal  may  be  demonstrated  in  the  absorbmg  villi 
of  the  intestine.  It  is  difficult  however  to  determine  exactly 
the  amount  of  iron  absorbed,  since  the  main  channel  for 
excretion  of  this  substance,  as  for  most  of  the  other  heav.y 
metals,  is  the  intestmal  tract.  Thus  the  amount  of  iron  which 
can  be  recovered  from  the  fneces  is  no  index  to  the  amount  of 
iron  which  has  escaped  absorption. 

Chemistry  of  White  Blood-corpiificlefi 

The  chemistry  of  the  white  blood-corpuscles  is  the  same 
as  that  of  any  indifferent  animal  cells.  It  can  be  studied 
most  conveniently  on  the  leucocytes  of  lymphatic  glands. 
These  are  found  to  consist  almost  entirely  of  bodies  belonging 
to  the  class  of  nucleo-proteins.  The  extract  of  these  cells 
on  treatment  with  acetic  acid  gives  a  precipitate  of  nucleo- 
protein,  which  is  soluble  in  weak  alkalies.  The  solution 
produces  intravascular  clotting  on  injection  into  the  blood- 
vessels. Besides  the  nucleo-proteins,  the  leucocytes  contain 
lecithin,  fats,  and  cholesterin,  and  often  glycogen,  in  addition 
to  various  nitrogenous  extractives.  Their  salts  are  similar 
to  those  of  the  red  corpuscles,  and  contain  a  preponderance 
of  potassium  salts  and  phosphates. 

Origin  of  White  Blood-corpuscles 

The  various  kinds  of  leucocytes  probably  differ  in  their 
mode  of  origin.  There  seems  little  doubt  that  the  hyaline 
corpuscles  are  derived  from  the  lymphocytes,  which  are  found 


74  PHYSIOLOGY 

ill  the  lymphatic  glands  and  enter  the  blood  witli  the  lymph- 
stream  by  way  of  the  thoracic  duct. 

Many  authorities  ascribe  a  similar  mode  of  origin  to  the 
chief  or  polynuclear  leucocytes.  These  corpuscles  occur 
mainly  in  the  blood,  and  have  been  seen  in  a  state  of  division, 
so  that  it  is  most  probable  that  they  reproduce  themselves  by 
direct  cell  division  in  the  blood-stream  itself. 

The  eosinophile  corpuscles  are  found  in  large  numbers  in 
the  connective  tissue  in  various  parts  of  the  body,  especially 
in  the  red  marrow  of  bones.  They  probably  represent  a  migra- 
tory tissue  sui  generic  (perhaps  of  a  glandular  nature),  and 
are  derived  from  similar  cells  by  division  in  the  blood-stream 
or  in  the  connective  tissues. 

Estimation  of  Blood-corpuacles 

In  order  to  count  the  corpuscles,  a  known  small  volume  of  blood  is  diluted 
with  some  indifferent  fluid  (such  as  0-9  per  cent.  NaCl  solution),  and  a  drop  of 
this  placed  in  a  small  cell  on  a  glass  slide,  the  bottom  of  which  is  ruled  with 
squares.  The  depth  of  the  cell  and  the  size  of  the  squares  being  known,  it  is 
easy  to  count  the  corpuscles  lying  on  each  square  under  the  microscope,  and 
from  this  to  estimate  the  number  present  in  a  cubic  millimetre  of  undiluted  blood. 

Thus  in  Gowers'  hremocytometer  the  graduated  glass  cell  is  I  millimetre 
deep,  and  each  square  is  -^  millimetre  each  way. 

Five  cubic  mni.  of  blood  are  drawn  into  a  graduated  capillary  tube,  and 
then  blown  into  a  '  mixing  vessel '  containing  sodium  sulphate  solution  (sp.  gr. 
1025).  The  mixture  is  well  stirred  and  a  drop  placed  in  the  middle  of  the  cell, 
and  the  cell  covered.  In  a  few  minutes  the  corpuscles  have  sunk  to  the  bottom 
of  the  cell,  and  rest  on  the  squares.  The  number  of  corpuscles  in  ten  squares 
is  counted,  and  this  multiplied  by  10,000  gives  the  number  of  corpuscles  in  a 
cubic  millimetre.  In  normal  blood  there  are  from  four  to  five  million  cor- 
puscles in  a  cubic  millimetre  (i.e.  an  average  of  forty  or  fifty  to  each  square  of 
Gowers'  instrument). 

Estimation  of  Hcpinoglobin 

To  estimate  the  amount  of  hremoglobinin  a  given  sample  of  blood,  20  cubic 
mm.  are  taken  and  diluted  with  water,  until  the  mixture  is  equal  in  tint  to 
a  permanent  standard  coloured  solution  made  with  glycerin  and  carmine,  and 
corresponding  in  tint  to  blood  diluted  100  times  (Gowers'  haemoglobinometer). 
Thus  the  number  of  times  the  blood  must  be  diluted  to  bring  it  to  a  standard 
tint  divided  by  100  gives  the  percentage  amount  of  haemoglobin  present,  the 
normal  amount  being  taken  as  100. 

In  von  Fleischl's  ha>moglobinometer  the  specimen  of  blood  is  always  diluted 
to  the  same  extent ;  but  the  standard  of  comparison  is  a  wedge  of  coloured 
glass,  which  can  be  slipped  to  and  fro  till  its  tint  exactly  equals  the  tint  of  the 
sample  of  diluted  blood  placed  in  a  glass  cell  by  the  side  of  the  wedge  for  com- 
parison. The  sliding  wedge  is  graduated  to  indicate  the  percentage  amount  of 
haemoglobin  present  compared  with  the  normal.  Thus  if  the  tint  of  the  blood 
is  equal  to  the  wedge  at  100,  the  blood  contains  the  normal  amount  of  hfemo- 
globin  ;  if  at  50,  half  that  amount ;  and  so  on. 


THE   BLOOD  75 

Section    2 
THE   COAGULATION   OF   THE   BLOOD 

The  most  striking  property  of  blood  is  that  of  clotting 
when  it  is  shed.  If  blood  be  drawn  from  an  artery  or  vein 
into  a  vessel,  in  from  two  to  three  minutes  it  becomes  rather 
viscid.  This  viscidity  increases  till  the  whole  mass  of  blood 
solidifies  to  form  a  jelly-like  mass,  exactly  occupying  the 
volume  of  the  original  fluid  blood. 

After  about  an  hour,  yellowish  drops  exude  from  the  sur- 
face of  the  clot,  and  this  exudation  continues  till  the  clot  has 
shrunk  to  half  its  former  dimensions,  and  floats  in  a  clear 
yellow  fluid  (the  serum). 

Thus,  as  a  result  of  standing,  the  blood  has  been  resolved 
into  solid  clot  and  fluid  serum.  On  examining  the  clot  under 
the  microscope,  we  find  that  it  consists  of  all  the  corpuscles 
enclosed  in  a  meshwork  of  fine  fibrils  (Fig.  24). 

Fig.  24. 


Network  of  fibrin,  after  washing  away  the  corpuscles  from  a  film 
of  blood  that  has  been  allowed  to  clot ;  many  of  the  filaments 
radiate  from  Httle  clumps  of  blood-platelets.     (Schiifer.) 

If  however,  directly  the  blood  is  drawn,  it  be  whipped  with 
a  bundle  of  twigs  or  anything  presenting  a  large  rough  surface, 
the  latter  becomes  covered  with  a  stringy  mass,  and  we  find 
that  the  blood  has  lost  its  power  of  setting  to  form  a  jelly. 
This  stringy  mass  is  called  fibrin,  and  it  is  evident  that  the 
coagulation  of  the  blood  is  due  to  the  appearance  in  it  of  these 
fibrils  of  fibrin,  which  form  a  network  enclosing  in  its  meshes 
the  corpuscles  and  the  remaining  fluid  part  of  the  blood. 
This  network  then  contracts,  squeezing  out  the  fluid,  which 
appears  on  the  surface  of  the  clot  as  the  serum. 

Fibrin  obtained  by  whipping  fresh  blood,  or  by  washing 
away  the  corpuscles  from  a  clot,  exhibits  the  following  pro- 
perties : 

It  is  insoluljle  in  water  or  in  dilute  saline  solutions.  In 
stronger  solutions,  such  as  10  per  cent,  potassium  nitrate,  it 


76  PHYSIOLOGY 

is  very  slowly  dissolved,  but  is  altered  in  the  process,  the 
solution  containing  not  fibrin  but  proteins  Ijelonging  to  the 
globulin  class.  It  swells  up  in  dilute  HCl  (0-2  per  cent.),  and 
if  digested  with  it  at  a  temperature  of  40'  C.  slowly  dissolves, 
with  the  formation  of  acid  albumen  or  syntonin.  If  shreds  of 
fibrin  are  suspended  in  water  and  heated  to  boiling,  they  are 
converted  into  coagulated  protein,  losing  the  property  of  swell- 
ing up  in  dilute  HCl.  The  general  reactions  and  constitution 
of  fibrin  show  that  it  belongs  to  the  class  of  proteins. 

The  serum,  the  other  end-product  of  the  reactions  which 
determine  coagulation,  is  a  transparent  yellowish  fluid,  con- 
taining about  7  to  9  per  cent,  total  solids.  Of  these,  nearly 
eight  parts  are  protein  in  nature.  Proceeding  on  the  lines  of 
the  conventional  classification  laid  down  in  the  second  chapter, 
we  may  separate  two  distmct  proteins  in  the  serum — serum - 
glol)ulin  or  paraglobulin,  and  serum-albumen.  The  serum- 
globulin  may  be  thrown  down  by  saturation  with  magnesium 
sulphate,  or  by  adding  to  the  serum  an  equal  volume  of  a 
saturated  solution  of  ammonium  sulphate.  It  is  also  thrown 
down  if  the  serum  be  dialysed  against  distilled  water,  show- 
ing that  it  is  insoluble  except  in  the  presence  of  a  certain 
amomit  of  neutral  salt.^  On  heating,  it  coagulates  at  about 
75°  C.  An  imperfect  separation  of  paraglobulin  may  be 
effected  by  diluting  the  serum  with  twenty  volumes  of  distilled 
water,  and  adding  a  trace  of  acetic  acid  or  passing  a  stream 
of  CO.,  through  the  liquid.  On  removing  the  globulin  pre- 
cipitate, the  filtrate  still  contains  3  to  5  per  cent,  of  a  protein 
belonging  to  the  albumin  class,  known  as  serum-albumen, 
which  is  solul)le  in  distilled  water,  not  precipitated  by  satura- 
tion with  magnesium  sulphate,  but  totally  precipitated  on 
saturatmg  with  solid  ammonium  sulphate.  The  filtrate  from 
this  last  precipitate  is  perfectly  free  from  proteins.  Solutions  of 
serum-albumen  coagulate  on  heating  between  77°  and  SS''  C. 

By  the  method  of  fractional  heat-coagnlation  three  varieties  of  serum- 
albumen  have  been  separated.  But  in  the  absence  of  further  chemical  i^roof 
the  value  of  the  separation  so  effected  must  be  regarded  as  doubtful. 


'  It  has  been  shown  lately  that  the  precipitate  obtained  from  serum  on 
saturation  with  MgSO^,  or  half  saturation  with  ammonium  sulphate,  really 
comprises  two  bodies  : — 

(a)  A  true  globulin,  called  euglobuHn,  insoluble  in  pure  water,  and  identical 
with  the  precipitate  obtained  on  dialysis. 

(b)  A  pseudo-globulin,  soluble  in  distilled  water,  and  therefore  remaining  in 
solution  when  the  salts  are  removed  by  dialysis. 


THE    BLOOD  77 

The  other  constituents  of  the  serum  mchide  extractives 
such  as  urea,  sugar  (in  small  traces),  and  about  1  per  cent, 
inorganic  salts,  of  which  sodium  chloride,  carbonate,  and 
phosphate  are  the  most  important. 

We  have  now  to  consider  the  processes  which  lead  to  the 
formation  of  fibrin  in  shed  blood.  In  order  to  analyse  these 
processes  it  is  necessary  to  slow  the  process  of  coagulation. 

Clotting  is  favoured  by  the  following  influences : 

Exposure  to  high  temperature  (up  to  50"  C). 

Contact  with  foreign  surfaces  (as  when  the  blood  is  whipped 
with  a  bundle  of  twigs). 

It  is  retarded  or  prevented  by- 
Exposure   to   cold  (l)lood   may  be   kept    fluid   almost  in- 
definitely at  a  little  above  0"  C). 

Mixture  with  various  salts,  such  as  magnesium  or  sodium 
sulphate,  or  common  salt.  The  blood  is  received  mto  one- 
third  its  volume  of  a  saturated  solution  of  magnesium  sul- 
phate, or  into  an  equal  volume  of  half-saturated  sodium 
sulphate  solution. 

By  receiving  the  blood  from  the  vessels  directly  into  a 
solution  of  a  soluble  oxalate  in  such  proportions  that  the 
resulting  mixture  contains  1  in  1,000  of  oxalate. 

Injection  of  albumoses  ('  peptone ')  or  of  leech  extract  (also 
an  albumose)  into  the  veins  before  the  blood  is  drawn. 

Contact  with  the  lining  membrane  of  a  living  blood-vessel. 
Thus  if  we  ligature  the  jugular  vein  in  a  horse  at  two  points, 
the  blood  in  the  intervening  part  will  remain  fluid  for  many 
hours.  In  fact,  two  such  '  living  test-tubes  '  may  be  prepared, 
and  the  blood  poured  in  a  thin  stream  from  one  to  the  other 
without  coagulating. 

If  blood  be  drawn  from  an  artery  or  vein  in  a  bird,  without 
coming  in  contact  with  the  surrounding  tissues,  it  does  not 
clot.  Clotting  can  be  at  once  induced  by  adding  a  small  frag- 
ment of  tissue,  or  a  watery  extract  of  any  of  the  bird's  tissues. 

If  blood  which  has  been  prevented  from  coagulating  by 
one  of  these  methods  be  allowed  to  stand  in  a  cool  place,  the 
blood-corpuscles,  which  are  heavier  than  the  plasma,  gradually 
sink  to  the  bottom,  leaving  a  clear  supernatant  layer  of  plasma, 
which  can  be  pipetted  or  siphoned  oil".' 

'  This  process  is  much  shoitened  by  using  a  centrifugal  machine.  This 
consists  essentially  of  a  horizontal  wheel,  with  slots  cut  in  it  in  which  tubes  are 


78  PHYSIOLOGY 

Plasma  prepared  in  this  way  perfectly  free  from  formed 
elements  can  be  easily  made  to  clot.  Cooled  plasma  clots 
directly  its  temperature  is  allowed  to  rise ;  salt  plasma  on 
simple  dilution  ;  oxalate  plasma  on  the  addition  of  a  soluble 
salt  of  lime ;  peptone  plasma  on  dilution  and  passage  of 
a  current  of  CO^. 

The  clot  formed  is  colourless,  and  contracts  after  forma- 
tion just  like  the  clot  formed  in  the  whole  blood.  It  only 
difiers  from  the  latter  in  containing  no  corpuscles :  in  fact  it 
is  pure  fibrin. 

Hence  it  is  evident  that  the  hluud-plasma  contains  within 
itself  the  precursors  of  Jihrui. 

We  may  therefore  represent  the  processes  occurring  in 
coagulation  schematically  as  follows  : 

Clotting  of  blood  at  rest 

Blood 


Blood-corpuscles  Plasma 

\ 

I  I 

Fibrin  Serum 


Clot 

Clotting  of  whip2)ed  blood 

Blood 


Blood-corpuscles  Plasma 


Serum  Fibrin 


Defibrinated  blood 


What  are  these  precursors  ?  If  plasma  prepared  in  either 
of  the  foregoing  methods  be  saturated  with  common  salt,  a 
sticky  white  precipitate  is  produced.  This  may  be  collected 
on  a  filter  and  washed  w'ith  saturated  salt  solution  to  remove 
all  traces  of  adhering  plasma.     If  we  dissolve  this  substance 

suspended.  These  tubes  are  filled  with  the  blood,  and  the  wheel  made  to  re- 
volve about  2,000  times  per  minute.  The  tubes  swing  out  to  a  horizontal 
position,  and  the  centrifugal  force  causes  all  the  heavier  particles  to  collect  at 
the  ends  of  the  tubes,  so  that  in  half  an  hour  the  blood-corpuscles  form 
a  compact  mass  at  the  bottom  of  the  tubes. 


THE   BLOOD  79 

in  dilute  salt  solution,  the  solution  (at  the  ordinary  tempera- 
ture) soon  becomes  viscid  and  clots,  the  clot  after  a  while  con- 
tracting and  separating  out  a  serum,  which  is  found  to  contain 
a  protein  belonging  to  the  globulins.  Denis,  the  discoverer  of 
this  precipitate,  called  it  plasmine,  and  supposed  that  clotting 
consisted  essentially  in  a  splitting  up  of  this  simple  soluble 
body  into  two  bodies,  one  of  which  was  insoluble  (fibrin)  and 
the  other  soluble  (serum  globulin). 

Later  on  Alexander  Schmidt  found  it  possible  to  separate 
this  plasmine  into  two  substances,  which  he  called  fibrinogen 
and  fibrinoplastin.  Now  the  latter  is  identical  with  the  para- 
globulin  of  the  serum,  and  this,  as  Hammarsten  showed,  takes 
no  part  in  the  process  of  clotting.  We  must  therefore  regard 
the  fibrinogen  as  the  precursor  of  fibrin,  since  it  entirely  dis- 
appears and  is  replaced  by  fibrin. 

Fibrinogen  was  originally  prepared  by  passing  a  current 
of  CO2  through  plasma  after  dilution  with  twenty  volumes  of 
water.  The  separation,  however,  is  much  better  eftected  by  a 
modification  of  Denis'  original  method.  Sodium  chloride  is 
added  to  the  plasma  until  it  reaches  15  per  cent.  This  is 
conveniently  done  by  adding  to  each  volume  of  the  plasma  an 
equal  volume  of  a  saturated  solution  of  NaCl,  containing  about 
30  per  cent,  of  the  salt.  A  precipitate  is  gradually  produced, 
at  first  granular  and  then  becoming  fiocculent  and  fibrinous. 
This  precipitate  may  be  washed  free  from  adherent  protein 
by  half-saturated  solution  of  sodium  chloride.  The  pure 
fibrinogen  thus  obtained  belongs  to  the  class  of  globulins, 
being  insoluble  in  distilled  water  but  soluble  in  dilute  salt 
solutions.  The  solutions  coagulate  on  heating  at  the  low 
temperature  of  56°  C. 

If  the  plasma  which  served  as  the  source  for  the  fibrinogen 
was  sodium  sulphate  or  sodium  chloride  plasma,  the  solution 
of  the  fibrinogen  may  clot  on  simple  standing,  being  almost 
entirely  converted  into  fibrin,  only  a  small  trace  of  protein 
being  left  in  solution.  The  change  therefore  from  fibrinogen 
to  fibrin  involves  a  process  of  splitting,  in  which  by  far  the 
larger  proportion  becomes  insoluble.  If  however  we  get  the 
fibrinogen  from  magnesium  sulphate  or  oxalate  plasma,  the 
solution,  although  in  all  other  respects  identical  with  that 
obtained  from  sodium  sulphate  plasma,  does  not  clot  on 
standing  ;  and  the  same  is  sometimes  found  with  the  fibrinogen 
solution    from    sodium    sulphate   plasma,  if  the  precipitated 


80  PHYSIOLOGY 

fibrinogen  has  been  very  thoroughly  washed.  In  all  cases 
however  a  pure  fibrinogen  solution  can  be  made  to  clot  by 
adding  to  it  a  drop  or  two  of  serum,  or  of  the  washings  of 
a  blood-clot.  It  is  evident  therefore  that  some  other  factor 
besides  fibrinogen  must  be  present  in  order  to  bring  about 
coagulation.  Since  this  other  factor  may  be  in  excessively 
small  quantities,  and  may,  if  given  time,  convert  almost  in- 
definitely large  amounts  of  fibrinogen  into  fibrin,  it  has  been 
regarded  as  a  ferment,  and  caAled  fibrin  ferment. 

A  fairly  pure  solution  of  the  fibrin  ferment  may  be  pre- 
pared in  the  following  way.  Serum,  or  chopped-up  blood - 
clot,  is  allowed  to  stand  with  about  twenty  times  its  volume  of 
absolute  alcohol  for  two  or  three  months.  The  proteins  by 
this  means  are  precipitated  and  rendered  insoluble  in  water, 
so  that  an  aqueous  extract  of  the  dried  precipitate  contains 
very  little  protein  matter,  but  is  rich  in  fibrin  ferment ;  that 
is  to  say,  it  possesses  the  power  of  converting  solutions  of 
fibrinogen  into  fibrin  ;  for  we  can  never  recognise  ferments 
except  by  their  action. 

We  must  conclude  therefore  that  the  coagulation  of  the 
blood  is  due  to  the  conversion  of  a  soluble  protein  present  in 
the  plasma — fibrinogen,  into  an  insoluble  protein — fibrin, 
under  the  agency  of  a  ferment,  lohich  is  known  as  fibrin 
ferment  or  throonbin. 

As  to  the  chemical  nature  of  the  ferment  we  know  practically  nothing.  It 
is  destroyed  on  heating  to  55°  C.  and  is  always  associated  with  a  certain 
amount  of  protein.  It  is  stated  by  some  authors,  on  rather  insufhcient  grounds, 
to  belong  to  the  class  of  nucleo-proteins. 

What  is  the  origin  of  this  fibrin  ferment?  It  is  not 
present  in  the  circulating  blood,  but  is  formed  after  the  blood 
has  left  the  vessels.  If  blood  be  received  straight  from  an 
artery  into  a  large  quantity  of  absolute  alcohol,  and  the 
precipitate  extracted  with  water  after  two  or  three  months,  the 
extract  is  not  found  to  have  any  power  of  causing  clotting  in 
solutions  of  fibrinogen. 

Schmidt  was  of  opinion  that  the  colourless  corpuscles  break 
down  as  soon  as  they  leave  the  vessels  and  liberate  the  ferment. 
If  horse's  blood  be  received  into  a  vessel  placed  in  ice,  and 
allowed  to  stand,  it  soon  separates  into  three  layers :  an  upper 
layer  of  pure  plasma,  a  thin  layer  of  leucocytes  and  granules, 
and  a  layer  of  red  corpuscles.  If  the  temperature  of  the  blood 
be  allowed  to  rise,  it  clots  throughout,  but  the  clotting  begins 


THE^BLOOD  81 

soonest  in  the  layer  of  leucocytes,  and    s  tirmest  there.     If 
the  clot  be  divided  into  three  portions,  and  treated  for  the 
extraction  of  ferment,  the  extract  from  the  part  of  the  clot 
enclosing  the  leucocytes  and  granular  matter  is  much  more 
active  than  that  from  either  of  the  two  ends  of  the  tube.     Of 
course  the  significance  of  this  experiment  depends  on  the  view 
we  take  of  the  origin  of  the  granular  matter.     Schmidt  looked 
on  it  as  the  debris  of  exploded  corpuscles,  while  Wooldridge 
regarded  it  as  a  precipitate   produced  by  the  effect  of  cold 
on  the  plasma.    This  latter  observer  therefore  considered  that 
the  ferment  was  produced,  not  from  the  leucocytes  but  from 
the  plasma,  and  that  the  granular  precipitate  represented  the 
precursor  or  zymogen  of  the  ferment.     Later  researches  by 
Hammarsten  on  the  clotting  of  oxalate  plasma  have  confirmed 
this  view  and  thrown  light  on  the  necessary  conditions  of   the 
change  from  precursor  to  ferment.     Oxalate  plasma  will  clot 
on  the  simple  addition  to  it  of  a  soluble  salt  of  lime,  e.g.  CaCL, ; 
and  the  same  statement  holds  good  for  the  fibrinogen  prepared 
from  this  plasma  by  the  ordinary  methods.     If  however  the 
plasma  be  cooled  to  0"  C.  for  two  or  three  days,  a  granulai 
discoid  precipitate  is  produced,   and    on    separating   oft'  the 
plasma  and  extracting  from  it  the  fibrinogen,  it  is  found  that 
the  solution  will  no  longer  clot  with  lime  salts,  but  needs  the 
addition  of  fibrin  ferment.     No  effect  is  produced  by  adding 
the  precipitate  to  the  fibrinogen,  but  if  a  little  calcium  chloride 
be  mixed  with  the  precipitate  and  the  mixture  added  to  the 
fibrinogen  solution,  clotting  ensues.     This  shows  that  plasma 
contains  in  solution  a  precursor  of  fibrin  ferment  which  has 
been  termed protJtrombin,  the  conversion  of  prothrombin  into 
thrombin  being  dependent  on  the  action   of   calcium    salts. 
The  origin  of  fibrin  ferment  is  however  rather  more  complex. 
Oxalate  plasma,  which  has  been  separated   from  the  preci- 
pitate of  '  prothrombin,'  can  be  made  to  coagulate  by  the  addi- 
tion of  extracts  of  almost  any  animal  tissues  together  with 
lime  salts,  and  these  were  therefore  supposed  to  contain  pro- 
thrombin similar  to  that  obtained  by  cooling  oxalate  plasma. 
But  these  extracts,  even  on  mixture  with  calcium  salts,  are 
found  to  be  without  effect  on  pure  solutions  of  fibrinogen.   More- 
over, the  precipitate  produced  by  cold,  if  thoroughly  washed 
before  treatment  with  lime  salts,  loses  its  power  of  evoking 
coagulation  in  fibrinogen  solutions.     It  is  therefore  concluded 

6 


82  PHYSIOLOGY 

that  three  factors  are  necessary  for  the  production  of  librin 
ferment  or  thrombin,  viz. :  (1)  lime  salts  ;  (2)  a  substance 
present  in  the  precipitate  of  *  prothrombin  '  as  well  as  in 
most  animal  tissues  (this  is  called  tkrombokinase)  ;  (3)  a 
substance  present  in  solution  in  oxalate  plasma,  and  carried 
down  adhering  to  the  precipitate  obtained  on  cooling,  which 
is  called  thromhugen. 

Our  present  views  as  to  the  essential  nature  of  the  pro- 
cesses concerned  in  coagulation  may  be  summed  up  as 
follows :  — 

When  blood  leaves  the  vessels  there  is  a  disintegration  of 
the  blood-platelets  (themselves  perhaps  derived  from  ante- 
cedent changes  in  the  plasma  or  from  an  unstable  form  of 
leucocyte)  with  a  liberation  of  thromboldnase.  This  acts  upon 
thrombogen,  already  in  solution  in  the  plasma,  and  in  the 
presence  of  lime  salts  gives  rise  to  thrombin.  Under  the 
influence  of  the  ferment  thrombin,  the  fibrinogen  present  in 
the  plasma  is  transformed  into  fibrin. 

These  changes  may  be  represented  by  the  following 
schema : 

Blood^Plasma  Blood-platelets  (?  pre-formed) 

Fibrinogen  Thrombogen  Lime  Salts         Thrombokinase 

Thrombin  (fibrin  ferment) 


Fibrin 

This  view  of  the  nature  of  the  changes  involved  in  coagulation  is  borne  out 
by  observations  on  other  forms  of  plasma,  especially  of  plasma  obtained  from 
birds'  blood.  This,  when  obtained  with  scrupulous  cleanliness,  so  as  to  avoid 
any  contamination  with  dust  or  with  the  tissues,  remains  permanently  un- 
coagulable.  In  the  plasma  got  by  centrifuging  the  blood,  no  blood-platelets 
are  to  be  seen  and  no  precipitate  is  produced  by  exposure  to  a  temperature  of 
0°  C.  We  may  say  therefore  that  blood-platelets,  with  their  contained  throm- 
bokinase, are  absent  from  birds'  blood,  and  with  them  the  property  of  sponta- 
neous coagulability.  The  blood  is  also  free  from  fibrin  ferment,  but  contains 
thrombogen  as  well  as  soluble  lime  salts.  It  is  only  necessary  therefore  to  add 
thrombokinase,  in  the  shape  of  a  watery  extract  of  any  tissue  or  cells,  in  order 
to  cause  the  formation  of  thrombin  and  the  conversion  of  the  fibrinogen  already 
present  into  fibrin. 

IntraiKiscular  Clotting 

If  fibrinogen  be  really  present  as  such  in  the  circulating 
blood,  one  would  expect  to  produce  intravascular  clotting  by 
the  injection  of  solutions  of  fibrin  fei'ment.     We  find  however 


THE   BLOOD  83 

that  with  our  ordinary  thrombin  solutions  practically  no 
effect  is  produced.  If,  however,  the  very  strong  fibrin  fer- 
ments contained  in  some  snake  venoms  be  injected,  universal 
intravascular  clotting  (thrombosis),  may  be  produced.  On 
the  other  hand,  injection  of  solutions  containing  throm- 
bokinase  causes  at  once  clotting  of  the  blood  in  the  vessels. 
This  is  probably  the  explanation  of  the  action  of  the  extracts 
of  cellular  tissues,  containing  the  so-called  tissue-fibrinogens. 

A  solution,  tissue-libriaogen,  is  prepared  by  extracting  chopped-up  thymus 
or  lymphatic  glands  with  water  or  normal  salt  solution.  After  separating 
the  suspended  cells  by  means  of  a  centrifuge,  the  clear  tiuid  is  treated  with 
acetic  acid ;  this  throws  down  a  precipitate  of  tissue-librinogen,  which 
may  be  dissolved  in  1  per  cent,  solution  of  sodium  carbonate.  A  few  c.c. 
of  this  solution,  injected  into  the  jugular  vein,  causes  extensive  intravascular 
clotting  within  30  seconds  of  the  beginning  of  the  injection.  In  the  rabbit 
intravascular  clotting  is  practically  universal,  and  this  is  also  the  case  in  the  dog 
if  in  full  digestion.  In  the  fasting  dog  the  clotting  may  be  limited  to  the  portal 
area.  In  this  case  the  rest  of  the  blood,  when  drawn  off,  is  found  to  be  incoagu- 
lable. We  may  thus  distinguish  two  phases  in  the  action  of  tissue-fibrinogens 
on  the  living  blood  :  first,  a  phase  of  increased  coagulability,  resulting  in  the 
production  of  a  clot — the  positive  phase ;  secondly,  a  negative  phase  of  dimi- 
nished coagulability.  If  the  injection  be  carried  out  very  slowly,  or  the 
solution  be  dilute,  only  the  negative  phase  may  be  produced.  Subsequent 
injections,  however  rapid,  of  a  strong  solution  have  no  effect,  the  animal  being 
for  the  time  protected  or  rendered  immune  by  the  first  injection. 

Certam  histological  observations  support  the  view  that 
coagulation  is  normally  inaugurated  by  the  disc-like  precipitate 
of  prothrombin,  which  appears  to  be  identical  with  the  blood- 
platelets  of  histologists.  If  a  small  vessel  be  observed  under 
the  microscope,  and  a  small  part  of  the  lining  endothelium 
injured,  it  is  noticed  that  blood-platelets  are  deposited  on  the 
injured  spot,  so  as  to  form  a  little  heap.  The  platelets  seem 
to  fuse  into  one  another,  and  finally  form  a  little  white  mass 
of  fibrin  (white  thrombus),  which  effectually  occludes  any 
opening  in  the  wall  of  the  vessel.  This  process  in  the  living 
body  always  occurs  when  a  vessel  is  injured,  and  is  a  means 
by  which  the  animal  is  protected  from  bleeding  to  death  from 
any  small  wound. 

It  may  be  well  liere  to  summarise  the  conditions  which  determine  the 
clotting  of  the  various  kinds  of  plasma  that  have  been  mentioned  in  this 
section. 

1.  Cooled  horse's  plasma  contains  all  the  fibrin  precursors,  and  clots  on 
simple  rise  of  temperature.  If  however  it  be  cooled  for  some  time  and 
filtered,  the  thrombokinase  is  separated,  and  the  filtered  plasma  will  not  clot 
without  the  addition  of  fibrin  ferment. 

2.  Sodium  sulphate  and  sodium  chloride  plasma  also  contain  all  the  fibrin 


84  PHYSIOLOGY 

precursors  and  clot  on  sirople  dilution,  the  salt  appearing  merely  to  inhibit 
the  action  of  the  ferment  on  the  fibrinogen.  Magnesium  sulphate  however 
gradually  precipitates  the  thrombokinase  and  thrombogen.  The  plasma  pre- 
pared by  the  use  of  this  salt  contains  therefore  only  fibrinogen,  and  needs 
the  addition  of  fibrin  ferment  as  well  as  dilution  to  make  it  clot. 

3.  Oxalate  plasma,  when  fresh,  contains  fibrinogen,  thrombokinase  and 
thrombogen.  It  needs  therefore  only  the  addition  of  lime-salts.  If  however  it 
be  cooled  and  filtered,  the  thrombokinase  is  separated,  and  to  induce  clotting, 
either  fibrin  ferment  or  a  mixture  of  thrombokinase  (or  a  tissue  extract)  and 
lime-salts  must  now  be  added. 

4.  Peptone  plasma  presents  many  peculiarities,  and  it  is  still  doubtful  how 
far  the  facts  gained  from  its  study  are  applicable  to  the  explanation  of  the 
coagulation  of  blood  under  normal  conditions.  The  mere  addition  of  peptone 
(i.e.  a  mixture  of  albumoses,  especially  hetero-albumose)  to  blood  does  not 
delay  clotting.  It  is  necessary  to  inject  the  peptone  in  the  proportion  of 
0'3  grm.  per  kilo  body-weight  into  the  veins  of  a  living  animal  (dog).  It  seems 
that,  under  the  infiuence  of  the  peptone,  the  liver  cells  or  vascular  endothelium 
secrete  into  the  blood  a  substance  which  hinders  clotting,  so  that  the  loss  of 
coagulability  is  merely  a  secondary  result  of  the  peptone  injection.  Peptone 
plasma  resembles  intravascular  plasma  in  that  it  will  clot  only  on  addition  of 
thrombokinase  or  tissue  fibrinogens,  and  is  unaffected  by  fibrin  ferment.  It 
apparently  contains  not  only  a  precursor  of  fibrin  ferment,  but  also  a  pre- 
cursor of  fibrinogen,  since  it  yields  no  precipitate  on  heating  to  56°  C, 
although,  by  precipitation  with  salt  and  re-solution,  the  typical  fibrinogen 
described  above  results. 

5.  Leech-extract  plasma  can  be  obtained  by  injecting  a  decoction  of  dried 
leeches  into  the  circulation,  or  by  adding  it  to  the  blood  as  it  leaves  the  vessels. 
The  active  principle  of  the  decoction  is  a  body  allied  to  an  iilbumose,  which  is 
secreted  by  the  buccal  glands  of  the  leech,  and  which  has  the  property  of 
neutralising  and  so  destroying  the  action  of  fibrin  ferment. 

Main  Points  in  the  Composition  of  the  Blood 

Specific  gravity  of  whole  blood  about  1055  ;  of  corpuscles 
about  1085  ;  of  serum  about  1035. 

The  specific  gravity  of  the  blood  may  be  estimated  clinically  in  the  follow- 
ing way : — A  series  of  mixtures  of  glycerin  and  water  are  prepared,  with 
specific  gravities  varying  from  1030  to  1070.  A  drop  of  blood  is  then  sucked 
up  into  a  capillary  pipette  with  its  point  bent  to  a  right  angle,  and  minute 
portions  of  this  drop  are  expelled  into  a  series  of  glasses  containing  glycerin 
and  water  mixtures  of  various  strengths.  The  red  drop  expelled  from  the 
pipette  will  rise  or  sink  in  the  fluid  so  long  as  its  specific  gravity  differs  from 
that  of  the  fluid.  The  specific  gravity  of  the  mixture  in  which  the  blood 
neither  rises  nor  sinks  is  equal  to  that  of  the  blood,  and  is  the  number  we  want 
to  know. 

The  blood  is  slightly  alkalme. 

This  is  best  demonstrated  by  placing  a  drop  on  a  piece  of  delicate  glazed 
litmus-paper,  and  then  wiping  it  off.  The  spot  where  the  blood  rested  is 
found  to  be  stained  blue. 

Blood  contains  from  one-third  to  half  its  weight  of  cor- 
puscles.    The  plasma  is  resolved  by  clotting  into  serum  and 


THE    BLOOD  85 

fibrin.  The  serum  contains  in  100  parts  proteins  (consisting 
of  serum  allmmen  and  paraglobulin)  8  parts ;  salts  about 
1  part ;  water  about  91  parts. 

Tlie  paraglobulin  and  serum  albumen  occur  in  varying 
proportions.  The  proportion  of  paraglobulin  to  albumen  in 
one  case  was  1 :  1*5  (man). 

The  chief  salt  present  is  sodium  chloride,  which  constitutes 
60  per  cent,  of  the  ash.  Next  to  this  comes  sodium  carbonate 
(about  30  per  cent.),  and  besides  these  two  we  find  traces  of 
potassium,  sodium,  and  calcium  chlorides  and  phosphates. 
Traces  of  fats,  cholesterin,  lecithm,  dextrose,  urea,  and  other 
nitrogenous  extractives  are  constantly  fomid  in  the  serum. 
The  fats  are  much  increased  after  a  meal  rich  in  them,  and 
may  give  the  serum  a  milky  appearance. 

The  red  corpuscles  contam  in  100  parts  — water  70  parts, 
solid  constituents  30  parts. 

Of  the  solid  constituents,  haemoglobin  forms  nme-tenths  ; 
the  other  tenth  corresponds  to  the  stroma,  consistmg  of  nucleo- 
proteiu,  lecithm  and  cholesterin,  and  salts.  There  is  a 
striking  contrast  between  the  salts  of  the  corpuscles  and  those 
in  the  serum  ;  the  former  consisting  chiefly  of  potassium 
phosphate,  the  latter  of  sodium  chloride,  which  ma}^  be  almost 
or  entirely  wanting  from  the  corpuscles. 

The  Biological  Significance  of  certain  Constituents  of 
the  Ser7im. 

It  has  long  been  known  that  the  serum  of  some  animals 
acted  as  a  poison  when  injected  into  others,  producing  exten- 
sive breaking  up  of  corpuscles,  appearance  of  haemoglobin  in 
solution  in  the  urine,  and  death.  The  same  destructive  efiect 
of  seram  on  foreign  corpuscles  may  be  observed  in  vitro. 
Thus  dog's  serum  causes  a  rapid  destruction  of  the  red 
corpuscles  when  added  to  defibrinated  rabbit's  blood,  although 
there  is  no  appreciable  difi^erence  between  the  concentrations 
of  the  sera  of  the  two  animals.  This  relative  glohidicidccl 
action  of  the  serum  may  be  artificially  produced.  Thus  injec- 
tion of  the  corpuscles  of  a  guinea-pig  into  a  rabbit  causes  the 
serum  of  the  latter  to  become  giobulicidal  for  the  corpuscles 
of  the  guinea-pig.  The  giobulicidal  property  is  destroj'ed  on 
heating  the  serum  to  55^^  C. 

The  giobulicidal  or  hceviolytic  power  of  the  serum  depends  on  the  presence 
in  the  latter  of  two  distinct  substances,  viz.  an  antibody  or  avihoceptor,  and  a 


86  PHYSIOLOGY 

complement.  Thus  when  rabbit's  serum  is  made  hn^molytic  for  guinea-pig's 
corpuscles,  by  repeated  injections  of  the  corpuscles  of  the  latter  into  the 
rabbit,  the  amboceptor  in  the  rabbit's  serum  is  the  direct  result  of  the  injection. 
It  is  found  that,  if  this  serum  be  heated  to  55°  C,  it  loses  its  ha!molytic 
properties.  These,  however,  are  at  once  restored  to  it  on  the  addition  of  some 
blood-serum  from  a  normal  guinea-pig.  It  is  therefore  concluded  that  serum 
normally  contains  a  substance,  toxic  for  corpuscles,  which  is  destroyed  at 
55°  C.  This  however  is  unable  to  act  except  in  the  presence  of  another 
substance,  the  immune  body  or  amboceptor,  which  is  specific  for  each  kind  of 
corpuscle,  and  is  produced  as  the  direct  result  of  the  injection  of  any  foreign 
corpuscles  into  an  animal.     The  amboceptor  is  not  destroyed  at  55°  C. 

The  blood  sera  of  various  animals  have  in  the  same  way  a 
destructive  power  on  certain  bacteria,  and  this  action  may  be 
increased  by  the  previous  injection  into  an  animal  of  the 
bacteria  in  question.  It  is  by  the  development  of  sucli 
bactericidal  substances  that  the  animal  is  in  many  cases 
enabled  to  react  to  an  infection,  or  to  develop  an  immunity 
after  one  attack  from  further  attacks  of  the  disorder. 

This  reaction,  viz.  the  production  of  '  antibodies '  in  the 
serum,  is  not  confined  to  the  effects  of  injecting  living 
corpuscles  or  bacteria.  Almost  any  protein,  injected  into  an 
animal's  veins,  evokes  the  production  in  the  animal  of  an 
antibody,  a  coaguUn,  which  has  the  property  of  inducing  a 
precipitate  in  solutions  of  the  protein  which  had  been  injected, 
but  in  no  other.  The  antibody  therefore  in  this  case  is 
specific.  In  the  same  way  the  injection  of  toxins,  the  poisonous 
products  of  bacteria,  e.g.  diphtheria  toxin,  or  tetanus  toxin, 
causes  the  development  in  the  blood  serum  of  a  corresponding 
antitoxin,  which  has  the  power  of  combining  with  and 
neutralising  the  toxin  in  question.  This  fact  is  made  use  of 
in  therapeutics.  Horses  are  injected  with  increasing  doses  of 
diphtheria  toxin.  After  some  time  they  are  bled,  and  the 
blood  serum  so  obtained,  when  injected  into  a  patient  the 
subject  of  diphtheria,  neutralises  the  poison  circulating  in  the 
body,  and  leads  in  this  way  to  arrest  of  the  disease. 

Thus  we  must  conclude  that  the  serum  of  any  individual, 
besides  the  definite  chemical  substances  already  described, 
contains  a  large  number  of  *  antibodies  '  of  complex  constitu- 
tion, which  have  been  produced  by  the  entry  into  the  blood  of 
bacterial  poisons  from  without,  or  of  ferments  from  the 
alimentary  canal,  or  of  proteins  from  the  disintegration  of 
tissues,  and  that  these  substances  play  an  important  part  in 
the  defence  of  the  individual  against  further  infection  or 
spread  of  any  process  of  destruction. 


87 


CHAPTER  IV 

THE   CONTRACTILE  TISSUES 

Section  1 
GENERAL   CHARACTERS   OP   MUSCLE 

The  means  b^^  which  the  organism  acts  on  its  environment  is 
furnished  by  the  contractile  tissues,  under  which  term  we 
inchide  all  the  varieties  of  muscle,  striated  and  unstriated. 

All  movements  that  require  to  be  sharply  and  forcibly 
carried  out  are  effected  by  means  of  striated  muscular  tissue, 
and  as  these  movements  are  in  nearly  all  cases  under  the  con- 
trol of  the  will,  the  muscles  are  often  spoken  of  as  voluntary. 

Unstriated  muscular  fibres  (often  termed  involuntary  ' ) 
form  sheets  or  closed  tubes  surrounding  the  hollow  viscera, 
and  by  their  slow  prolonged  contractions  serve  to  maintain 
and  regulate  the  flow  of  the  contents  of  these  organs. 

Intermediate  in  properties  between  these  two  classes  we  find 
heart  muscle ;  this,  though  striated,  presents  important  histo- 
logical differences  from  striated  voluntary  muscle.  We  shall 
study  this  form  more  fully  when  we  come  to  consider  the 
physiology  of  the  whole  vascular  s^'stem. 

The  properties  of  the  contractile  tissues  have  been  most 
fully  investigated  in  voluntary  muscles,  the  most  highly  dif- 
ferentiated members  of  the  group  ;  we  shall  therefore  consider 
this  part  of  the  subject  at  length,  merely  indicating  at  the  end 
in  what  points  the  unstriated  involuntary  muscles  differ  from 
the  striated. 

Voluntary  Muscle 

The  voluntary  or  striated  muscles  form  a  large  part  of  the 
body,  and  are  known  as  the  flesh  or  meat.     Each  muscle  is 

'  The  ciliary  muscle  furnishes  an  example  of  a  muscle  which,  though 
unstriated,  is  under  the  control  of  the  will. 


88  PHYSIOLOGY 

eml)edded  in  a  layer  of  connective  tissue,  and  is  made  up  of 
an  aggregation  of  muscular  fibres,  which  are  united  into 
bundles  ])y  means  of  areolar  connective  tissue.  The  individual 
fibres  vary  much  in  length,  and  may  be  as  long  as  4  or  5  cm. 
At  each  end  of  the  muscle  the  fibres  are  firmly  united  to  tough 
bundles  of  white  fibres,  which  form  the  tendon  of  the  muscle, 
and  are  attached  as  a  rule  to  bones.  Eunning  in  the  con- 
nective tissue  framework  of  the  muscle  we  find  a  number  of 
blood-vessels,  capillaries,  and  nerves. 

On  examination  of  a  living  muscle,  each  fibre  is  seen  to 
consist  of  a  series  of  alternate  light  and  dark  strife,  arranged 
at  right  angles  to  its  long  axis,  and  enclosed  in  a  structure- 
less sheath — the  sarcolemma.  Each  band  may  be  considered 
to  be  made  up  of  a  number  of  prisms  (sarcomeres)  side  by 
side,  with  interstitial  substance  between  them.     The  muscle 

Fig.  25. 


Muscular  fibre  of  a  mammal,  examined  fresh  in  serum,  highly 
magnified.     (Schiifer.) 

prisms  of  adjacent  discs  are  connected  to  form  long  columns 
(primitive  fibrillse,  or  sarcostyles).  Each  muscle  prism  is 
more  transparent  at  the  two  ends  than  in  the  middle,  thus 
giving  rise  to  the  appearance  of  light  and  dark  striae.  In  the 
middle  of  the  light  band  is  a  line  or  row  of  dots  (often  appear- 
ing double),  called  Krause's  membrane. 

The  development  of  this  regular  cross  and  longitudinal 
striation  is  closely  connected  with  the  evolution  and  specialisa- 
tion of  the  muscular  function,  i.e.  contraction.  Contractility 
is  among  others  a  function  of  all  undifferentiated  protoplasm. 
Cells  so  constituted  can  only  effect  slow  and  weak  contrac- 
tions. Directly  a  specialisation  of  function  is  necessary  and 
some  cell  or  part  of  a  cell  has  to  contract  rapidly  in  response 
to  some  stimulus  from  within  or  without,  we  find  a  differentia- 
tion both  of  form  and  of  internal  structure.     In  many  cases. 


THE    CONTRACTILE   TISSUES 


89 


as  in  the  devfiloping  muscle  of  the  embryo  or  the  adult 
muscles  of  manj^  mvertebrates,  this  difterentiation  affects  only 
part  of  the  cell,  so  that  while  one  part  presents  the  ordinary 


Fig.  27. 


Fig.  -2(5. 


Fig.  2G.— Muscle-fibre  of  an  ascaris.  a.  The  difi'erentiated  contractile 
portion  of  the  cell.     (After  Hertwig.) 

Fig.  27. — Muscle-fibres  from  the  small  intestine,  showing  the  fine 
longitudinal  striation.     (Schiifer.) 


granular  appearance,  the  other  half  is  finely  and  longitu- 
dinally striated,  the  striation  being  apparently  due  to  the 
development  of  special  contractile  fibrillte.  In  the  slowly 
contracting  unstriated  muscle  of  the  vertebrate  intestine,  the 


90  PHYSIOLOGY 

longitudinal  striation  is  with  difificulty  made  out  ;  but  as  the 
muscle  rises  in  the  scale  of  efficiency,  the  longitudinal 
striation  becomes  more  apparent,  and  in  the  striated  muscle  of 
vertebrates,  and  still  more  in  the  wonderful  wing-muscles  of 
insects,  which  can  perform  three  hundred  complete  contrac- 
tions in  a  second,  the  longitudinal  is  associated  with  and 
often  apparently  subordinated  to  a  transverse  striation,  due 
to  the  regular  segmentation  of  the  contractile  fibrillae  or 
sarcostylefi.  Every  muscular  fibre,  which  presents  any  trace 
of  histological  differentiation,  maj^  be  said  to  consist  of  con- 
tractile fibrill?e  (sarcostyles),  each  composed  of  a  series  of 
contractile  elements  {sarcoufi  elementfi  or  sarcomerefi),  and 
embedded  in  a  granular  material  known  as  sarcoplaam.  The 
enormous  variation  in  the  aspect  of  muscular  fibres  from 
different  parts  of  the  animal  kingdom  is  largely  conditioned 
by  the  varying  relations,  spatial  and  quantitative,  of  the 
sarcoplasm  to  the  sarcostyles.  Thus  in  the  higher  vertebrates, 
two  types  of  voluntary  muscular  fibre  are  distinguished, 
according  to  the  amount  of  sarcoplasm  they  contain  :  one 
rich  in  sarcoplasm,  more  granular  in  cross-section,  and 
generally  containing  htemoglobin  ;  and  the  other  poor  in 
sarcoplasm,  clear  in  cross-section,  and  containing  no  haemo- 
globin. From  the  fact  that  the  granular  fibres  are  found 
chiefly  in  those  muscles  which  have  to  carry  out  long- 
continued  and  powerful  contractions,  it  seems  reasonable  to 
regard  the  interstitial  sarcoplasm  as  the  local  food-supply  of 
the  active  sarcostyles,  although  some  authors  have  endowed 
the  sarcoplasm  with  a  contractile  power  of  its  own,  differing 
only  by  its  extremely  prolonged  character  from  the  quick 
twitch  of  the  sarcostyles.  The  connection  between  structure 
and  activity  of  the  muscle-fibres  is  well  shown  by  Fig.  28. 

In  some  animals,  such  as  the  rabbit,  we  find  muscles  consisting  almost 
entirely  of  one  or  other  of  these  varieties;  but  in  most  animals  (amongst  which 
we  may  reckon  frog  and  man)  the  two  varieties  occur  together  in  one  muscle, 
so  that  what  we  have  to  say  about  the  properties  of  voluntary  muscle,  which 
rests  nearly  entirely  on  experiments  with  frog's  muscle,  really  has  reference  to 
a  mixed  muscle,  i.e.  muscle  containing  both  red  and  white  fibres. 

Since  the  sarcous  element  represents  the  contractile  unit 
of  the  muscle,  a  knowledge  of  its  intimate  structure  should 
be  of  great  importance  for  the  theory  of  muscular  contraction. 
Unfortunatelv,    however,   we  are   here  at   the   limits   of  the 


THE    CONTEACTILE   TISSUES 


91 


demonstrably  visible.  Where  every  worker  has  liis  oavh  in- 
terpretation, it  is  only  possible  in  this  book  to  select  one, 
which  appears  to  be  the  most  suggestive  from  the  physio- 
logical standpoint.  Schafer,  working  on  the  highly  diffe- 
rentiated  wing-muscle   of    the    wasp,    concludes    that    each 

Fig.  28. 


Transverse  sections  of  the  pectoral  muscles  of  a,  the  falcon,  b,  the 
goose,  and  c,  the  domestic  fowl.  It  will  be  noticed  that  the 
relative  amount  of  granular  or  red  fibres  present  varies  directly 
as  the  bird's  power  of  sustained  flight.     (After  Knoll.) 

sarcostyle  is  divided  by  Krause's  membranes  (the  lines  in  the 
middle  of  each  light  stripe)  into  sarcomeres.  Each  sarcomere 
contains  a  darker  substance  near  tlie  centre  divided  into  two 
parts  by  Hensen's  disc.  At  each  end  of  the  sarcomere  the 
contents  are  clear  and  hyaline.     In  the  act  of  contraction,  the 

Fig,  29. 


S.E^ 


Diagram  of  a  sarcomere  in  a  moderately  extended  condition,  a, 
and  in  a  contracted  condition,  n ;  k,  k,  membranes  of  Krause ; 
H,  line  or  plane  of  Hensen ;  se,  poriferous  sarcous  element. 
(Schafer.) 


clear  material  flows,  according  to  Schafer,  into  tubular  pores 
in  the  central  dark  material. 

When  a  muscle-fibre,  killed  by  osmic  acid  or  alcohol,  is 
examined  under  the  microscope  by  polarised  light,  it  is  seen 
to  be  made  up  of  alternate  bands  of  singly  and  doubly 
refracting   material.     The   doubly   refracting    {anifiotropo^is) 


92 


PHYSIOLOGY 


substance  corresponds  to  tlie  dark  band,  and  the  singly 
refracting  (isotropoiis)  to  the  light  band.  If  however  the 
living  fibre  be  examined  in  the  same  way,  it  is  fomid  that 
nearly  the  whole  of  it  is  doubly  refracting,  the  singly  refract- 
ing substance  appearing  only  as  a  meshwork  with  long  parallel 
meshes  corresponding  to  the  muscle  prisms.  In  short,  in  a 
living  fibre  the  muscle  prisms  are  anisotropous,  the  sarcoplasm 
isotropous. 

When  a  muscle-fibre  contracts,  there  is  an  apparent 
reversal  of  the  situations  of  the  light  and  dark  stripes,  owing 
to   the  fact  that  the  interstitial  sarcoplasm  is  squeezed  out 


Fig,  30. 


sole. 

end  arborisation, 
muscle-fibre. 


sarcolemnia. 
—  muscle. 


Motor  end-plates  of  lizard. 

from  between  the  bulging  sarcomeres,  and  accumulates  on 
each  side  of  the  membranes  of  Krause.  The  accumulation 
of  sarcoplasm  in  this  situation  makes  the  previously  light 
strife  appear  dark,  and  the  dark  strife  by  contrast  lighter  than 
they  were  before.  That  there  is  no  true  reversal  of  the  striae 
is  shown  by  examining  the  muscle  by  polarised  light,  the 
two  substances,  isotropous  and  anisotropous,  retaining  their 
relative  positions. 

Every  skeletal  muscle  is  connected  with  the  central 
nervous  system  by  nerve-fibres,  some  conveying  impressions 
from  the  muscle  to  the  centre,  the  others  acting  as  the  path 
of  the  motor  impulses  from  the  centre  to  the  muscle.     These 


THE   CONTRACTILE   TISSUES 


93 


latter  -the  motor  nerves — end  in  the  muscular  fibre  itself, 
by  means  of  a  special  end-organ  -the  motor  end-plate.  The 
neurilemma  of  the  nerve-fibre  becomes  continuous  with  the 
sarcolemma,  the  medullary  sheath  ends  suddenly,  while  the 
axis  cylinder  ramifies  in  a  mass  of  undifferentiated  protoplasm, 
containing  nuclei,  and  lying  in  contact  with  the  contractile 
substance  of  the  muscle  immediately  under  the  sarcolemma. 

Fig.  31. 


Tib.  ant.  Ion 


Gastrocnemius. 
Tib.  post. 
Tibia. 


Sartorius. 

Add.  magn. 
Gracilis. 


Tendo  Acbillis. 


Muscles  of  hinder  extremity  of  frog  (after  Eckcr) 


So  far  as  we  can  tell  at  present,  the  ultimate  ramifications 
of  the  axis-cylinder  end  freely  and  do  not  enter  into  organic 
connection  with  the  contractile  substance  itself. 

Most  of  our  knowledge  on  the  subject  of  muscle  has  been 
derived  from  the  study  of  the  gastrocnemius  and  sartorius 
muscles  of  the  frog.  The  position  of  these  muscles  is  shown 
in  the  accompanying  diagram  (Fig.  31).     The  gastrocnemius 


94  PHYSIOLOGY 

which,  with  the  attached  sciatic  nerve,  is  most  frequently 
employed  as  a  nerve-muscle  preparation,  forms  a  thick  belly 
immediately  under  the  skin  at  the  back  of  the  leg,  and  arises 
by  two  tendons  from  the  lower  end  of  the  femur  and  the 
outer  side  of  the  knee-joint.  The  two  tendons  converge 
towards  the  centre  of  the  muscle,  uniting  about  its  middle, 
and  from  them  a  number  of  short  muscular  fibres  arise, 
passing  backwards  and  dorsally  to  be  inserted  into  a  flat 
aponeurosis  covering  the  lower  half  of  the  muscle,  which  ends 
in  the  tendo  Achillis.  On  account  of  this  irregular  arrange- 
ment of  the  muscular  fibres,  the  gastrocnemius  can  only  be 
employed  when  the  contraction  of  the  muscle  as  a  whole  is 
the  object  of  investigation.  The  effective  cross-area  of  the 
fibres  is  much  greater  than  the  actual  cross-section  of  the 
muscle,  so  that,  while  the  actual  shortening  of  the  gastro- 
cnemius is  but  small,  its  strength  of  contraction  is  con- 
siderable. 

The  sartorius  muscle  consists  of  a  thin  band  of  muscle- 
fibres  running  parallel  from  one  end  of  the  muscle  to  the 
other.  It  lies  on  the  ventral  surface  of  the  thigh,  arising 
from  the  symphysis  pubis  by  a  thin  flat  tendon,  and  is 
inserted  by  a  narrow  tendon  into  the  inner  side  of  the  head 
of  the  tibia.  On  account  of  the  regularity  with  which  its 
fibres  are  disposed,  this  muscle  is  of  especial  value  in 
experiments  on  the  local  conditions  of  a  muscle-fibre  accom- 
panying its  activity.  When  a  greater  mass  of  approximately 
parallel  fibres  is  necessary,  recourse  may  be  had  to  a  pre- 
paration consisting  of  the  gracilis  and  semi-membranosus 
muscles  together.  This  latter  muscle  lies  dorsally  to  the 
gracilis  muscle  which  is  shown  in  the  illustration. 


THE  CONTEACTILE  TISSUES  95 


Section  2 
EXCITATION  OF  MUSCLE 


A  muscle  may  be  caused  to  contract  in  various  ways. 
Normally  it  contracts  only  in  response  to  impulses  starting 
in  the  central  nervous  system  and  transmitted  down  the 
nerves.  But  contraction  may  be  artificially  excited  in 
various  ways  m  a  muscle  removed  from  the  body.  If  we 
make  a  muscle-nerve  preparation  {i.e.  a  muscle  with  as  long 
a  piece  of  its  nerve  as  possible  attached  to  it),  such  as  the 
gastrocnemius  of  the  frog  with  the  sciatic  nerve,  we  find  we 
can  cause  contraction  by  various  forms  of  stimuli — mechanical, 
thermal,  or  electrical  -applied  to  the  muscle  or  the  nerve 
(direct  and  indirect  stimulation).  Thus  the  muscle  responds 
with  a  twitch  if  we  pass  an  induction  shock  through  it  or  its 
nerve,  or  pinch  either  with  a  pair  of  forceps.  Or  we  may 
use  chemical  stimuli,  and  cause  contraction  by  the  application 
of  strong  glycerin  or  salt  solution  to  the  nerve. 

These  experiments  do  not  prove  conclusively  that  muscle 
itself  is  irritable.  It  might  be  urged  that,  when  we  pinched 
or  burnt  the  muscle,  we  stimulated,  not  the  muscle  substance 
itself,  but  the  terminal  ramifications  of  the  nerve  in  the 
muscle,  and  that  these  in  their  turn  incited  the  muscle  to  con- 
tract. But  the  independent  excitability  of  muscle  is  shown 
clearly  by  the  following  experiment. 

A  frog,  whose  brain  has  been  previously  destroyed,  is 
pinned  on  a  board,  and  the  sciatic  nerves  on  each  side 
exposed.  A  ligature  is  then  passed  round  the  right  thigh 
underneath  the  nerve,  and  tied  tightly  so  as  to  close  eflectu- 
ally  all  the  blood-vessels  supplying  the  limbs,  without  inter- 
fering with  the  blood-supply  to  the  nerve.  Two  drops  of 
a  1  per  cent,  solution  of  curare  are  then  injected  into  the 
dorsal  lymph-sac.  After  the  lapse  of  a  quarter  of  an  hour 
it  is  fomid  that  the  strongest  stimuli  may  be  applied  to  the 
left  sciatic  nerve  without  causing  any  contraction  of  the 
muscles  it  supplies.  On  the  right  side  however,  stimulation 
of  the  nerve  is  as  efficacious  as  before.  Both  gastrocnemii 
respond    readily    to   direct    stimulation,    showing    that    the 


96 


PHYSIOLOGY 


muscles  are  not  affected  by  the  drug.  Since  both  sciatic 
nerves  have  been  exposed  to  the  influence  of  the  curare,  it 
is  evident  that  the  difference  on  the  two  sides  cannot  be  due 
to  any  deleterious  effect  on  them  by  the  curare.  We  have 
also  excluded  the  muscles  themselves  ;  so  we  must  conclude 
that  the  curare  paralyses  the  muscles  by  affecting  the  termi- 
nations of  the  nerve  within  the  muscle,  and  probably  the 
end-plates  themselves. 

This  experiment  therefore  teaches  us  that  muscle  can  be 
excited  to  contract  by  direct  stimulation,  even  when  the 
terminal  ramifications  of  the  nerve  within  it  are  paralysed, 
so  that  stimulation  of  them  would  be  without  effect. 

The  same  fact  may  be  demonstrated  in  a  different  way  by 
means  of  chemical  stimuli.     It  is  found  that  whereas  strong 

Fig.  32. 


The  ramification  of  the  nerve-fibres  within  the  sartorius  muscle  of 
the  frog,  showing  the  freedom  of  the  lower  portion  of  the  muscle 
from  nerve-fibres.     (Kiihne.) 


glycerin  excites  nerve-fibres,  it  is  without  effect  on  muscle- 
fibres  ;  while  on  the  other  hand  weak  ammonia  is  a  strong 
excitant  for  muscle,  but  is  without  effect  on  nerve.  Thus  if 
the  frog's  sartorius  be  dissected  out  and  the  lower  end  dipped 
in  glycerin,  no  effect  is  produced.  On  snipping  off  the  lower 
third  of  the  muscle  and  then  immersing  the  cut  end  in  glycerin, 
a  twitch  at  once  occurs.  The  lower  end  contains  no  nerve- 
fibres  (Fig.  32),  and  it  is  only  when  a  section  contaming 
nerve-fibres   is   exposed  to  the  action  of   glycerin  that   con- 


THE    CONTRACTILE   TISSUES  97 

traction  takes  place.  On  the  other  hand,  mere  exposure  of 
muscle  to  the  vapour  of  dilute  ammonia  causes  contraction 
(and  subsequent  death),  although  the  nerve  to  the  muscle 
can  be  immersed  in  the  solution  without  any  excitation  being 
produced. 

Of  all  the  different  stimuli  that  we  have  mentioned  as 
capable  of  exciting  muscular  contraction,  the  electrical  is 
that  most  frequently  employed.  It  is  easy,  using  this  form, 
to  graduate  accurately  the  intensity  and  duration  of  the 
stimulus.  At  the  same  time  the  stimulus  may  be  applied 
many  times  to  any  point  on  the  muscle  or  nerve  without 
killing  the  part  stimulated,  whereas  with  other  forms  of 
stimulus  it  is  difficult  to  obtain  excitatory  effects  without 
injuring  to  a  greater  or  less  extent  the  part  stimulated. 

Two  forms  of  electrical  stimuli  are  employed, — the  make 
and  break  of  a  constant  current,  and  the  induction  currents 
of  high  intensity  and  short  duration  obtained  from  an  induc- 
tion coil. 

Constant  current. — As  a  source  of  constant  current  a 
Daniell's   cell   is   generally   employed    {vide   Appendix).     In 

Fig.  33. 


Kaihodej  Anode. 


this  cell  the  copper  is  the  negative  and  the  zinc  the  positive 
element.  The  current  therefore  passes  in  the  cell  from  zinc 
to  copper,  and  outside  the  cell  from  copper  to  zinc.  If 
therefore  wires  be  attached  to  the  zinc  and  copper,  the  wire 
attached  to  the  former  will  be  the  negative,  and  that  to  the 
latter  the  positive  pole.  If  we  connect  such  wires  with  the 
nerve  or  muscle  of  a  nerve-muscle  preparation  (as  in  Fig.  33), 
the  current  will  flow  from  copper  to  the  nerve  at  A,  and  along 
the  nerve  from  A  to  K.  At  K  the  current  will  leave  the 
nerve  to  flow  to  the  zinc  of  the  battery,  so  completing  the 
circuit.  The  point  at  which  the  current  enters  the  nerve 
{i.e.  the  point  of  the  nerve  connected  with  the  positive  pole 

7 


98  -  PHYSIOLOGY 

of  tlie  battery)  is  called  the  anode,  and  the  point  at  which 
the  current  leaves  the  nerve  is  called  the  kathode.  The  wires 
by  which  the  current  is  conducted  to  and  from  the  nerve  are 
called  the  electrodes. 

If  a  weak  current  from  a  Daniell's  cell  (or  any  other  form 
of  battery)  be  passed  through  a  muscle  or  any  part  of  its 
nerve,  we  find  that  at  the  make  of  the  current  the  muscle 
gives  a  single  sharp  contraction — a  muscle-twitcli.  No  effect 
is  produced  during  the  passage  of  the  current  or  when  it  is 
broken,  the  muscle  remaining  perfectly  quiescent.'  If  the 
current  is  now  increased  we  find  that  the  muscle  responds  to 
both  make  and  break,  remaining  however  quiescent  during 
the  passage  of  the  current.  Using  a  current  of  moderate 
strength,  we  find  the  contraction  due  to  make  is  more  energetic 
than  that  due  to  break. 

Thus  stimulation  is  caused  l)y  the  make  and  break  of  a 
constant  current,  the  make  stimulus  being  more  effective  than 

Fig.  34. 


Saitorius  clamped  in  middle  and  attached  to  levers  at  either  end. 

the  break.  Besides  this  difference  in  intensity,  there  is  a 
difference  in  the  point  from  which  excitation  starts.  A  make 
contraction  starts  from  the  hatliode,  a  break  contraction 
from  the  anode.  This  is  well  shown  by  the  two  following 
experiments. 

a.  A  curarised  sartorius  muscle  of  the  frog  (Fig.  34),  with 
its  bony  insertions  still  attached,  is  fastened  at  the  two  ends 
to  two  electrodes,  which  are  able  to  swing  when  the  muscle 
contracts,  and  are  attached  by  threads  to  levers  which  serve 
to  record  the  contraction.  The  middle  of  the  muscle  is  then 
fixed  by  clamping  it  lightly.     A  circuit  is  arranged  so  that 

'  This  statement  is  not  absolutely  correct.  No  propagated  contraction,  as 
a  rule,  is  produced  during  the  passage  of  a  constant  current.  Careful  observation 
will  show  however  that  there  is  a  state  of  continued  contraction  limited  to  the 
immediate  neighbourhood  of  the  kathode,  which  lasts  as  long  as  the  current  is 
passed  through  the  muscle. 


THE   CONTRACTILE   TISSUES  99 

a  constant  current  can  be  sent  through  the  electrodes  and 
the  whole  length  of  the  muscle.  It  is  found,  on  making  the 
current,  that  the  lever  attached  to  the  kathode — that  is,  to  the 
electrode  by  which  the  current  leaves  the  muscle — rises  before 
the  other  lever.  On  the  other  hand,  on  breaking  the  current, 
the  lever  at  the  anode  rises  iirst,  showing  that  the  anodic  half 
of  the  muscle  contracts  before  the  kathodic  half. 

h.  The  irritability  of  a  muscle,  i.e.  its  power  of  responding 
to  a  stimulus  by  contracting,  is  intimately  dependent  on  the 
life  of  the  muscle.  If  the  muscle  be  injured  or  killed  at  any 
spot,  its  irritability  at  this  spot  will  be  therefore  diminished 
S)r  destroyed.  Hence,  if  we  stimulate  a  muscle  at  the  injured 
spot,  no  contraction  will  ensue.     This  fact  may  be  used  to 

Fig.  35. 

kathode    j^  anode(injured) 

contraction  at  make 


anode        rn  kathode  (injured) 

no  contraction  at  make. 


b 
Diagram  to  show  the  effect  of  local  injury  on  the  excitability  of  a 
muscle,     b,  battery  ;  wj,  muscle.     The  arrows  indicate  the  direction 
of  the  current. 


demonstrate  the  production  of  excitation  at  kathode  on  make, 
and  at  anode  on  break  of  a  constant  current. 

A  muscle  with  parallel  fibres,  such  as  the  sartorius,  is 
injured  at  one  end,  and  a  constant  current  passed,  first  from 
the  injured  to  the  uninjured  end,  and  then  in  the  reverse 
direction.  It  is  found  in  the  former  case,  when  the  anode 
is  on  the  injured  part  (which  is  therefore  less  excitable),  that 
break  of  the  current  is  ineffective,  and  in  the  latter,  when  the 
kathode  is  on  the  injured  surface,  that  the  make  stimulus  is 
ineffective,  showing  that  the  part  excited  corresponds  to  the 
kathode  at  make  and  to  the  anode  at  break. 

Induced  currents. — In  using  these  the  muscle  or  nerve  is 
stimulated  by  the  current  of  momentary  duration  produced 
in  the  secondary  circuit  of  an  induction  coil  by  the  make  or 


100  PHYSIOLOGY 

break  of  a  constant  current  in  the  primary.  The  strength  of 
the  shock  is  graduated  by  moving  the  secondary  nearer  to  or 
farther  away  from  the  primary  coil. 

Using  this  mode  of  stimulus,  it  is  found  that  the  con- 
traction on  break  of  the  constant  current  is  much  stronger 
than  that  on  make.  It  must  not  be  imagined  however  that 
there  is  any  contradiction  between  this  and  the  fact  that  the 
make  of  a  constant  current  is  a  stronger  stimulus  than  the 
break. 

When  we  put  a  muscle  in  the  secondary  circuit  and  make 
a  current  in  the  primary,  there  is  a  current  of  momentary 
duration  induced  in  the  secondary  ;  so  that  there  is  a  current 
made  and  hrohen  through  the  muscle,  and  the  same  thing 
takes  place  again  when  the  primary  circuit  is  broken.     It  has 


been  shown  that,  when  we  use  currents  of  such  short  dura- 
tion, the  break  stimulus  is  ineffective ;  so  in  both  cases, 
whether  we  make  or  break  the  current  in  the  primary  circuit, 
we  are  dealing  with  a  make  stimulus  in  the  muscle.  The 
difference  in  the  efficacy  of  make  and  break  induction  shocks 
is  purely  physical,  and  depends  on  the  fact  that  the  current 
induced  in  the  secondary  coil  on  make  is  of  slower  rise  and 
smaller  potential  than  that  produced  at  break.  (See 
Appendix.) 

We  see  therefore  that  the  efficiency  of  an  electrical  stimu- 
lus depends  on  the  rate  of  variation  of  the  current  employed, 
and  within  wide  limits  is  proportional  to  this  rate.  Tf,  instead 
of  suddenly  diminishing  and  increasing  the  current  passing 
through  an  irritable  structure,  we  carry  out  the  change 
gradually,  no  excitatory  efi'ect  is  produced,  even  although  the 
current  may  finally  attain  a   considerable  strength.      This 


THE   CONTEACTILE   TISSUES  101 

fact  may  be  demonstrated  with  the  help  of  the  rheocord 
(Fig.  36),  which  consists  of  a  simple  wire  ab  through  which 
a  current  is  led.  Two  wires  pass  from  a  and  a  movable  rider  c 
to  the  nerve  or  muscle.  It  is  evident  that  if  c  be  in  close 
proximity  to  a,  no  current  will  flow  through  the  muscle. 
But  as  c  is  pushed  towards  b,  the  current  through  the  nerve 
gradually  increases.  It  will  be  found  that  if  the  rider  be 
moved  slowly  from  a  to  b,  or  in  the  reverse  direction,  no 
effect  is  produced,  whereas  a  quick  movement  in  either 
direction  causes  excitation.  We  may  say  therefore  that  the 
excitatory  effect  of  a  current  increases  with — 

1.  The  intensity  of  the  current. 

2.  The  rate  of  change  of  the  current. 

3.  (In  the  case  of  nerve)  The  length  of  nerve  through 
which  the  current  passes. 

The  second  of  these  conditions  needs  however  some 
correction.  As  we  increase  the  rate  of  change  of  current,  by 
employing  in  the  case  of  induced  currents  more  and  more 
rapid  alternations,  we  find  that  the  excitatory  eftect,  instead 
of  increasing,  begins  to  diminish  and  finally  disappears,  so 
that  high  frequency  currents  of  enormous  tension  can,  as  in 
Tesla's  experiments,  be  led  through  the  body  without  any 
apparent  physiological  effect.  On  the  other  hand,  by  using 
more  sluggish  forms  of  irritable  tissue,  we  may  find  that 
even  our  induction  shocks  are  too  rapid  for  effective  excita- 
tion. Thus  the  red  muscles  of  the  slow-moving  tortoise  react 
better  to  the  slow  make  than  to  the  sudden  break  induction 
shock,  and  many  forms  of  unstriated  muscle  are  unaffected 
by  either  make  or  break  shock.  We  must  conclude  there- 
fore that  for  each  tissue  there  is  an  optimum  rate  of  change 
varying  with  the  character  of  the  tissue,  at  which  the  energy 
necessary  to  produce  a  response  is  at  a  minimum.  This 
optimum  rate  of  change  is  spoken  of  by  Waller  as  the 
*  characteristic '  of  an  irritable  tissue,  and  has  been  deter- 
mined by  him  for  nerve. 

A  minimal  stimulus  is  the  weakest  stimulus  that  will  produce  a  contraction. 
A  maximal  stimulus  is  one  that  produces  the  strongest  contraction  a  muscle  is 
capable  of  under  the  effects  of  a  single  stimulus.  A  suhmaximal  stimulus  is  any 
strength  of  stimulus  between  these  two  extremes. 


102 


PHYSIOLOGY 


Section  3 

THE   MECHANICAL   CHANGES   THAT   A   MUSCLE 
UNDEEGOES   WHEN   IT   CONTEACTS 

Change  of  Form 

The  most  evident  change  about  a  muscle  when  it  con- 
tracts is  of  form.  It  becomes  shorter  and  thicker,  its  bulk 
remaining  unaltered. 

To  study  this  change  more  closely,  it  is  necessary  to 
obtain  a  graphic  record  of  the  contraction.  For  this  purpose 
the  femur,  to  which  the  gastrocnemius  of  the  muscle-nerve 
preparation  is  attached,  is  clamped  firmly,  and  the  tendo 
Achillis  attached  by  a  thread  to  a  light  lever,  free  to  move 

Fig.  37. 


Arrangement  of  apparatus  for  recording  simple  muscle- twitch. 


round  an  axis  at  one  end.  The  point  of  this  lever  is  armed 
with  a  bristle  (anything  that  is  stifi'  and  pointed  will  do), 
which  just  touches  the  blackened  surface  of  a  piece  of  glazed 
paper.  This  paper  is  stretched  round  a  cylinder  (drum)  which 
can  be  made  to  rotate  at  any  constant  speed  required.  If  the 
drum  is  moving,  the  point  of  the  bristle  draws  a  horizontal 
white  line  on  the  smoked  paper. 

If  however  a  single  induction  shock  be  sent  through  the 
nerve  of  the  preparation,  the  lever  is  jerked  up,  falling  again 
almost  directly,  and  a  curve  is  drawn  like  that  shown  in 
Fig.  38. 


THE   CONTEACTILE   TISSUES 


103 


A  similar  curve  is  obtained  if  the  muscle  be  stimulated 
directly. 

In  all  such  graphic  records  we  should  have  also — 

1.  A  1 17)16  record. — This  is  furnished  by  means  of  a  small 
electro-magnet,  armed  with  a  pointed  lever  writing  on  the 
smoked  surface.  This  electro-magnet  (time  marker  or  signal) 
is  made  to  vibrate  100  times  a  second  (more  or  less  as 
may  be  required)  by  putting  it  in  a  circuit  which  is  made 
and  broken  100  times  a  second  by  means  of  a  tuning-fork 
vibrating  at  that  rate.  The  tmiing-fork  is  maintained  in 
vibration  in  the  same  way  as  the  Wagner's  hammer  of  an 
induction  coil. 

2.  A  record  of  the  exact  point  at  which  the  nerve  or  muscle 
is  stimulated. — This  may  be  obtained  in  two  ways : 

a.  When  using   the   pendulum  or  trigger  myograph,  in 

Fig.  38. 


Curve  of  single  muscle-twitch  taken  on  a  rapidly  moving 
surface  (pendulum  myograph)  (from  Yeo). 


both  of  I  which  the  recording  surface  is  a  smoked  Hat  surface 
on  a  glass  plate,  this  latter  is  so  arranged  that  it  knocks  over 
a  key  as  it  shoots  across,  and  so  breaks  the  primary  circuit 
and  excites  the  nerve  or  muscle  of  the  preparation.  As  we 
know  the  exact  pomt  that  the  plate  reaches  when  it  knocks 
over  the  key,  we  can  mark  on  the  contraction  curve  the  exact 
moment  at  which  stimulation  took  place. 

h.  If  we  wish  to  make  and  break  the  primary  circuit  at 
will  by  means  of  a  key,  a  small  electro-magnetic  signal,  inter- 
posed in  the  circuit,  is  arranged  to  write  on  the  revolving 
drum,  and  so  mark  the  point  of  stimulation. 

In  the  figure  (Fig.  38)  the  upper  line  is  the  curve  drawn 
by  the  lever  of  the  muscle  as  it  contracts  ;  the  small  upright 
line  shows  the  point  at  which  the  muscle  was  stimulated  ; 


104 


PHYSIOLOGY 


and  the  second  line  is  the  tracing  of  the  chronograph,  every 
vibration  representing  -5^  of  a  second. 

In  the  pendulum  myograph  (Fig.  39)  a  smoked  glass  plate  is  carried  on  a 
heavy  iron  pendulum.  At  each  side  the  pendulum  is  armed  with  a  catch, 
which  fits  on  to  other  catches  at  the  side  of  the  triangular  box,  from  the  apex 
on  which  the  pendulum  is  suspended.  At  its  lower  part  the  pendulum  carries 
a  projecting  piece  which  can  knock  over  the  '  kick-over '  key  (k),  thus  breaking 
a  circuit  in  which  is  included  the  primary  coil  of  an  induction  coil.  The  lever 
attached  to  the  muscle  is  arranged  so  as  to  write  lightly  on  the  glass  plate, 
Everything  being  ready,  and  the  key  (k)  closed,  the  pendulum  is  raised  to  a,  the 

Fig.  39. 


Simple  form  of  pendulum  myograph. 


catch  (a)  is  then  released,  and  the  pendulum  falls  at  an  ever-accelerating  rate 
and  then  rises  again,  gradually  slowing  off  until  it  is  caught  again  at  b. 
As  it  passes  by  the  key  it  breaks  the  circuit.  A  break  induction  shock  is 
sent  into  the  muscle  or  nerve,  which  contracts,  and  a  curve  is  obtained 
similar  to  that  shown  in  Fig.  38.  Since  the  rate  of  the  pendulum  is  constantly 
varying  throughout  its  course,  it  is  necessary  to  have  a  tuning-fork,  or  time- 
marker  actuated  electrically  by  a  tuning-fork,  writing  just  below  the  muscle- 
lever. 

In  the  spring  myograph,  otherwise  known  as  the  trigger  or  shooter  myograph 
(Fig.  40),  a  smoked  glass  plate  is  also  used.  '  The  frame  supporting  the  glass 
plate  slides  on  two  horizontal  steel  wires.  To  make  the  instrument  ready  for 
use,  the  frame  is  moved  to  one  side,  which  compresses  a  short  spring.  When 
the   catch   holding   it.  in  this  position  is  released  by  the  trigger,  the  spring 


The  contractile  tissues 


105 


which  only  acts  for  a  short  space,  gives  the  frame  and  the  glass  plate  a  rapid 
horizontal  motion ;  and  the  momentum  carries  the  glass  plate  through  the 
rest  of  the  distance,  till  stopped  by  the  buffers.  The  velocity  during  this 
time  is  nearly  constant,  as  the  friction  of  the  guides  is  small.  Two  keys 
are  knocked  over  by  pins  on  the  frame  and  break  electric  circuits.  The 
relative  positions  at  which  the  circuits  are  broken  can  be  altered  by  a  con- 
venient adjustment.  A  tuning-fork  vibrating  about  100  per  second  fixed  to  the 
base  of  the  instrument  marks  the  time ;  its  prongs  are  sprung  apart  by 
a  block  between  their  ends,  and  the  same  action  which  releases  the  glass 
plate  also  frees  the  fork  by  removing  the  block  and  allows  it  to  vibrate ;  a 
writing  style  then  draws  a  sinuous  line  on  the  smoked  surface  of  the  moving 
glass  plate.  A  muscle  lever  with  a  scale-pan  attached  also  forms  part  of  the 
nstrument.' ' 


Fig.  40. 


(T^ 


Diagram  of  spring  myograph,  or  '  shooter.' 


It  will  be  seen  that  a  simple  muscular  contraction  or 
twitch,  such  as  we  have  in  Fig.  38,  produced  by  a  momentary 
stimulus,  consists  of  three  main  phases  : 

1.  A  phase  during  which  no  apparent  change  takes  place 
in  the  muscle,  or  at  any  rate  none  which  gives  rise  to  any 
movement  of  the  lever.     This  is  called  the  latent  period. 

2.  A  phase  of  shortening,  or  contraction. 

3.  A  phase  of  relaxation,  or  return  to  the  original  length. 
The  small  curves  seen   after  the  main  curve  are  due  to 

elastic  vibrations  of  the  lever,  and  do  not  indicate  any  changes 
occurring  in  the  muscle  itself.  From  the  time-marking 
below  the  tracing,  we  see  that  the  latent  period  occupies  about 

'  From  Catalogue  of  Camb.  Sci.  Inst.  Co. 


106  PHYSIOLOaY 

j-g-o  of  a  second,  the  phase  of  shortening  y^^,  and  the  relaxa- 
tion j^ff  second. 

Thus  a  single  muscle-twitch  is  completed  in  about  yV  of 
a  second.  It  must  be  remembered  however  that  this  number 
is  only  approximate,  and  varies  with  the  temperature  of  the 
muscle  and  its  condition,  being  much  longer  in  a  fatigued 
muscle. 

It  is  generally  said  that,  during  the  latent  period,  invisible 
preparatory  changes  are  taking  place  in  the  muscle,  and 
these  changes  are  supposed  to  be  indicated  by  the  electrical 
change  accompanying  a  muscular  contraction,  which  is 
generally  described  as  taking  place  durmg  the  latent  period. 
But  recent  experiments  have  tended  more  and  more  to 
shorten  the  latent  period,  and  it  now  seems  probable  that 
nearly  the  whole  of  the  latent  period  is  due  partly  to  instru- 
mental inertia,  and  partly  to  the  mechanical  inertia  of  the 
muscle  itself.  For  living  muscle  is  elastic  and  very  exten- 
sible, and  the  effect  of  contraction  of  any  given  part  of  it  will 
be  first  to  stretch  the  adjacent  part,  and  afterwards  to  move 
the  part  of  the  muscle  to  which  the  lever  is  attached.' 

Sanderson  and  Burch  have  measured  the  mechanical 
latency  of  muscle  by  a  photographic  method.  The  thicken- 
ing of  the  muscle  at  the  point  stimulated  was  recorded 
graphically  by  photographing  the  outline  of  the  muscle  on  a 
slit,  behind  which  was  a  moving  sensitive  plate.  Thus  avoid- 
ing all  instrumental  inertia,  and  diminishing  the  inertia  of 
the  muscle  to  a  minimum,  the  mechanical  latent  period  was 
found  to  be  only  0*0025  second.  The  electrical  change,  how- 
ever, began  at  the  moment  of  stimulation,  and  had  reached 
its  culminating  point  when  the  mechanical  change  was 
commencing. 

As  we  should  expect  from  the  precedmg  paragraph,  the 
latent  period  is  much  increased  by  increasing  the  load.     It 

'  The  effect  of  the  extensibility  of  muscle  in  lengthening  the  latent 
period  will  perhaps  be  more  intelligible  if  illustrated  by  an  example.  If  we 
have  a  weight  supported  by  a  rigid  wire,  and  suddenly  pull  the  upper  end  of 
the  wire  so  as  to  raise  the  weight,  the  latter  will  rise  instantaneously.  If 
however  the  weight  be  suspended  by  a  piece  of  elastic,  it  will  not  follow  the 
pull  exactly,  but  will  lag  behind,  the  first  part  of  the  pull  being  occupied 
with  stretching  the  indiarubber,  and  only  when  this  is  stretched  to  a  certain 
degree  will  the  weight  begin  to  rise.  The  same  retardation  of  the  pull 
would  be  observed  if,  instead  of  indiarubber,  we  used  a  piece  of  living  muscle. 


THE   CONTRACTILE  TISSUES  107 

is  also  lengthened  by  cold  and  fatigue.  It  is  longer  in  the 
red  than  in  the  white  muscles. 

The  height  of  the  contraction  depends — 

1.  On  the  strength  of  the  stimulus. 

2.  On  the  load  ;  the  height  being  smaller,  the  greater  the 
weight  the  muscle  has  to  raise.  If  the  load  is  graduall}' 
increased,  a  point  is  reached  when  the  muscle  can  no  longer 
raise  it.  This  weight  represents  the  '  absolute  force  '  of  the 
muscle.  In  determining  it  the  muscle  is  '  after-loaded,'  that 
is  to  say,  the  lever  is  supported  by  a  screw  in  its  unweighted 
position,  so  that  the  weight  cannot  act  on  the  muscle  till 
the  latter  begins  to  contract.  In  human  muscle  the  absolute 
force  has  been  found  to  amount  to  6,000  to  8,000  grams  per 
square  centimetre  of  cross  area  of  the  muscle. 

The  relaxation  of  muscle  is  helped  by  a  moderate  load, 
and  in  a  normal  condition  is  complete.  It  is  not  active — 
that  is  to  say,  is  not  due  to  a  contraction  in  the  transverse 
direction  —  but  is  a  passive  efiect  of  extension  and  elastic 
rebound.  This  may  be  shown  by  allowing  a  muscle  to  con- 
tract while  floating  on  mercury.  The  subsequent  lengthening 
on  relaxation  is  very  incomplete. 

Isotonic  and  Isometric  Contractions 

In  order  to  obtain  a  true  curve  of  a  muscle-twitch  special  precautions  are 
necessary  to  avoid  the  effects  of  instrumental  inertia.  When  the  muscle 
begins  to  contract  it  imparts  a  very  rapid  movement  to  the  lever,  which 
therefore  tends  to  overshoot  the  mark  and  deform  the  curve.  This 
source  of  error  may  be  almost  avoided  by  making  the  lever  as  light  as 
possible,  and  hanging  the  extending  weight  in  close  proximity  to  the  axle 
of  the   lever,  as  shown  in  Fig.  41.     Since   the  energy  of  a  moving  mass  is 

\ ,   and  the  tension   due   to 

the  weight  as  well  as  the  velocity  on  contraction  is  directly  proportional  to  the 
distance  of  the  weight  from  the  axis,  it  follows  that  it  is  better  to  load  the 
muscle  with  40  grams  1  millimetre  from  the  axis  than  with  1  gram  40  milli- 
metres from  the  axis,  though  the  tension  put  on  the  muscle  will  be  the  same  in 
both  cases. 

In  the  first  case  the  energy  of  the  moving  mass  will  be  proportional  to 

J^  ^  '  =  20,  and  in  the  second  to  -^^ — '-  =  800,  and  it  is  this  energy  which 
2  2 

determines  the  overshooting  of  the  lever  and  the  deformation  of  the  curve. 

Since  throughout  the  contraction  the  lever  follows  the  muscle  in  its  movement, 

the  tension  on  the  muscle  remains  the  same  throughout,  and  the  method  is 

therefore  known  as  the  isotonic  method. 

In  many  cases  it  is  of  importance  to  be  able  to  record  the  development  of 

the  energy  (i.e.  the  tension)  of  the  active  muscle  apart  from   any  changes  in 


108 


PHYSIOLOGY 


its  length.  For  this  purpose  the  muscle  is  allowed  to  contract  against  a  strong 
spring,  the  movements  of  which  are  magnified  by  means  of  a  very  long  lever. 
Thus  the  shortening  of  the  muscle  is  almost  entirely  prevented,  but  the  in- 
crease in  its  tension  causes  a  minute  but  proportionate  movement  of  the  spring, 
which  is  recorded  by  means  of  the  lever.  Since  in  this  case  the  length  or 
measurement  of  the  muscle  remains  approximately  constant,  while  the  tension 
is  continually  varying  throughout  the  contraction,  it  is  known  as  the  isometric 
metJiod.  The  great  magnification  necessary  in  this  method  introduces  serious 
sources  of  error ;  but  it  seems  that,  if  all  due  precautions  be  taken  to  avoid 
these  errors,  the  isometric  curve  differs  very  little  in  form  from  the  isotonic, 
displaying  only  a  somewhat  quicker  development  of  energy  at  the  beginning  of 
contraction. 


Blix  apparatus  for  recording  isometric  and  isotonic  curves  syn- 
chronically  (Miss  Buchanan),  p,  the  steel  cylindrical  support 
with  jointed  steel  arm  to  bear  the  isotonic  lever  Z,  which  consists 
of  a  strip  of  bamboo  with  an  aluminium  tip.  t,  the  isometric 
lever,  also  of  bamboo,  except  for  a  short  metal  part  V  in  which 
are  holes  for  fixing  the  muscle.  The  two  wires  from  an  induction 
coil  are  brought,  one  to  x  ,  which  is  in  connection  with  the  sup- 
port and  hence  with  the  metal  bar  V ,  the  other  to  y,  which  is 
insulated  from  the  support  but  connected  by  a  copper  wire  with 
a  thin  piece  of  copper  surrounding  the  isotonic  lever  at  the  point 
where  the  muscle  is  attached  to  it.  CI.,  clamp  for  fixing  the 
lower  end  of  the  nmsele  when  an  isometric  curve  is  to  be  taken. 
The  axis  of  the  isotonic  lever  is  at  x,  close  to  which  is  hung  the 
weight  of  50  grms. 

Propagation  of  Contraction.     The  Contraction  Wave 

The  whole  muscle  does  not  as  a  rule  contract  simul- 
taneously. When  excited  from  its  nerve  the  contraction 
begins  at  the  end -plates  and  spreads  in  both  directions 
through   the   muscle.     The  rate  of  propagation   of  the  con- 


THE   CONTRACTILE   TISSUES 


109 


traction  wave  can  only  be  measured  by  employing  a  cm-arised 
muscle,  so  as  to  avoid  the  wide  spreading  of  the  excitatory 
change  by  means  of  the  intra-muscular  nerve-endings.  For 
this  purpose  a  curarised  sartorius  muscle  is  taken,  stimulated 
at  one  end,  and  the  thickening  of  the  muscle  recorded  by 
means  of  two  levers  placed,  one  near  the  exciting  electrodes 
and  the  second  at  the  other  end  of  the  muscle,  as  shown 
in  the  diagram  (Fig.  42).  The  difference  between  the  latent 
periods  of  the  two  curves  represents  the  time  taken  by  the 
contraction  wave  in  travelling  from  a  to  h.  By  measure- 
Fir;.  42. 


Diagram  of  arrangement   for   recording   the   contraction   wave   in   a 
curarised  sartorius. 


ments  carried  out  in  this  way  it  is  found  that  the  rate  of  pro- 
pagation of  the  contraction  in  frog's  muscle  is  3  to  4  metres 
per  second  ;  in  the  muscle  of  warm-blooded  animals  it  may 
amount  to  6  metres. 

The  actual  duration  of  the  contraction  at  any  given  point 
is  necessarily  smaller  than  that  of  the  whole  muscle,  and 
amounts  in  frog's  muscle  to  only  0-05-0-09  second,  about 
half  the  duration  of  the  contraction  of  a  whole  muscle  of 
moderate  length.  The  length  of  the  wave  is  obtained  by 
multiplying  the  rate  of  transmission  by  the  duration  of  the 
wave  at  any  one  point.     It  varies  therefore  in  frog's  muscle 


110  PHYSIOLOGY 

between  3,000  x  -05  (  =  150)  and  4,000  x  -09  (  =  360)  millimetres. 
Thas  the  muscle-fibres  in  the  frog  are  much  too  short  to 
accommodate  the  whole  length  of  the  wave,  and  the  contrac- 
tion of  the  whole  muscle  must  be  made  up  of  the  summated 
effects  of  the  contraction  wave  as  it  passes  from  point  to  point. 
Hence  the  longer  the  muscle,  the  more  must  the  contraction 
be  lengthened  by  the  time  taken  up  in  propagation  from  one 
end  to  another. 

Summation  of  Contractions 

If  a  muscle  is  stimulated  twice  in  succession,  so  that  the 
second  stimulus  follows  the  first  before  the  muscle  has  reached 
its  maximum  shortening,  we  get  a  combination  of  the  effects 
of  the  two,  which  results  in  a  stronger  contraction.  The 
second  stimulus  has  the  same  effect  on  the  muscle  as  if  the 
condition  of  contraction,  which  this  latter  has  attained  when 
the  stimulus  reaches  it,  were  its  normal  length. 

Thus,  if  the  period  that  elapses  between  the  two  stimuli  is 
equal  to  the  duration  of  the  first  part  of  the  contraction  (from 


Muscle  curves  showing  summation  of  stimuli,  r  and  r',  the  points 
at  which  the  stimuli  were  sent  into  the  nerve.  From  the  first 
stimulus  alone  the  curve  a  b  c  would  be  obtained.  From  r'  the 
curve  d  e  f  is  obtained.  These  two  curves  are  summated  to  form 
the  curve  a  g  h  i  k  when  both  stimuli  are  sent  in  at  the  interval 
rr'. 


its  beginning  to  the  maximum  height),  the  shortening  of  the 
muscle  may  be  doubled.  Fig.  43  shows  the  effect  of  two  suc- 
cessive stimuli  at  an  interval  of  about  J^^  second.  The  two 
lower  curves  represent  the  contractions  which  would  have 
resulted  from  either  of  the  stimuli  alone. 

This  piling  up  of  one  contraction  on  the  other  is  spoken  of 
as  siimmation. 


THE   CONTEACTILE   TISSUES  ]11 


Tetanus 


If  a  muscle  be  stimulated  so  many  times  in  a  second  {e.g. 
with  the  interrupted  current  of  an  ordinaiy  induction  coil) 
that  it  has  no  time  to  relax  between  one  stimulus  and  another, 
we  get  a  prolonged  steady  contraction,  which  is  much  stronger 
than  the  maximal  muscle-twitch,  owing  to  the  summation  of  the 
rapidly  following  stimuli.     This  condition  is  called  tetanus. 

The  rapidity  of  stimulation  needed  to  produce  an  unbroken 
tetanus  depends  on  the  duration  of  a  single  muscle-twitch,  and 
varies  therefore  according  to  the  kind  and  condition  of  the 


Fig.  44. 


Curves  showing  formation  of  tetanus  (from  frog's  gastrocnemius), 
a.  Six  stimuli  per  sec.  b.  Ten  stimuli  per  sec.  c.  Thirty  stimuli 
per  sec. 

muscle.  Thus  the  rapidity  need  only  be  small  in  the  case  of 
cooled  and  tired  muscles,  or  of  the  red  muscles  of  the  rabbit 
and  tortoise.  The  rate  varies  from  about  1.5  in  the  case  of 
red  muscles  to  30  or  40  for  white  muscles.  For  the  much 
more  highly  differentiated  muscles  of  insects  the  rate  is 
probably  very  much  greater. 

Extensibility 

Besides  the  change  of  form,  we  find  changes  in  the 
elasticity  and  extensibility  of  muscle  taking  place  during 
contraction. 

Living  muscle  in  a  perfectly  normal  condition  is  distin- 


112  PHYSIOLOGY 

guished  by  its  slight  l)ut  perfect  elasticity ;  that  is  to  say,  it 
is  considerably  stretched  by  a  slight  force  (in  the  longitudinal 
direction),  but  returns  to  its  original  length  when  the  extend- 
ing weight  is  removed.  The  length  to  which  muscle  is  stretched 
is  not  proportional  to  the  weight  used,  but  any  given  increment 
of  weight  gives  rise  to  less  elongation,  the  more  the  muscle  is 
already  stretched.  The  accompanying  curves  show  the  elonga- 
tion of  muscle  as  compared  with  a  piece  of  indiarubber,  when 
the  weight  on  it  is  uniformly  increased. 

Dead  muscle  is  less  extensible  and   its   elasticity  is  less 
perfect.     A  given  weight  applied  to  a  dead  muscle  will  not 

Fig.  45. 


^ 


Extensibility   of  indiarubber   (a)   compared   with    that   of    a   frog's 
gastrocnemius  muscle  (b). 

stretch  it  so  much  as  when  the  muscle  was  alive  ;  but  the  dead 
muscle  does  not  return  to  its  original  length  when  the  weight 
is  removed. 

A  contracted  muscle  on  the  other  hand  is  more  extensible 
than  a  muscle  at  rest.  A  gram  applied  to  a  tetanised  gastro- 
cnemius will  cause  greater  lengthening  than  if  it  were  applied 
to  the  same  muscle  at  rest.  At  the  same  time  the  elasticity 
is  more  perfect — that  is  to  say,  when  the  weight  is  removed, 
the  muscle  returns  more  quickly  to  its  original  length. 


THE   CONTKACTILE   TISSUES 


113 


Section  4 

THE   PRODUCTION   OP   WORK  AND   HEAT   BY 
VOLUNTARY  MUSCLE 

The  External  Work  done  by  a  Muscle 

We  have  already  seen  that  the  height  of  contraction  of 
a  muscle  diminishes  as  the  load  is  increased.  This  diminution 
in  height  is  at  first  very  slight  and  is  not  proportional  to  the 
load,  so  that  the  work  done  hy  the  muscle,  which  is  measured 
hy  the  product  of  the  weight  lifted  and  the  height  to  which  it 
is  raised,  w  x  h,  with  increase  of  weight  rises  at  first  quickly 
then  more  slowly  to  a  maximum,  and  then,  on  further  increasing 
the  load,  sinks. 

Fig.  46. 


Curve  showing  the  length  of  a  muscle  under  various  loads  in  the 
contracted  condition  b,  and  uncontracted  condition  a.  The 
double  lines  a  b  etc.  represent  the  contracted  muscle,  while  the 
long  single  lines  a  c  etc.  show  the  length  of  the  inactive  muscle. 


This  will  be  rendered  clearer  by  reference  to  the  diagram 
(Fig.  46)  representing  the  lengths  of  the  resting  and  contracted 
muscle  with  various  loads.  The  lines  h^„  h,,  etc.,  are  the  actual 
height  of  contraction  of  the  muscle  when  loaded  with  weights 
of  0,  10,  20  grams,  etc.  The  work  in  each  case  is  given  by 
ho  X  0,  h,  X  10,  h.^  X  20,  hg  x  30,  etc.  By  inspection  it  will  be 
seen  that — 

O.h^  <  lO.h^  <  20.h,  <  SO.hg  >  40.h ,  >  SO.b^. 


114  PHYSIOLOGY 

In  this  case  therefore  the  maximum  of  mechanical  work  is 
obtained  when  the  muscle  is  loaded  with  about  30  grams. 
This  increase  of  work  with  increased  load  shows  that  the 
amount  of  external  work  performed  by  a  muscle  is  not  a  con- 
stant quantity,  nor  one  determined  solely  by  the  strength  of 
stimulus,  but  is  essentially  conditioned  by  the  tension  mider 
which  the  muscle  contracts.  The  muscle  is  in  fact  endowed 
with  a  certain  power  of  adaptation,  so  that  it  can  respond 
with  increased  efforts  or  expenditure  of  energy  when  it  has 
more  work  set  it  to  do.  It  might  be  thought  that  the  in- 
creased mechanical  energy  evolved  under  these  conditions 
had  its  origin  at  the  expense  of  some  other  form  of  energy, 
such  as  heat  or  electrical  changes,  but  it  is  found  that 
increased  tension  augments  all  the  processes  of  muscle, 
including  chemical  changes  and  the  production  of  heat. 
This  excitatory  effect  of  tension  on  skeletal  muscle  is  aided 
in  all  the  higher  animals  by  impulses  which  pass  through 
the  central  nervous  system,  the  nature  of  which  we  shall 
have  to  discuss  later  on  when  dealing  with  the  question  of 
so-called  *  tendon  reflexes.'  The  phenomenon  however  is 
common  to  all  forms  of  contractile  tissues,  and  is  indeed 
much  better  marked  in  such  forms  as  the  heart-muscle  and 
the  unstriated  muscular  fibres  of  the  viscera.  One  may 
occasionally  find  that  the  application  of  a  slight  load  to  a 
skeletal  muscle  actually  increases  the  height  of  the  contrac- 
tion, especially  if  the  muscle  be  not  after-loaded.  In  the 
heart-muscle  an  increase  of  tension  within  physiological  limits 
causes  invariably  increased  contraction — a  fact  of  very  great 
importance  for  the  physiology  of  compensation  in  heart 
disease.  This  excitatory  influence  affects  not  only  the  strength 
of  contraction  but  also  the  automatic,  rhythmic,  and  conduct- 
ing power  of  the  muscle  ;  and  m  some  cases,  as  in  the  snail's 
heart,  the  rate  of  beat  is  absolutely  determined  by  the  tension, 
the  heart  stopping  altogether  if  the  tension  be  reduced  to 
nothing. 

The  Production  of  Heat 

The  experience  of  everyday  life  teaches  us  that  muscular 
exercise  is  associated  with  increased  production  of  heat.  A 
man  walks  fast  on  a  frosty  day  to  keep  himself  warm ;  and 


THE    CONTEACTILE   TISSUES  115 

we  find  this  observation  confirmed  when  we  investigate  the 
contraction  of  an  isolated  muscle  outside  the  body. 

Thus,  if  a  frog's  muscle  is  tetanised,  its  temperature  rises 
from  0-14"  to  0-18°  C,  and  for  each  single  twitch  from  0-001° 
to  0-005°  C. 

It  is  evident  that  such  small  changes  in  temperature  as 
0-001"  cannot  be  estimated  by  ordinary  thermometric  methods. 
For  this  purpose  a  thermopile  must  be  used. 

The  construction  of  a  thermopile  depends  on  the  fact  that,  when  the 
junctions  of  a  circuit  made  of  two  metals  are  at  different  temperatures,  a 
current  of  electricity  generally  flows  through  the  circuit.  This  current  can  be 
measured  by  means  of  a  galvanometer,  and  is  proportional  to  the  difference  of 
temperature  between  the  two  junctions.     Thus  in  the  circuit  (Fig.  47)  composed 

Fig.  47. 

COOL 

Antimony    ■  1  ]    Bismuth 


Q 


WARM 

of  two  metals,  antimony  and  bismuth,  if  the  upper  junction  be  cooled,  there 
will  be  a  current  flowing  from  antimony  to  bismuih  in  the  direction  of  the 
arrow,  and  this  current  will  within  limits  ;^be  proportional  to  the  difference  of 
temperature. 

To  measure  the  production  of  heat  during  muscular  contraction,  a  small 
flat  thermopile  (containing  four  or  six  elements  composed  of  iron  and  German 
silver)  is  fixed  with  one  of  its  ends  between  two  frog's  gastrocnemii.  Another 
exactly  similar  pile,  but  reversed,  is  placed  between  two  other  gastrocnemii, 
which  are  kept  resting  and  at  a  perfectly  constant  temperature.  The 
arrangement  of  the  piles  is  shown  in  the  diagram  (Fig.  48).  So  long  as  the 
two  piles  are  at  the  same  temperature  no  current  flows  ;  but,  with  a  sensitive 
galvanometer,  the  slightest  difference  of  temperature,  such  as  that  caused  by 
the  contraction  of  one  pair  of  muscles,  at  once  causes  a  deflection  of  the  galvano- 
meter, the  extent  and  direction  of  which  enable  us  to  estimate  exactly  the 
seat  and  amount  of  heat  produced. 

Another  method  of  measuring  the  heat  production  in  muscle  takes  advan- 
tage of  the  fact  that  the  electrical  resistance  of  a  wire  increases  with  a  rise  of 
temperature.  In  both  methods  we  convert  heat  into  electrical  changes,  since 
our  means  of  judging  of  electrical  differences  are  more  sensitive  than  is  the 
case  with  any  other  physical  change. 

In  large  animals  the  production  of  heat  in  muscular  con- 
traction can  be  easily  shown  bj^  inserting  the  bulb  of  a  thermo- 
meter between  the  thigh  muscles,  and  stimulating  the  spinal 
cord.  The  rise  of  temperature  i:)roduced  in  this  way  may 
amount  to  several  degrees. 


116  PHYSIOLOGY 

The  discovery  of  exact  means  of  measuring  the  heat  pro- 
duction during  contraction  was  naturally  utilised  to  determine 
the  relation  between  the  heat  produced  and  the  work  done 
under  varying  conditions.  In  the  muscle  as  in  a  steam 
engine,  we  have  a  conversion  of  potential  energy  stored  up  in 
carbon  compounds  into  kinetic  energy,  which  may  appear  as 
work  and  heat.  In  the  engine  there  is  a  definite  ratio  be- 
tween work  and  lieat.  Only  a  certain  small  proportion  of  the 
total  energy  can  be  utilised  as  work,  the  rest  being  dissipated 
as  heat.  The  exact  proportion  depends  on  the  difference  of 
temperatures  that  is  available  in  the  machine,  and  in  the  best 
engines  at  our  disposal  amounts  to  one-tenth.  If  the  machine 
does  no  work,  the  heat  production  is  increased  by  the  amount 

Pre.  48. 


ACTIVE  MUSCLE  RESTING    MUSCLE 

Diagram  of  the  arrangement  for  showing  the  development  of  heat 
during  muscular  contraction.  A  B,  B  A,  two  thermo-electric 
junctions ;  G,  galvanometer.     (After  Waller.) 


corresponding  to  the  work.  The  same  is  true  to  a  certain 
extent  in  muscle.  If  a  muscle  be  allowed  to  contract  and  relax 
twenty  times  when  loaded  by  a  weight,  the  total  external  work 
done  will  be  nothing.  If  however  the  weight  be  attached  to 
the  axle  of  a  wheel,  which  is  provided  with  a  catch  so  that 
the  weight  can  only  be  drawn  up  (Fig.  49),  and  the  muscle 
be  allowed  to  pull  at  each  contraction  on  the  circumference  of 
the  wheel,  at  each  contraction  work  is  done.  It  is  found  that 
in  the  latter  case  the  muscle  is  less  heated  than  in  the  former, 
and  the  difference  is  equivalent  to  the  work  done  in  raising 
the  weight.  But  as  soon  as  we  begin  to  alter  the  work  by 
altering  the  weight,  we  are  at  once  met  by  the  difficulty  that 
increased  tension  augments  all  the  properties  of  the  muscle, 


THE  CONTEACTILE  TISSUES 


117 


and  with  the  same  stimuhis  both  work  and  heat  production 
are  raised  by  increasing  the  load.  In  fact  the  maximum 
amount  of  heat  is  produced  when  the  muscle  is  made  to 
contract  against  a  strong  spring,  so  that  it  cannot  shorten  at 
all  (isometric  contraction). 

In  view  of  the  comparison  of  the  muscle  to  a  heat  engine, 
it  becomes  interesting  to   inquire  into  its  efficiency,  i.e.  the 

I'lG.    4'J. 


Diagram  of  Fick's  '  Arbeitsanimler '  or  muscle  crank,  a  6  is  a 
counterbalanced  lever,  attached  to  the  muscle  M  at  m.  When 
the  muscle  contracts,  the  catch  c  carries  round  the  circumference 
of  the  wheel  D  and  so  coils  up  the  weight  W  round  the  axle  of 
the  wheel.  When  the  muscle  relaxes,  if  c._,  is  in  the  situation  of 
the  dotted  line,  the  weight  pulls  the  wheel  and  lever  back  to  its 
original  position.  If,  however,  c,  be  applied  to  D,  the  backward 
movement  of  the  wheel  is  prevented,  and  the  nmscle  is  extended 
simply  by  the  weight  of  the  lever  a  b.  Thus  at  each  contraction 
the  weight  is  drawn  a  httle  higher,  and  external  work  is  per- 
formed by  the  muscle. 

relation  of  the  work  to  the  total  energy  expended.  This 
amount  is  found  to  vary  within  very  wide  limits.  In  a  fresh 
muscle  the  heat  energy  may  be  twenty-five  times  as  great  as 
the  work  energy,  but  the  heat  evolved  with  each  contraction 
diminishes  with  fatigue  more  rapidly  than  the  work  done,  so 
that  the  proportion  may  fall  to  as  low  as  three  to  one.     In 


118  PHYSIOLOGY 

the  intact  animal,  in  the  clog  fed  on  a  pure  flesh  diet,  Pfliiger 
has  calculated  that  the  efficiency  may  be  as  great  as  48  per 
cent.  The  efficiency  of  a  heat  engine  is  determined  by  the 
difference  of  absolute  temperatures  obtaining  on  the  two  sides 
of  the  machine  ;  and  since  we  cannot  imagine  even  minutely 
localised  changes  of  temperature  in  the  animal  body  of  more 
than  a  few  degrees  Centigrade,  we  must  discard  altogether 
the  analogy  of  the  steam  engine,  and  seek  some  other  explana- 
tion of  the  mechanism  by  which  the  muscle  is  enabled  to 
transmute  the  chemical  energy  of  its  food  into  work  or  heat. 
It  seems  probable  that  the  two  products,  heat  and  work,  are 
simultaneous  and  independent  in  their  origin,  and  that  any 
proportion  between  them,  therefore,  is  accidental.  The  muscle 
is,  in  fact,  not  a  heat  engine  but  a  chemical  engine.  The 
chemical  changes  are  converted  directly  into  pressure  changes 
leading  to  change  of  form  of  the  ultimate  contractile  element, 
and  it  seems  reasonable  to  suppose  that  these  pressure  changes 
are  directly  induced  by  the  osmotic  pressure  of  the  products 
of  chemical  change  produced  as  an  immediate  result  of 
excitation. 


THE   CONTEACTILE   TISSUES  111) 


Section  5 
THE   ELECTEICAL   CHANGES   IN   MUSCLE 

If  a  current  from  a  battery  be  passed  between  two  plates 
of  platinum  immersed  in  acidulated  water  or  salt  solution, 
electrolysis  of  the  water  takes  place,  bubbles  of  oxygen 
appearing  on  the  positive  plate  (anode),  and  bubbles  of  hydrogen 
on  the  negative  plate  (kathode).  If  now  we  remove  the 
battery,  and  connect  the  two  plates  (electrodes)  by  wires 
with  a  galvanometer,  it  will  be  seen  that  a  current  is  passing 
through  the  galvanometer  and  water  in  the  reverse  direction 
to  the  previous  battery  current.  This  current  is  called  the 
polarisation  current,  and  is  due  to  the  electrolysis  of  the 
water  that  has  taken  place.  The  vessel  in  which  the 
electrodes  are  immersed  has  in  fact  become  a  galvanic  cell, 
the  platinum  covered  with  hydrogen  bubbles  being  the  positive 
element,  and  that  covered  with  oxygen  bubbles  the  negative 
element.  Exactly  the  same  process  of  electrolysis  or  polarisa- 
tion takes  place  when  we  pass  currents  through  the  tissues  of 
the  body  by  means  of  metallic  electrodes. 

Hence  before  we  can  study  accurately  the  delicate  elec- 
trical changes  that  may  occur  normally  m  living  tissues,  it 
is  necessary  to  have  some  form  of  electrodes  in  which  this 
polarisation  will  not  occur.  The  '  non-polarisable  '  electrodes 
which  are  most  generally  used  for  this  purpose  are  made  in 
the  following  way.  A  glass  tube  (Fig.  50)  is  closed  at  one 
end  with  a  plug  of  kaolin  made  into  a  paste  with  a  saturated 
solution  of  zinc  sulphate.  The  rest  of  the  tube  is  filled  with 
a  similar  solution.  Dipping  into  the  zinc  sulphate  solution 
is  a  rod  of  pure  zinc,  amalgamated.  Just  before  use,  a 
plug  of  china  clay  made  with  normal  saline  solution  is  put 
on  the  end  of  the  tube,  so  as  to  effect  a  connection  between 
the  zinc  sulphate  clay  and  the  nerve  or  muscle  which  it  is 
desired  to  stimulate  or  lead  off".  In  these  electrodes  there 
is  no  contact  of  metals  with  fluids  that  can  produce  dis- 
similar ions  {e.g.  hydrogen  or  oxygen  bubbles)  at  the  surface 
of  contact,  and  hence  they  may  be  regarded  as  practically 
non-polarisable. 


120 


PHYSIOLOGY 


A  more  convenient  form  is  that  devised  by  Bm'don- 
Sanderson,  in  which  the  glass  tube  is  bent  into  a  U  (Fig.  51). 
The  mouth  of  the  tube  is  closed  by  a  smaller  glass  tube 
plugged  with  clay,  and  bearing  a  plug  of  normal  saline  clay. 

If  a  muscle  such  as  the  sartorius  be  removed  from  the 
body,  and  two  non-polarisable  electrodes  connected  with  a 
delicate  galvanometer  be  applied  to  two  points  of  its  surface, 
there  will  be  a  deflection  of  the  mirror  attached  to  the 
galvanometer,  showing  the  presence  of  a  current  in  the 
muscle  from  the  ends  to  the  middle,  and  in  the  external 
circuit  from  the  middle  (or  equator)  to  the  ends.  It  was 
formerly  thought  that  this  current  was  always  present  in  all 
normal  muscles,  and  it  was  spoken  of  as  the  '  natural  muscle 


Fig.  50. 
S-yinw-     a. 

C. 


Fig.  51. 


Fig.  50. — Diagram  of  non-polarisable  electrode,  a.  Covered  wire. 
h.  Amalgamated  zinc  rod.  c.  Glass  tube.  d.  Saturated  ZnSO  solu- 
tion,    e.  Plug  of  zinc  sulphate  clay.    /.  Plug  of  normal  saline  clay. 

Fig.  51. — U-shaped  non-polarisable  electrodes. 


current ; '  the  muscle  was  said  to  be  made  up  of  a  series  of 
electromotive  molecules,  the  equator  of  each  molecule  being 
positive  to  the  two  poles  (Du  Bois  Eeymond).  It  has  been 
conclusively  shown  however  (by  Hermann  and  others)  that 
this  current  of  resting  muscle  is  not  a  natural  current  at 
all,  but  is  due  to  the  effects  of  injury  in  making  the  pre- 
paration. The  less  the  preparation  is  injured,  the  smaller 
is  the  current  to  be  obtained  from  it,  and  in  some  contractile 
tissues,  such  as  the  heart,  there  may  be  absolutely  no  current 
during  quiescence. 

Hermann  describes  the  fact  of  the  existence  of  currents  of 
rest  thus  : — '  In  partially  injured  muscles  every  point  of  the 
injured  part  is  negative  towards  the  points  of  the  uninjured 


THE    CONTKACTILE   TISSUES  121 

surface.'     Fig.  52  shows  the  direction  of   the   current   in  a 
muscle  with  two  cut  ends. 

When  the  whole  muscle  is  quite  dead,  this  current  of 
rest,  or  'demarcation  current'  (Hermann),  disappears.  The 
current  is  due  to  the  electrical  differences  at  the  junction  of 
living  and  dying  (not  dead)  tissue.  If  the  sartorius  of  the 
frog  be  cut  out  and  immersed  for  twenty-four  hours  in  0'6  per 

L'lu.  52. 


Current  of  rest. 

cent.  NaCl  solution  made  with  tap  water  {i.e.  containing 
lime),  all  the  injured  fibres  die,  and  the  uninjured  fibres  are 
then  found  to  be  iso-electric  and  therefore  currentless. 

The  existence  of  this  current  may  be  demonstrated  with- 
out using  a  galvanometer.  If  the  nerve  of  a  sensitive 
muscle-nerve  preparation  (a.  Fig.  53)  be  allowed  to  fall  on  an 
excised  muscle  (b),  so  that  two  points  of  the  nerve  are  in 

Fig.  53. 


contact  with  the  cut  end  and  with  the  surface  of  the  second 
muscle  (b),  the  muscle  (a)  will  contract  each  time  the  nerve 
touches  (b)  so  as  to  complete  the  circuit. 

Whatever  be  the  explanation  of  this  current  of  resting 
muscle,  there  is  no  doubt  that  a  very  definite  electrical  change 
occurs  in  a  muscle  when  it  contracts. 

To  show  this  change,  we  may  lead  off  two  points,  one  on 
the  cut  end  and  one  on  the  surface  of  the  muscle  of  a  muscle- 


122 


PHYSIOLOGY 


nerve  prei^aration,  to  a  galvanometer.  We  shall  then  obtain 
a  deflection  of  the  mirror  of  the  magnet,  due  to  the  current  of 
rest  or  demarcation  current.  If  now  the  nerve  be  stimulated 
with  an  interrupted  current  so  as  to  throw  the  muscle  into  a 
tetanus,  the  ray  of  light  from  the  galvanometer  mirror  is 
observed  to  swing  back  towards  the  zero  of  the  scale,  showing 
that  the  current  which  was  present  before  is  diminished. 
When  the  excitation  of  the  nerve  is  discontinued,  the  galvano- 
meter indicates  once  more  the  original  current  of  rest.  Thifi 
diminution  of  the  current  of  rest  during  activity  of  a  muscle 
is  spoken  of  as  the  *  negative  variation.'' 

In  carrying  out  this  experiment  it  is  usual  to  compensate  the  demarcation 
current  by  sending  in  a  small  fraction  of  the  current  from  a  constant  cell. 
The  arrangement  of  the  apparatus  is  represented  in  the  accompanying  dia- 
gram.    Two   non-polarisable  electrodes   (np)  are  applied  to  the  surface  and 

Fig.  54. 


cross-section  of  a  muscle  (m).  These  are  connected  with  the  shunt  of  the 
galvanometer,  one  of  the  wires  however  being  connected  with  a  Pohl's  re- 
verser  (p),  and  this  in  its  turn  with  the  shunt  (s).  The  two  end  terminals  of  the 
reverser  are  connected  with  a  rheochord,  through  the  wire  of  which  (a,  b)  a 
constant  current  is  passing  from  the  Daniell  cell  (d).  By  means  of  the  rider  (c) 
the  fraction  of  current  passing  through  the  reverser  can  be  modified  to  any 
extent.  The  key  (k)  being  open,  the  muscle  is  connected  with  the  shunt  and 
galvanometer,  and  the  direction  and  extent  of  the  swing  noticed.  The 
key  (k)  is  then  closed,  and  by  means  of  the  reverser  the  current  is  sent  through 
the  galvanometer  in  the  opposite  direction  to  the  demarcation  current,  and  the 
rider  (c)  shifted  until  the  two  currents  exactly  balance  each  other,  and  the 


THE   CONTRACTILE  TISSUES 


123 


needle  of  the  galvanometer  returns  to  zero  of  the  scale.  This  adjustment  is 
first  made,  using  only  —^  of  the  total  current,  and  then  by  means  of  the 
shunt,  Y^,  ^,  and  finally  the  whole  current  is  thrown  into  the  galvanometer. 
If  this  precaution  be  not  taken,  much  too  large  a  current  may  in  the  first 
case  be  sent  through  the  galvanometer,  to  the  detriment  of  the  instrument. 
If   we  know  the  difference  of   potential  between  the   two   ends  of  the   wire. 


the  proportion  ^  ^  will  give  us  the  E.M.F.  of  the  demarcation  current. 
ab 


The 


galvanometer  needle  having  by  compensation  been  brought  to  zero,  stimu- 
lation of  the  nerve  at  (e)  by  interrupted  currents  causes  the  needle  to  swing 
at  once  in  the  opposite  direction  to  the  first  variation.  This  swing  is  the 
measure  of  the  negative  variation  or  current  of  action. 

The  negative  variation  may  also  be  observed,  by  means  of 
a  YGYy  lightly  moving  galvanometer,  to  accompany  a  single 
twitch  of  the  muscle.  It  has  been  found  to  occupy  about 
.j^  second,  and  to  occur  immediately  after  stimulation. 

Since  a  delicate  galvanometer  takes  some  seconds  to  come  to  rest  when 
a  current  is  sent  through  it,  the  whole  of  the  variation  is  over  at  a  time 
when  the  magnet  has  hardly  commenced  its  swing.  Hence  to  analyse  more 
fully  the  electrical  change  accompanying  each  separate  twitch  of  the  muscle, 
recourse  must  be  had  to  other  methods  than  the  direct  galvanometric  method. 

For  this  purpose  we  generally  use  an  instrument  called  the  rheotome,  by 
which  we  can  connect  the  electrodes  on  the  muscle  with  the  galvanometer  at 


Diagram  of  rheotome  (Hermann). 


varying  intervals  after  stimulation,  and  by  observing  the  galvanometer  readings 
at  each  j^  second  after  stimulation,  can  map  out  the  exact  course  of  the  current 
of  action. 

The  instrument  is  represented  diagrammatically  in  Fig.  55. 

By  means  of  a  clock  or  motor  the  rod,  a  b,  is  made  to  rotate  at  any 
required  rate  round  a  vertical  axis  at  its  centre.  On  either  end  it  carries 
two  brushes  made  of  fine  wire  and  connected  together.  The  brushes  at  each 
rotation  come  in  contact  with  the  pieces  of  copper,  r  r',  and  when  this 
happens  the  primary  circuit,  k,  r',  a,  r,  p,  is  rapidly  closed  and  broken  again. 


124  PHYSIOLOGY 

thus  giving  rise  to  a  momentary  current  in  the  secondary  coil,  s,  and  ex- 
citing the  muscle,  m.  In  the  same  way  the  brushes,  b,  close  the  galvano- 
meter-electrode-uiuscle  circuit,  g,  t,  b,  t',  1,  ii,  ([,  each  time  they  brush  on  the 
copper  banks,  t  t'.  By  turning  the  disc,  a,  round,  the  interval  at  which  the 
brushes,  b,  pass  t  t',  after  the  brushes,  a,  pass  r  r',  can  be  altered  at  will,  and 
therefore  the  interval  between  stimulation  and  leading  off  the  current  to  the 
galvanometer. 

But  there  is  no  need  for  any  demarcation  current  to  be 
present  in  order  to  show  an  electrical  change  accompanying 
contraction.  In  fact  we  learn  much  more  about  the  nature 
of  the  excitatory  change  if  we  study  the  electrical  behaviour 
of  a  perfectly  normal  (and  therefore  currentless)  muscle  on 
stimulation.  This  can  be  easily  done  by  means  of  the  rheo- 
tome  or  capillary  electrometer. 

If  a  perfectly  miinjured  regular  muscle  (such  as  the 
sartorius)  be  stimulated  with  a  single  induction  shock  at  one 
end  (x)  (Fig.  56),  and  the  relative  electrical  conditions  of  the 

Fig.  56. 


Diagram  showing  diphasic  variation  of  uninjured  muscle. 

points  (a)  and  (b)  investigated,  it  will  be  found  that  as  soon  as 
the  excitatory  process  reaches  (a),  this  point  becomes  negative 
to  (b),  and  there  is  thus  a  current  in  the  galvanometer  from 
(b)  to  (a).  A  moment  later  the  two  points  are  equipotential, 
as  shown  by  the  fact  that  no  current  passes  through  the 
galvanometer.  A  thousandth  of  a  second  later  this  balance 
is  upset,  and  now  (b)  is  negative  to  (a),  and  the  galvano- 
meter needle  swings  in  the  opposite  direction. 

Thus  every  excitation  of  a  normal  muscle  gives  rise  to 
a  diphasic  variation,  of  such  a  direction  that  the  point 
stimulated  first  becomes  negative  '  to  all  other  points  of  the 

'  The  statement  that  the  excited  portion  of  the  muscle  becomes  '  negative,' 
though  sanctioned  by  long  usage,  is  not  very  exact  and  may  give  rise  to  mis- 
conception. When  we  lead  off  the  terminals  of  a  copper  zinc  couple  or  cell  to 
a  galvanometer,  a  current  flows  outside  the  cell  from  copper  to  zinc  and  inside 
the  cell  from  zinc  to  copper.     In  this  case  we  know  that  the  zinc  is  electro- 


THE    CONTRACTILE   TISSUES 


12^ 


muscle,  and  this  '  negativity '  (to  use  a  loose  but  convenient 
expression)  passes  as  a  wave  down  the  muscle,  accompanying 
or  preceding  the  wave  of  contraction,  and  travelling  at  the 
same  rate. 

This  diphasic  current  of  action  is  shown  much  more 
clearly  and  easily  on  a  slowly  contracting  tissue  such  as  the 
frog's  ventricle. 

Fig.  57  represents  the  photograph  of  the  variation  of 
the  frog's  heart,  as  shown  by  a  capillary  electrometer,  one 
terminal  (acid)  of  which  is  connected  with  the  base  of  the 
ventricle,  and  the  other  (Hg)  with  the  apex.  The  con- 
traction of  the  ventricle  begins  at  the  base.  The  base 
therefore  becomes  negative,  and  the  column  of  mercury 
moves   up  (1  to   2).     A   moment  later   the   contraction  has 


Fig.  57. 


Tracing  of  diphasic  variation  of  frog's  heart  taken  with 
capillary  electrometer. 

extended  to  the  apex.  There  is  now  an  equalisation  of  the 
potential  between  the  two  terminals,  so  the  mercury  comes 
back  quickly  to  the  base  line.  Here  it  stops  for  about  one 
and  a  half  seconds.  During  this  time  the  whole  heart  is 
contracting  equally ;  both  base  and  apex  are  thus  in  a  similar 
condition,  and  there  can  be  no  difference  of  potential  between 
them.  The  contraction  then  goes  off,  but  the  relaxation,  just 
as  the  contraction,  begins  at  the  base,  and  proceeds  thence 
to  the  apex.     There  is  thus  a  small  period  in  which  the  apex 

positive  to  the  copper,  and  in  the  same  way  we  must  assume  that  the  excited 
portion  of  a  muscle  is  really  electropositive  to  the  unexcited  portions.  When 
therefore  we  speak  of  any  part  of  a  tissue  being  negative,  we  are  using  a  con- 
ventional expression  to  indicate  the  direction  of  the  current  in  the  outer 
circuit,  and  not  the  electrical  condition  of  the  tissue  itself.  In  order  to  avoid 
the  confusion  which  might  result  from  an  attempt  to  replace  the  loose  ex- 
pression 'negative'  by  the  correct  expression  'electropositive,'  Waller  has 
suggested  the  employment  of  the  term  '  zincative '  to  indicate  the  electrical 
condition  accompanying  excitation.  This  term  also  serves  to  emphasise  the 
fact  that  the  excited  portion,  like  the  zinc  in  a  zinc-copper  cell,  is  the  chief 
seat  of  chemical  change. 


1'2G 


PHYSIOLOGY 


is  still  contracted  while  the  base  is  relaxed,  and  the  apex  is 
therefore  negative  to  the  base.  This  terminal  negativity  of 
the  apex  is  shown  on  the  photograph  by  the  excursion  of  the 
column  of  mercury  away  from  the  point  of  the  capillary  at  (4). 
The  only  difference  between  the  electrical  changes  in  this 
case  and  in  that  of  voluntary  muscle  is  that  in  the  latter 
all  processes  are  very  much  quicker,  so  that  as  a  rule  the 
point  (a)    (Fig.    56)    has   ceased   to   be   negative   before   the 

Fm.  58. 


■  11'' 


.-aci(I 


Diagram  of  capillary  electrometer.  Hg.,  Mercury.  The  two 
terminals  are  represented  as  leading  off  two  points  at  the  base 
and  apex  of  a  frog's  heart,  a  b. 


negativity  of  (b)   has  attained   its  full  height,  and  there  is 
thus  no  prolonged  equipotential  stage. 

Although  in  the  case  of  the  slowly  contracting  ventricle  of  the  tortoise, 
the  record  obtained  of  the  electrical  changes  accompanying  its  contraction  by 
means  of  the  capillary  electrometer  shows  with  great  clearness  the  diphasic 
nature  of  the  variation,  and  therefore  the  wave  character  of  the  electrical 
change,  considerable  dilliculty  is  at  first  experienced  when  we  attempt  to  inter- 
pret in  the  same  way  the  electrometer  record  of  the  electrical  response  of 
voluntary  muscle.  In  this  case  the  electrical  change  at  any  spot  only  lasts  about 
^7^5  of  a  second,  and  there  is  not  a  prolonged  equij)otential  period,  as  in  the  case 
of  the  heart.  The  diphasic  nature  of  the  variation  is  however  obvious,  if  we 
compare  the  electrometer  record  of  an  intact  and  therefore  currentless  muscle 
with  that  of  the  change  produced  by  a  single  stimulus  in  a  muscle  in  which 
one  of  the  leading-off  points  has  been  injured,  so  as  to  give  rise  to  a  demar- 
cation current.  The  two  curves  are  given  in  Fig.  .59,  the  upper  shadowy 
tracing  being  that  obtained  from  the  injured  muscle.     It  will  be  seen  that  the 


THE   CONTRACTILE   TISSUES  127 

distinguishing  character  of  a  diphasic  variation  in  the  rapidly  contracting 
striated  muscle  consists  in  the  fact  that  the  downstroke  of  the  image  of  the 
meniscus  is  as  rapid  as  the  upstroke,  whereas  the  monophasic  variation  of  the 
injured  muscle  presents  a  slow  fall  produced  by  the  gradual  leakage  of  the 
charge  imparted  to  the  instrument  back  through  the  electrodes  and  muscle. 
Knowing  the  constants  of  the  instrument  used,  it  is  possible,  by  measuring 
the  curvature  of  this  '  spike,'  as  the  record  of  the  diphasic  variation  is 
termed,  to  determine  accurately  the  electromotive  force  of  the  action  current 
producing  the  excursion  of  the  electrometer.  An  exactly  similar  curve  can  be 
obtained  by  putting  in  a  current  of  similar  E.M.F.  fi'om  a  battery,  first  in  one 
direction  for  t^^-,  of  a  second,  and  then  in  a  reverse  direction  for  another  ^^^  of 
a  second. 

It  must  be  remembered  that  a  diphasic  variation  does  not  mean  that  one 
part  of  a  muscle  changes  from  normal  in  one  direction,  and  then  swings  back 

Fig.  59. 


Superimposed  photographs  of  the  electrical  variation  of  the  sartorius 
in  response  to  a  single  stimulus.     (Burdon-Sanderson.) 

past  the  normal  in  another  direction,  but  that  a  change  in  one  direction  at  one 
electrode  dies  away  and  is  succeeded  by  a  similar  change  in  the  same 
direction,  which  also  dies  away,  at  the  second  electrode :  that  is  to  say,  a 
diphasic  variation  implies  a  progression  of  a  wave  of  electrical  change  between 
the  leading-off  points.  It  is  found  that  the  rate  of  transmission  of  this 
electrical  change  in  muscle  is  exactly  the  same  as  the  rate  of  propagation 
of  the  wave  of  contraction,  and  amounts  at  ordinary  temperatures  to  about 
3  metres  per  second. 

We  may  now  return  for  a  moment  to  the  consideration  of 
the  current  of  rest  observed  in  injured  muscle. 

Hermann  considers  that  muscle  (or  contractile  tissue) 
becomes  negative  under  two  conditions  : 

(1)  In  activity. 

(2)  When  dying. 

But  it  must  be  confessed  that  the  second  may  be  easily 
placed  under  the  first  head.  Section  or  injury  of  a  muscle 
causes  a  constant  stimulation  of  the  adjacent  parts.  These 
parts  therefore  become  negative  to  the  other  parts  that  are 
further  away  from  the  seat  of  injury,  and  we  thus  get  a 
demarcation   current.      Hence    we   come   to   the   conclusion 


128  PHYSIOLOGY 

(only  paradoxical  in  terms)  that  the  currents  of  rest  are  cur- 
rents of  action,  and  are  due  to  excitation  around  the  injured 
spot.' 

Electrical  Organs 

An  interesting  light  has  been  thrown  on  this  question  by 
a  study  of  the  electrical  organ  in  the  torpedo  and  other  electric 
fishes.  In  the  torpedo  is  found  on  each  side  of  the  middle 
line  a  kidney-shaped  organ  forming  a  considerable  mass  of 
the  body- substance  and  extending  from  dorsal  to  ventral  side. 
Viewed  from  the  surface  each  organ  presents  a  honeycombed 
appearance,  due  to  the  fact  that  it  is  made  up  of  a  number  of 
prismatic  columns  running  dorso-ventrally,  each  column  con- 
sisting of  an  enormous  number  of  hexagonal  discs  placed  one 
above  the  other.  Each  column  is  richly  supplied  with  nerves, 
which  are  derived  from  large  ganglion  cells,  situate  in  a 
distinct  lobe  of  the  brain.  Each  nerve  divides  up  into  eighteen 
or  twenty  branches  as  it  approaches  the  column,  as  shown  in 
the  diagram  (Fig.  60).  Each  plate  receives  nerve-fibres  at  all 
its  six  corners.  On  tracing  the  nerve-fibre  into  the  plate,  we 
find  that  every  element  consists  practically  of  a  gigantic  motor 
end-plate.  The  axis-cylinders  branch  dichotomously  and  form 
a  close  network  embedded  in  granular  protoplasm,  and  rest- 
ing on  a  granular  material  which  represents  all  that  is  left  of 
the  striated  muscles  of  the  embryo,  out  of  which  the  electrical 
organs  have  been  developed.  We  have  therefore  a  series  of 
end-plates  separated  from  one  another  by  laminae  of  connect- 
ive tissue,  and  so  arranged  that  any  electrical  changes  in  the 
nerve  terminations  will  be  summated  somewhat  after  the 
analogy  of  the  summation  of  efl'ect  in  a  Volta's  pile.  As  a 
result  we  find  that,  although  the  electromotive  force  of  the 
action  current  in  a  single  element  is  only  about  0*025  volt  (no 
greater  than  that  observed  in  an  ordinary  nerve  or  muscle), 
yet  the  E.M.F.  of  the  whole  organ  compounded  of  thousands 
of  these  plates  attains  to  several  hundred  volts,   so  that,  in 

'  If  the  demarcation  current  is  really  only  due  to  excitation,  we  should 
expect  to  find  it  weaker  than  the  action  current  obtained  by  exciting  the 
whole  muscle  to  contract.  And  this  is  the  case.  The  E.M.F.  of  the  demar- 
cation current  of  a  sartorius  equals  about  0-05  of  a  Daniell  cell  The  action 
current  of  the  same  muscle  may  attain  to  an  E  M.F.  =  0-08  of  a  Daniell  cell 
(Gotch). 


THE   CONTEACTILE   TISSUES 


129 


spite  of  the  short  circuiting  of  the  current  by  the  water  in 
which  the  animal  is  immersed,  a  discharge  of  the  organ  would 
give  a  painful  shock  to  any  one  touching  the  fish,  and  may 
even  kill  small  animals. 

We  have  here  an  excitable  tissue  in  which  the  action 
current  is  from  the  anatomical  arrangements  of  the  organ 
always  monophasic.  If  an  organ  or  segment  of  an  organ  be 
cut  out  of  the  body,  it  will  be  found  to  present  a  resting 
current,  which  however  declines  quickly  and  may  finally 
disappear  altogether.     The  direction  of  this  resting  current 

Fi«.  60. 


Diagram  of  the  structure  of  a  column  of  the  electrical  organ  of  the 
Torpedo,  showing  its  division  into  diacs,  and  the  distribution  of 
its  nerve  supply.     (Fritsch.) 


is  always  the  same  as  that  of  the  discharge.  Injury  to  any 
part  of  the  organ  or  at  either  electrode  alwaj^s  increases  the 
resting  current,  whereas  of  course  in  a  muscle  the  direction  of 
the  resting  current  is  entirely  determined  by  the  situation  of 
the  injury.  In  the  torpedo  therefore  the  resting  current  is 
always  a  current  of  action  due  to  the  slight  constant  irritation 
of  the  injury  ;  and  there  seems  no  reason  why  we  should 
assume  a  difierent  explanation  for  the  resting  current  in  striated 
muscle  or  in  nerve. 

9 


130  PHYSIOLOGY 


Secondary  Contraction.     Rheoscojjic  Frog 

The  negative  variation  of  one  muscle  may  be  used  to  make 
another  contract. 

If  the  nerve  of  the  preparation  (a)  (in  Fig.  Gl)  be  kid  so 
as  to  touch  at  two  points  the  cut  end  and  surface  of  the 
muscle  (b),  and  the  nerve  of  (b)  then  stimulated  with  single 
induction  shocks,  every  contraction  of  (b)  will  be  attended  by 
a  contraction  of  (a),  excited  by  the  negative  variation  of  the 

Fig.  61. 


Rhcoscopic  frog. 

current  passing  through  its  nerve  from  the  point  touching  the 
cut  end  to  that  in  contact  with  the  equator  of  (b). 

If  the  nerve  of  (b)  is  tetanised,  (a)  as  well  as  (b)  enters 
into  a  continued  contraction.  This  '  secondary  tetanus  '  is 
of  interest  as  showing  that,  although  the  contractions  of  (b) 
are  fused,  the  excitatory  process  and  negative  variations  are 
still  quite  distinct. 


THE    CONTRACTILE   TISSUES  181 


Section    6 
THE   CHEMISTEY   OP   MUSCLE 

Chemical  Composition  of  Voluntary  Muscle 

It  is  impossible  to  speak  with  certainty  about  the  chemical 
composition  of  any  living  tissue,  since  in  the  act  of  analysis 
we  destroy  the  life  of  the  tissue  ;  all  we  can  do  in  most  cases 
is  to  find  the  proximate  principles  present  in  the  dead  tissue. 
But,  by  using  certain  precautions,  we  may  learn  some  interest- 
ing facts  about  the  chemistry  of  living  muscle.  Muscle  of 
cold-blooded  animals  mav  be  cooled  below  0""  C.  without  losing 
its  irritability  on  re-warming  and  therefore  we  may  say  with- 
out its  life  being  destroyed.  If  the  living  muscle  of  frogs  be 
frozen,  then  minced  with  ice-cold  knives  as  finely  as  possible 
and  pounded  in  a  mortar  with  four  times  its  weight  of  snow 
containing  0-6  per  cent,  of  common  salt,  and  the  mixture 
thrown  on  to  a  filter  and  kept  at  a  little  over  0°  C,  an 
opalescent  fluid  filters  through.  The  filters  soon  get  clogged 
and  therefore  must  be  frequently  changed.  Their  temperature 
must  not  be  allowed  to  rise  over  2°  or  3°  C.  This  fluid  is 
called  muscle-plasma.  If  its  temperature  be  allowed  to  rise 
to  that  of  the  room,  it  clots,  and  the  clot  soon  contracts, 
squeezing  out  a  serum,  just  as  in  the  case  of  blood-plasma. 

The  muscle-plasma  is  neutral  or  slightly  alkaline.  When 
coagulation  takes  place  however,  it  becomes  distinctly  acid, 
and  this  acidity  has  been  shown  to  be  due  to  the  formation  of 
sarcolaetic  acid  in  the  process. 

Arguing  chiefly  from  analogy  with  the  blood-plasma,  the 
muscle-plasma  has  been  said  to  contain  a  body,  myosinogen, 
which  is  converted  when  clotting  takes  place  into  myosin,  and 
perhaps  other  bodies,  of  which  lactic  acid  may  be  one. 

The  exact  nature  of  the  proteins  in  muscle-plasma,  as  well  as  of  the  pro- 
tein constituent  of  the  clot,  which  we  have  called  myosin,  is  still  a  subject  of 
debate.  Kiihne,  to  whom  we  owe  our  first  acquaintance  with  muscle-plasma, 
described  the  clot  as  consisting  of  myosin,  a  globulin,  soluble  in  5  per  cent, 
solutions  of  neutral  salts,  such  as  NaCl  or  MgSo^,  precipitated  by  complete 
saturation  with  ]MgSo4,  and  coagulated  on  heating  to  56°  C.  In  the  muscle 
serum,  obtained  after  separation  of  the  clot,  he  found  three  proteins,  one 
coagulating  at  45°  C,  one  he  called  an  albumate  (i.e.  a  derived  albumen),  and 


132  PHYSIOLOGY 

the  third  coagulating  about  75°  C,  and  apparently:  identical  with  serum 
albumen.  Halliburton  extended  these  researches  to  the  muscles  of  warm- 
blooded animals.  He  described  four  proteins  as  existing  in  muscle-plasma, 
of  which  two,  paramyosinogen  and  myosinogen,  gave  rise  to  the  clot  of 
myosin. 

In  no  case  however  is  it  possible  to  entirely  dissolve  up  the  clot  when  once 
formed,  and  it  seems  that  the  so-called  solution  in  dilute  salt  solutions  was 
merely  an  extraction  of  still  soluble  proteid  in  the  meshes  of  the  clot.  The 
latest  work  on  the  subject  by  von  Fiirth  has  shown  that  if  the  muscles  of  a 
mammal  are  washed  free  of  adherent  lymph  and  blood,  the  plasma  obtained 
by  extraction  with  normal  salt  solution  contains  only  two  proteins.  These 
proteins  are  extremely  unstable,  and  are  gradually  transformed  on  standing 
into  insoluble  protein,  giving  rise  to  a  precipitate  in  dilute  solutions,  or 
forming  a  jelly-like  clot  in  strong  solutions.  The  properties  of  these  proteins 
may  be  summarised  as  follows  : 

1,  Myosin  (paramyosinogen  of  Halliburton).  A  globulin,  coagulating  at 
about  47°-  50°  C,  precipitated  by  half  saturation  with  ammonium  sulphate  or 
on  dialysis.  Transformed  slowly  in  solution,  rapidly  on  precipitation,  into  an 
insoluble  protein,  myosin  fibrin. 

2.  Myogen  (myosinogen  of  Halliburton).  A  protein  allied  to  the  albumens 
in'that  it  is  not  precipitated  by  dialysis.  Coagulates  on  heating  at  55°-G0°  C. 
It  changes  slowly  into  an  insoluble  protein,  myogen  fibrin,  but  passes  through 
an  intermediate  soluble  stage  called  soluble  myogen  fibrin.  This  latter  body 
coagulates  on  heating  to  40°  C,  being  instantly  converted  at  this  temperature 
into  insoluble  myogen  fibrin.  It  does  not  seem  that  any  ferment  action  is 
associated  with  these  changes,  which  we  may  represent  by  the  following 
schema  : 

Muscle-plasma. 


I  I 

Myosin  or  Paramyosinogen.  |  Myogen  (Myosinogen  of  Halliburton, 

I  Albumate  of  Kiihne). 

I  Soluble  myogen  fibrin. 

Myosin  fibrin.  Insoluble  myogen  fibrin. 

Muscle  clot. 

Soluble  myogen  fibrin,  which  in  mammalian  muscle-plasma  forms  only  on 
standing,  exists  apparently  preformed  in  frog's  muscle.  Hence  the  instan- 
taneous clotting  of  frog's  muscle-plasma  on  warming  to  40°  C. 

The  residue  left  after  the  expression  of  the  muscle-plasma 
consists  chiefly  of  connective  tissue,  sarcolemma,  and  nuclei, 
and  as  such  contains  gelatin  (or  rather  collagen),  mucin, 
nuclein,  and  adhering  traces  of  the  proteins  of  the  muscle- 
plasma  itself. 

The  muscle  serum  contains  the  greater  part  of  the  soluble 
constituents  of  muscle.     These  are — 

A.  Colouring  matters. — All  red  muscles  contain  a  con- 
siderable amount  of  haemoglobin.    In  many,  a  special  pigment, 


THE    CONTRACTILE   TISSUES  133 

probably  allied  to  hsemoglobm,  is  also  present.  This  has  been 
named  mijolicematin  (MacMunn). 

B.  Nitrogenous  extractives. — Of  these,  the  most  important 
is  creatin  (C4HgN302  +  H20),  which  occurs  to  the  extent  of  0*2 
to  0*3  per  cent.  This  substance  is  found  only  in  muscular 
and  nervous  tissues.  Its  significance  we  shall  discuss  later  on 
when  inquiring  into  the  history  of  the  formation  of  urea. 

Other  nitrogenous  extractives  are — 

Hypoxanthin  or  sarcin,  xanthin  (both  bodies  allied  to  uric 
acid),  carnin,  and  a  trace  of  urea. 

c.  Non-nitrogenous  constiticents. — Fats. 

Glycogen.  The  amount  of  this  is  very  variable.  In  the 
embryo  the  muscles  may  contain  large  quantities,  but  in  the 
adult  they  contain  only  from  0*4  to  1  per  cent. 

Inosit  (CyH,20^  +  2K,0),  or  '  muscle-sugar,' which  occurs 
in  minute  traces,  is  non-fermentable,  does  not  rotate  polarised 
light,  and  does  not  reduce  Fehling's  solution.  It  does  not 
belong  to  the  group  of  carbohydrates  at  all,  being  a  derivative 
of  benzene. 

Dextrose.  It  is  doubtful  whether  this  is  present  in  fresh 
resting  muscle. 

D.  Inorganic  constituents. — Muscle  contains  about  75  per 
cent,  of  water.  The  ash  forms  1  to  1*5  per  cent,  and  consists 
chietly  of  salts  of  potassium  and  phosphoric  acid.  There  are 
small  traces  of  calcium,  magnesium,  chlorine,  and  iron. 

Bigor  Mortis 

All  muscles,  within  a  short  time  of  their  removal  from  the 
body,  or  if  left  in  the  body  after  general  death,  lose  their  irri- 
tability, and  this  is  succeeded  by  an  event  which  occurs  rather 
suddenly,  and  is  known  as  rigor  mortis.  The  muscle,  which 
was  before  translucent,  supple,  extensible,  becomes  more 
opaque,  rigid,  and  inextensible,  and  shortens.  The  shortening 
is  not  very  powerful,  and  can  be  prevented  by  loading  the 
muscle  moderately  Chemical  changes  also  take  place.  The 
muscle,  which  was  previously  alkaline,  becomes  distinctly  acid, 
the  acidity  being  due  to  the  formation  of  sarcolactic  acid. 
There  is  also  pruduction  of  COj  and  an  evolution  of  heat. 

It  is  generally  believed  that  this  change  is  identical  with 
the  clotting  of  muscle-plasma,  and  that  the  rigidity  as  well  as 


134  PHYSIOLOGY 

the  contraction  of  the  muscle  is  due  to  the  coagulation  of  the 
muscle-proteins.  That  there  is  at  any  rate  a  close  connection 
between  the  two  sets  of  phenomena  is  shown  by  Brodie's  obser- 
vations on  the  heat-contraction  of  muscle.  This  observer  found 
that,  if  a  living  muscle  be  lightly  loaded  and  then  warmed  very 
gradually,  a  series  of  stages  in  the  heat-contraction  could  be 
distinguished  corresponding  to  the  coagulation  temperatures  of 
the  different  proteins  described  by  von  Fiirth  in  muscle-plasma. 

The  onset  of  rigor  is  closely  connected  with  the  post-mortem 
production  of  sarcolactic  acid.  If  the  formation  of  this  acid 
be  prevented  or  diminished  (as  may  be  effected  by  keeping 
the  muscle  in  an  atmosphere  of  pure  oxygen),  rigor  may 
be  retarded  or  prevented  altogether.  The  same  result  is 
obtained  if  the  accumulation  of  the  acid  in  the  muscle  is  pre- 
vented by  washing  out  its  blood  vessels  with  a  slightly  alka- 
line normal  saline  solution.  Under  these  circumstances,  the 
muscle  gradually  loses  its  irritability,  and  may  finally  be 
regarded  as  dead,  but  it  will  then  begin  to  putrefy  without  at 
any  time  showing  any  signs  of  rigor.  The  influence  of  lactic 
acid  in  causing  the  coagulation  of  muscle  proteins,  distinctive 
of  rigor  mortis,  is  shown  by  the  fact  that  after  severe  muscular 
fatigue,  as  in  hunted  animals,  where  there  has  already  been  a 
considerable  formation  of  these  waste  products  of  muscular 
contraction,  rigidity  may  come  on  almost  immediately  after 
death.  Eigor  mortis  therefore  may  be  regarded  as  a 
coagulation  of  muscle  proteins  induced  by  the  accumulation 
of  the  products  of  metabolism  in  the  surviving  excised 
muscle.  Of  course,  a  rigidity  of  muscle  may  be  also  brought 
about  by  the  direct  agency  of  heat,  as  by  plunging  into 
boiling  water.  In  this  case  there  has  been  no  time  for  any 
chemical  change  to  take  place,  and  the  scalded  muscle  remains, 
like  fresh  muscle,  slightly  alkaline  to  litmus. 

A  rigid  muscle  never  recovers.  It  is  dead,  and  in  dying 
chemical  and  physical  changes  have  taken  place,  giving  rise 
to  shortening  and  rigidity,  and  converting  a  fluid  complex 
substance  into  an  aggregate  of  insoluble  protein,  with  the 
formation  of  CO.^  and  lactic  acid.  The  unstable  living  molecule 
has  broken  down  into  dead  stable  molecules,  the  potential 
energy  of  the  former  appearing  as  heat  and  work. 

The  lactic  acid  formed  in  nauscle  (sarcolactic  acid)  is  a  i^hysical  isomer  of 
the  lactic  acid  formed  in  the  fermentation  or  souring  of  milk.     They  both 


THE   CONTRACTILE   TISSUES  135 

have  the  formula  CH3.GH(0H).C00H,  i.e.  they  are  ethylidene  lactic  acids. 
The  lactic  acid  of  fermentation  is  optically  inactive ;  sarcolactic  acid  rotates 
polarised  light  to  the  right ;  while  a  third  isomer  which  is  laevorotatory  is 
produced  by  the  action  of  various  bacilli  and  vibriones  on  cane  sugar. 


The  Gliemical  Changes  lohich  accompany  Activity 

The  principle  of  the  conservation  of  energy  teaches  us  that 
the  energy  of  the  contraction  of  muscle  must  be  derived  from 
chemical  changes,  probably  processes  of  decomposition  and 
oxidation,  occurring  in  the  muscle  itself.  In  seeking  out  the 
nature  of  these  changes,  three  methods  are  open  to  us : 

1.  We  can  examine  the  changes  in  the  muscle  itself,  avoid- 
ing so  far  as  possible  reintegrative  changes  by  working  on 
excised  muscles. 

2.  We  can  investigate  the  changes  in  the  medium  of  the 
muscle.  Muscle  may  be  exposed  in  a  vacuum  or  in  a  confined 
space  of  air,  and  its  gaseous  interchanges  during  rest  and 
activity  compared.  Or  we  may  lead  a  current  of  defibrinated 
blood  through  excised  muscles,  and  determine  the  change  in 
the  composition  of  the  blood  before  and  after  passing  through 
the  muscle  under  various  conditions. 

3.  A  method  which,  although  apparently  complex,  has 
rendered  the  utmost  service  to  the  physiology  of  muscle,  is  to 
use  the  changes  in  the  total  metabolism  of  the  animal  during 
rest  and  muscular  work  as  a  clue  to  the  muscular  metabolism 
itself.  In  such  a  case  the  respiratory  exchanges  of  the  animal 
are  determined  (viz.  its  oxygen  intake  and  its  CO.,  output), 
and  the  urine  and  faces  are  carefully  analysed,  in  order  to 
judge  of  the  action  of  muscular  work  on  the  carbon  and 
nitrogen  metabolism  of  the  body. 

By  one  or  other  of  these  methods  it  has  been  found  that 
the  main  products  of  muscular  activity  are  the  same  as  those 
which  are  produced  during  the  death  of  a  muscle,  viz.  sarco- 
lactic acid  and  carbon  dioxide. 

It  was  shown  long  ago  by  Helmholtz  that  when  a  muscle 
was  tetanised  to  exhaustion,  the  total  amount  of  its  watery 
extractives  diminished,  while  the  amount  of  its  alcoholic 
extractives  increased  ;  and  there  is  no  doubt  that  part  of  this 
difference  is  due  to  the  formation  of  lactic  acid.  The  souring 
of  muscle  during  activity  can  be  easily  demonstrated  by 
stimulating  the  muscle  for  some   time  and  then  crushing  a 


136  PHYSIOLOGY 

fragment  of  the  excised  muscle  on  litmus  paper.  The  litmus 
is  at  once  turned  red.  Or  we  may  inject  a  solution  of  acid 
fuchsin  under  the  skin  of  a  frog,  and  the  next  day  expose  a 
sciatic  nerve  and  stimulate  it  for  fifteen  or  twenty  minutes. 
On  skinning  the  hind  legs,  a  difference  in  colour  will  be  at 
once  apparent,  the  leg  which  has  been  active  being  of  a  deep 
rose  colour,  owing  to  the  action  of  the  acid  on  the  fuchsin. 

Sarcolactic  acid  is  not  present  in  a  free  state  in  muscle,  the  acidity  being, 
like  that  of  urine,  due  to  the  presence  of  acid  phosphates.  The  sarcolactic 
acid  can  be  extracted  from  the  muscle  by  means  of  alcohol.  It  is  generally 
separated  in  the  form  of  the  zinc  sarcolactate,  by  boiling  its  partially  puri- 
fied solution  with  zinc  carbonate.  Its  presence  may  be  tested  for  by  means  of 
Uffelmann's  reagent,  which  is  made  by  the  addition  of  ferric  chloride  to  dilute 
carbolic  acid.  The  purple  solution  thus  produced  is  at  once  changed  to  yellow 
by  the  addition  of  even  traces  of  lactic  acid. 

We  get  a  similar  formation  of  lactic  acid  in  excised 
mammalian  muscles,  which  are  kept  alive  by  an  artificial 
circulation.  We  do  not  know  how  far  the  formation  of  lactic 
acid  occurs  under  normal  circumstances  in  the  living  body. 
At  all  events  any  lactic  acid  produced  by  the  muscle  during 
activity  in  the  normal  animal,  is  further  transformed  (to  CO.,) 
before  it  leaves  the  body.  During  severe  muscular  exercise 
however,  lactic  acid  may  be  formed  in  the  muscles  and  escape 
oxidation  in  the  body,  so  that  it  is  excreted  in  the  urine. 

The  second  substance,  carbon  dioxide,  is  continually  being 
formed  by  all  living  tissues,  and  is  the  end-product  of 
practically  all  the  carbon  metabolism  of  the  body.  If  a 
muscle  be  hung  up  in  a  confined  space,  it  will  be  found  to 
take  up  oxygen  and  give  off  CO.^ ;  and  these  interchanges  are 
quickened  by  causing  the  muscle  to  contract. 

It  is  found  however  that  this  effect  of  activity  is  dependent 
on  the  composition  of  the  gas  surrounding  the  muscle.  If  it 
be  hung  up  in  a  vacuum  or  in  an  atmosphere  of  nitrogen 
or  hydrogen,  there  is  a  slow  evolution  of  COo,  which  is  not 
appreciably  quickened  during  contraction,  and  seems  to  be 
conditioned  by  a  gradual  driving  off  of  CO2  from  the  alkaline 
carbonates  in  the  muscle,  as  a  result  of  the  steady  production 
of  lactic  acid  which  precedes  the  onset  of  rigor.  If  however 
the  muscle  be  suspended  in  an  atmosphere  of  pure  oxygen,  the 
formation  of  acid  is  diminished  or  abolished ;  but  now  each 
contraction  of  the  muscle  is  followed  by  an  increased  evolution 
of  carbon  dioxide. 


THE   CONTRACTILE   TISSUES  137 

We  see  therefore  that,  accordmg  to  the  environment  of 
the  muscle,  its  activity  is  attended  by  the  formation  either  of 
lactic  acid  or  of  carbon  dioxide,  the  latter  substance  being  the 
sole  product  if  sufficient  oxygen  be  supplied  to  the  muscle.  If 
the  supply  of  oxygen  be  inadequate,  both  substances  are  pro- 
duced, the  proportion  of  lactic  acid  varying  according  to  the 
relative  inadequacy  of  the  oxygen  supply.  This  relation  holds 
good  both  in  rest  and  activity,  the  effect  of  activity  being 
merely  to  increase  the  chemical  changes  which  are  going  on 
spontaneously  in  the  surviving  resting  muscle. 

It  is  an  interesting  point  to  determine  whether  we  have 
here  really  two  alternative  chemical  mechanisms  for  the  pro- 
duction of  energy.  We  know  that  sugar  can  be  utilised  by 
muscle  as  a  food  and  source  of  energy.  It  has  been  suggested 
therefore  that,  in  the  absence  of  oxygen,  the  energy  for  con- 
traction was  derived  from  a  process  of  disintegration,  each 
molecule  of  grape  sugar  breaking  down  into  two  molecules  of 
lactic  acid,  thus  : 

C,H,A=2C3H,303. 

Sugar        Lactic  acid 

On  the  other  hand,  in  the  presence  of  sufficient  oxygen,  the 
sugar  would  be  entirely  oxidised  with  the  formation  of  COo 
and  water,  thus : 

C,H,  A  +  602=6C02  +  6H,0. 

Sugar 

The  change  from  sugar  to  lactic  acid  involves,  however, 
practically  no  evolution  of  energy — so  that  in  the  absence  of 
oxygen  the  energy  of  contraction  must  be  derived  from  some 
other  source. 

It  seems  more  probable  that  we  are  dealing  here  with  two 
stages  of  one  process,  and  that  in  the  muscle  under  normal 
conditions  {i.e.  richly  supplied  with  oxygen)  the  first  chemical 
change  is  one  of  disintegration,  leading  to  the  formation  of 
lactic  acid  (and  probably  other  substances),  and  tliat  this  is 
followed  by  a  process  of  oxidation,  in  which  all  the  products  of 
the  first  stage  are  converted  into  CO.,  which  can  be  rapidly 
eliminated  from  the  muscle.  If  the  supply  of  oxygen  is 
deficient,  the  products  of  the  first  stage  remain  in  the  muscle, 
giving  rise  to  the  phenomena  of  fatigue,  and  finally  inducing 


138  PHYSIOLOGY 

the  coagulation  of  the  muscle  proteins  which  determines  rigor 
mortis. 

We  may  therefore  conceive  of  the  living  protoplasm  of 
the  muscle  during  contraction  as  using  stored-up  carbo- 
hydrate or  allied  substances,  and  oxygen  to  furnish  the 
necessary  energy  ;  but  it  is  also  possible  that  the  whole  living 
molecule  breaks  down  with  the  formation  of  a  still  living 
protein  residue,  and  waste  products  such  as  COo  and  lactic 
acid.  Immediately  afterwards  this  living  residue  takes  up 
oxygen  and  carbon-containing  substances  from  the  surround- 
ing lymph  or  interstitial  substance  of  the  muscle,  and  is 
ready  once  more  to  break  down  and  furnish  energy  in  the 
process. 

A  hypothetical  muscle  molecule  of  this  nature  is  spoken 
of  as  inogen. 

The  facts,  that  the  main  products  of  muscular  activity  are 
CO2  and  lactic  acid,  and  that  no  change  has  been  found  to 
occur  in  the  creatin  or  other  nitrogenous  extractives  of  the 
muscle  during  contraction,  have  produced  a  widespread  idea 
that  the  sole  source  of  muscular  energy  is  to  be  sought  in 
the  combustion  of  carbohydrate  or  fatty  material,  the  proteins 
of  the  body  taking  no  part  in  the  process.  The  whole 
question  will  have  to  be  discussed  more  fully  later  on,  when 
dealing  with  the  general  metabolism  of  the  body.  I  may  say 
here  however  that,  so  far  as  the  evidence  goes,  we  cannot 
draw  any  qualitative  distinction  between  the  metabolism 
which  results  in  muscular  work,  and  the  metabolism  of  the 
resting  animal.  Thus  the  relative  proportion  of  the  CO2 
produced   to  the   oxygen  taken  in,  the  so-called  respiratory 

CO 

quotient  ^  '\  will  vary  according  to  the  food  that  is  being  con- 
sumed, being  unity  with  carbohydrates,  less  than  unity  with 
proteins,  and  still  less  with  fats.  It  is  found  that  muscular 
work  does  not  alter  the  respiratory  quotient  :  i.e.  during 
work  the  qualitative  metabolism  of  the  whole  body  is  the 
same  as  during  rest.  We  must  conclude  therefore  that  the 
muscle  derives  its  energy  from  the  combustion  of  all  three 
classes  of  foodstuffs,  although  in  the  absence  of  excess  of 
food,  it  will  perform  its  work  at  the  expense  of  stored-up  fat 
or  carbohydrate. 

The  absence  of  change  in  the  respiratory  quotient  during 


THE   CONTRACTILE   TISSUES  139 

exercise  shows  moreover  that  in  a  muscle  mider  normal 
conditions  the  two  processes,  viz.  the  taking  in  of  oxygen  and 
the  giving  out  of  CO,,,  keep  pace  one  with  the  other.  In 
warm-blooded  animals,  the  shutting  off  of  the  oxygen  supply 
rapidly  induces  paralysis  and  loss  of  irritability  of  the  muscles. 
This  fact,  coupled  with  the  fact  that,  as  mentioned  above,  the 
final  results  of  muscular  activity  differ  according  as  the 
muscle  is  or  is  not  supplied  with  oxygen,  suggests  that  the 
oxygen  takes  part  in  the  process  of  activity  only  at  a  late 
stage,  after  the  disintegration  of  the  complex  living  molecule 
has  already  begun. 

Such  a  conclusion  is,  liowever,  opposed  to  the  generally  accepted  views  on 
the  nature  of  the  oxidation  processes  in  the  cell.  According  to  Hermann, 
Pfliiger,  Verworn  and  others,  there  is  during  rest  a  building  up  both  of  oxygen 
and  food  material  into  the  living  molecule.  Activity  consists  in  a  rearrange- 
ment of  the  molecule,  with  the  assumption  of  more  stable  positions  by  the 
oxygen  and  carbon  atoms,  and  a  consequent  production  of  C0._,  (Cp.  the 
explosion  of  gun-cotton  or  nitroglycerin).  The  presence  of  this  intramolecular 
oxygen  in  an  unstable  position  would  be  a  necessary  condition  both  for  the 
irritability  as  well  as  for  the  activity  of  all  forms  of  living  tissue,  especially 
muscle  and  nerve. 

If  the  muscle  can  use  all  classes  of  foodstuffs  in  its  meta- 
bolism, one  would  expect  to  find  some  change  in  the  nitro- 
genous constituents  as  the  result  of  activity.  Physiologists 
have  searched  in  vain  however  for  any  evidence  of  the 
formation  of  creatin  or  urea  in  excised  muscle  during  con- 
traction. Schondorft"  has  recently  shown  that  if  excised 
muscle  be  kept  alive  by  perfusion  of  defibrinated  blood,  its 
activity  is  associated  with  increased  formation  of  ammonia. 
The  formation  of  ammonia  is  however  the  natural  mode  of 
protection  of  the  whole  organism  against  acid  poisoning,  and 
it  seems  quite  probable  that  in  Schondorff's  experiments  the 
ammonia  formation  was  simply  a  secondary  result  of  the  lactic 
acid  formation,  and  not  a  direct  expression  of  the  metabolism 
of  the  active  muscle. 


140  PHYSIOLOGY 


Section    7 

CONDITIONS    MODIFYING    THE     lERITABILITY     AND 

CONTEACTION   OP  MUSGULAE  TISSUE 

Temperature 

Speaking  generally,  the  effect  of  warming  a  muscle  is  to 
quicken  all  its  processes.  The  latent  period  becomes  shorter 
and  the  muscle  curve  steeper  and  shorter. 

It  is  very  often  observed  that  the  height  of  contraction  of  tlie  warmed  muscle 
is  greater  than  tliat  obtained  at  ordinary  temperatures.  It  seems  that  this 
apparent  increase  in  height  is  really  instrumental  in  origin,  the  quicker-moving 
muscle  jerking  the  lever  beyond  the  real  extent  of  the  contraction.  If  proper 
means  are  taken  to  eliminate  this  overshooting  of  the  lever,  it  is  found  that  the 
height  of  contraction  is  unaltered  between  5°  and  20^  C,  the  only  change  being 
in  the  time-relations  of  the  curves. 

If  a  muscle  be  heated  gradually  (without  stimulation)  up 
to  about  45°  C.,  it  begins  to  contract  slowly  at  about  34°  C., 
and  this  contraction  reaches  its  maximum  at  45°  C.,  at  which 
point  the  muscle  has  entered  into  pronounced  rigor  mortis. 

Cold  has  the  reverse  effect.  The  intra-molecular  processes 
which  lie  at  the  root  of  the  muscular  activity  are  slowed,  so 
that  the  latent  period  as  well  as  the  contraction  period  is 
prolonged.  Curiously  the  action  of  cold  on  the  excitability  of 
muscle  is  to  increase  it,  so  that  any  form  of  stimulus  is  more 
effective  at  5°  C.  than  at  25°  C.  Moreover,  when  maximal 
stimuli  are  being  used,  and  the  muscle  is  heavily  loaded,  the 
first  effect  of  the  application  of  cold  may  be  to  increase  the 
height  as  well  as  the  duration  of  contraction,  for  the  same 
reason  that  a  gentle  prolonged  push  is  more  efficacious  in 
closing  a  door  than  would  be  a  heavy  blow  with  a  hammer. 

If  however  a  muscle  be  cooled  for  a  short  time  to  zero 
or  a  little  below,  it  loses  its  irritability,  which  returns  if  the 
muscle  be  gradually  warmed  again. 

Prolonged  exposure  to  severe  cold,  sufficient  to  cause  the 
formation  of  ice  in  the  muscle  fibres,  irrevocably  destroys  its 
irritability.  Warming  the  muscle  now  will  simply  bring  about 
rigor  mortis. 


THE   CONTRACTILE   TISSUES  141 

Fatigue 

A  muscle  will  not  go  on  contracting  indefinitely.  If  it  be 
repeatedly  stimulated,  changes  soon  become  apparent  in  the 
curve  of  contraction.  The  latent  period  is  prolonged,  as  well 
as  the  length  of  the  contractions  ;  the  absolute  height  and 
work  done  are  diminished.  At  the  same  time  the  muscle 
does  not  return  to  its  original  length  ;  the  shortening  which 
remains  is  spoken  of  as  'contraction  remainder.'' 

After  an  initial  rise  during  the  first  few  contractions, 
these  diminish  uniformly  in  height  till  they  are  no  longer 
apparent,  so  that  the  muscle  is  now  said  to  have  lost  its 
irritability, 

Fic;.  r,2. 


Muscle  curves  showing  fatigue  in  consequence  of  repeated  stimula- 
tion. The  first  six  contractions  are  numbered,  and  show  the 
initial  increase  of  the  first  three  contractions.     (Brodie.) 


At  the  same  time  there  is  a  great  prolongation  of  the 
curve,  occasioned  almost  entirely  by  a  retardation  of  the 
relaxation,  so  that  after  forty  or  fifty  contractions  several 
seconds  may  elapse  before  the  lever  returns  to  the  base  line. 

The  fact  that  the  relaxation  part  of  the  muscle  citrve  is  affected  by  various 
conditions,  especially  fatigue,  apparently  independently  of  the  contraction  part, 
led  Fick  to  put  forward  a  theory  that  two  distinct  processes  were  concerned  in 
the  response  of  a  muscle  to  excitation,  one  process  causing  the  active  shortening 
and  the  other  the  relaxation.  (It  must  be  noted  that  this  is  not  the  same  as 
saying  that  the  lengthening  is  an  active  process,  a  statement  negatived  by  the 
behaviour  of  a  muscle  when  caused  to  contract  on  mercury.)  He  suggested  that 
the  disintegration  associated  with  activity  might  be  conceived  as  occurring  in  two 
stages  :  the  first  resulting  in  the  production  of  sarcolactic  acid  and  the  active 
shortening  of  the  muscle ;  the  second  in  the  further  conversion  of  the  acid  into 


142  PHYSIOLOaY 

CO.,,  with  a  consequent  relaxation.  A  retardation  of  this  second  pliase  would 
cause  the  prolonged  curve  with  '  contraction  remainder '  observed  in  a  fatigued 
muscle.  The  absence  of  any  appreciable  evolution  in  the  conversion  of  glucose 
to  lactic  acid,  shows  however  that  the  formation  of  lactic  acid  cannot  repre- 
sent the  whole  of  the  chemical  changes  involved  in  the  phase  of  shortening. 

If  left  to  itself,  the  muscle  which  has  been  exhausted  by 
repeated  stimulation  will  recover.  The  recovery  is  hastened 
by  passing  a  stream  of  blood,  or  even  of  salt  solution,  through 
the  blood-vessels  of  the  muscle.  Recovery  however  in  a 
muscle  outside  the  body  is  never  complete. 

The  phenomena  of  fatigue  prol)ably  depend  on  two 
factors : 

(1)  The  consumption  of  the  contractile  material  or  the 
substances  available  for  the  supply  of  potential  energy  to  this 
material. 

(2)  The  accumulation  of  waste  products  of  contraction. 
Of  these  waste  products  the  lactic  acid  is  of  great  import- 
ance. Fatigue  may  be  artificially  induced  in  a  muscle  by 
'  feeding  '  it  with  a  dilute  solution  of  lactic  acid,  and  this 
again  removed  by  washing  out  the  muscle  with  normal  saline 
solution  containing  a  small  percentage  of  alkali.  After  a 
certain  time  the  mere  removal  of  waste  products  by  means 
of  an  artificial  circulation  of  salt  solution  becomes  inadequate 
to  restore  contractile  power  to  the  muscle.  In  this  case  the 
muscle  can  be  made  to  contract  once  more  by  supplying  it 
with  fresh  food  material,  as  by  the  circulation  of  serum  or 
diluted  blood. 

The  Action  of  Drugs  upon  Vohintary  Muscle 

Of  the  drugs  that  have  a  direct  action  on  muscle,  the 
most  remarkable  is  veratrin,  which  causes  an  excessive  pro- 
longation of  a  muscular  contraction  (produced  by  a  single 
stimulus) .  Thus  the  '  twitch  '  of  a  muscle  poisoned  with 
veratrin  may  last  fifty  or  sixty  seconds,  instead  of  the  normal 
one-tenth  of  a  second  (Fig.  63). 

Barium  salts  have  a  similar  though  less  marked  effect. 

In  order  to  carry  out  the  poisoning  with  veratrin,  very 
weak  solutions  (1  in  100,000  or  1  in  1,000,000  of  normal 
saline)  should  be  used  and  the  muscle  exposed  to  its  action 
for  some  hours.  We  get  then  on  a  single  stimulus  a  resj)onse 
lasting  many  seconds  and  exactly  similar  in  height  and  form  to 


THE   CONTRACTILE   TISSUES 


143 


a  tetanus  obtained  by  discontinuous  stimulation.  If  stronger 
solutions  be  used,  the  action  of  the  drug  is  apt  to  affect  the 
fibres  unequally,  so  that  we  may  have  a  sharp  normal  twitch 
preceding  the  prolonged  contraction  (Fig.  64).     If  the  muscle 


Fi<;.  (33. 


A.  Tracing  of  the  contraction  of   a  frog's   sartorius,  poisoned  with 

veratrin,  in  response  to  a  momentary  stimulus.  The  time- 
marking  indicates  seconds. 

B.  Tetanic  contraction  of   normal  sartorius   in   response   to   rapidly 

interrupted  stimuli.  (The  duration  of  the  stimulus  is  indi- 
cated by  the  words  '  on '  and  '  off.')  It  will  be  noticed  that 
the  two  curves  are  practically  identical.     (Miss  Buchanan.) 

be  excited  immediately  after  the  prolonged  contraction  has 
passed  away,  it  responds  with  a  single  twitch  like  a  normal 
muscle,  but  if  allowed  to  rest  a  few  minutes,  stimulation  is 
again  followed  by  the  peculiar  long  drawn  out  contraction. 

Fig.  64. 


Excitation. 
'uvj"^\A.<.vla7\jLv_«-^^_/lA-/u>_ajlA_aji.aj'  Seconds. 

Tracing  of  the  contraction  of  a  muscle  poisoned  by  the  injection  of 
a  strong  solution  of  veratrin,  showing  the  double  contraction  due 
to  unequal  poisoning  of  different  fibres.     (Biedermann.) 

The  action  of  sodium  salts  on  muscle  is  of  considerable 
interest.  We  are  accustomed  to  use  a  0*6  per  cent,  solution 
of  NaCl  as  a  '  normal   fluid '   to   keep   muscle  preparations 


144  PHYSIOLOGY 

moist.  If  however  the  solution  be  made  with  distilled  water, 
it  has  a  distinctly  excitatory  effect  upon  the  muscle,  so  that 
single  induction  shocks  may  cause  tetaniform  contractions. 
The  same  excitatory  effect  is  still  better  marked  with  solutions 
of  Na.^CO.,  (Fig.  65).  If  a  thin  muscle,  such  as  a  frog's  sar- 
torius,  be  immersed  in  a  solution  containing  0*5  per  cent.  NaCl, 
0*2  per  cent.  Na.^HPO^,  and  0-04  per  cent.  Na^COg  (Biedermann's 
fluid),  the  muscle  enters  into  a  series  of  frequent  contractions, 
so  that  it  may  wriggle  from  side  to  side,  or  may  even  '  beat ' 
for  a  time  with  the  regularity  of  heart  muscle,  though  at  a 
much  greater  rate. 

This  excitatory  action  of  sodium  salts  is  neutralised  by  the 
addition  of  traces  of  calcium  salts.  Hence  the  normal  saline 
used  in  the  laboratory  should  always  be  made  with  tap  water, 
containing  calcium  salts. 

Potassium  salts,  although  forming  so  important  a  con- 
stituent of  the  ash  of  muscle,  act  as  muscle  poisons,  quickly 
and  permanently  destroying  its  irritability.  If  a  muscle  be 
transfused  with  normal  fluids  containing  minute  traces  of 
potassium  salts,  it  at  once  shows  all  the  signs  of  fatigue, 
signs  which  may  be  removed  by  washing  out  the  potassium 
salts  by  means  of  0-6  per  cent.  NaCl  solution.  It  is  possible 
that  the  setting  free  of  potassium  salts  may  be  one  of  the 
factors  involved  in  the  development  of  the  normal  fatigue  of 
muscle. 


THE   CONTEACTILE   TISSUES  145 


Section  8 
VOLUNTARY   CONTEACTION 

In  the  light  of  our  knowledge  gamed  chiefly  by  electrical 
excitation  of  muscles,  we  have  now  to  inquire  into  the  nature 
of  ordinary  voluntary  contraction  as  it  occurs  in  the  muscles 
in  the  body. 

The  fact  that  it  is  difficult  to  get  an  artificial  prolonged 
contraction  in  excised  muscles  except  by  repeated  stimula- 
tion— that  is,  by  summation  of  single  twitches  to  form  a 
tetanus — has  led  us  to  view  almost  every  voluntary  contraction 
as  a  tetanus.     What  further  evidence  is  there  for  this  view  ? 

Muscle- sound. — If  we  place  the  end  of  a  stethoscope  over 
the  biceps  muscle,  and  listen  while  we  voluntarily  contract  the 
muscle,  a  low  sound  is  heard.  This  is  the  muscle-sound,  and 
invariably  accompanies  any  voluntary  contraction.  Its  tone 
corresponds  to  about  thirty-six  vibrations  a  second,  and  it 
was  thought  to  be  the  first  overtone  of  a  note  of  eighteen 
vibrations  per  second,  which  therefore  was  looked  upon  as  the 
rhythm  of  '  voluntary  tetanus.' 

But  one  hears  exactly  the  same  note  when  listening  to 
any  irregular  sound  of  low  intensity.  The  roar  of  London 
that  we  hear  in  the  middle  of  Hyde  Park  has  the  same 
pitch  as  the  muscle-sound  of  our  contracting  biceps.  And 
really  this  muscle-sound  proves  nothing  about  the  number  of 
contractions  composing  voluntary  tetanus,  for  it  is  the  natural 
resonance-tone  of  the  ear,  and  therefore  selected  and  inten- 
sified in  the  ear  when  we  listen  to  any  irregular  mixture  of 
tones  and  noises. 

Tracings  of  most  voluntary  contractions  show  superficial 
vibrations  of  eight  to  twelve  a  second,  and  this  rhythm  is 
seen  in  many  movements,  such  as  the  clonic  convulsions  of 
epilepsy,  and  some  diseases  of  the  spinal  cord.  This  irregu- 
larity would  quite  well  account  for  the  muscle-sound. 

But  we  can  get  tracings  of  natural  contractions  showing 
eight  or  ten  complete  twitches  in  the  second  (contraction  and 
relaxation),  or  a  continued  contraction  with  eight  or  ten 
superficial  waves. 

10 


146  PHYSIOLOGY 

So  it  is  not  sufficient  to  say  that  voluntary  contraction  is 
a  tetanus  of  eight  or  twelve  per  second.  If  it  is  a  tetanus, 
there  must  be  some  means  by  which  each  individual  contrac- 
tion can  be  shortened  till  it  is  distinct,  or  lengthened  till 
it  fuses  with  the  next  contraction  to  produce  an  unbroken 
tetanus.  And  we  must  remember  that  the  electrical  stimulus 
differs  in  many  of  its  effects  from  the  natural  stimulus,  and 
so  not  transfer  all  our  results  of  electrical  stimulation  too 
unreservedly  to  the  contraction  of  muscles  in  the  living  body 
in  response  to  the  will. 

We  have  in  fact  definite  evidence  that  discontinuity  is  not 
essential  for  the  production  of  prolonged  contraction.  Thus 
we  have  already  seen  that,  during  the  passage  of  a  constant 
current  through  muscle,  there  is  a  continuous  contraction  in 

Fig.  65. 


/V/lA/wtwA^. 


Continued  contraction  followed  by  rhythmic  contractions  of  a 
muscle  in  response  to  a  constant  stimulus  (Biedermann).  The 
muscle  was  excited  by  the  ^lassage  of  a  constant  current,  the 
kathodal  end  having  been  moistened  with  a  weak  solution  of 
Na,CO,. 


the  neighbourhood  of  the  kathode.  If  the  irritability  of  the 
muscle  at  this  point  be  increased  by  the  application  of  a 
solution  of  sodium  carbonate,  this  excitation  is  propagated  to 
the  rest  of  the  muscle  and  we  get  a  prolonged  contraction  on 
closure  of  the  current,  followed  by  rhythmic  contractions 
(Fig.  65). 

Moreover  in  excitable  frogs  (frogs  which  have  been  kept 
at  about  2°  or  3°  C.  for  some  days)  the  closure  of  a  descend- 
ing current  through  the  nerve  causes  continued  contraction 
of  the  muscle,  and  in  the  same  way  there  may  be  a  prolonged 
contraction  at  the  opening  of  an  ascending  current  through 
the  nerve. 

Attempts  have  been  made  to  decide  the  question  by 
recording  the  electrical   changes   accompanying   the  natural 


THE    CONTEACTILE   TISSUES 


147 


contractions  of  a  muscle,  i.e.  those  excited  reflexly  through 
the  central  nervous  system.  It  was  long  ago  shown  by 
Loven  that  a  certain  discontinuity  could  be  seen  in  records 
of  such  changes,  as  of  the  mechanical  changes  ;  but  a 
renewed   investigation  of   the*  sul)ject  by  Burdon- Sanderson 

Fk;.  m. 


Electrometer  records  of  the  electrical  variations  of  frog's  muscle 
under  different  conditions  (Burdon-Sanderson).  a,  excited  by 
induction-shocks  14  times  per  second ;  i;,  excited  for  periods  of 
i  sec.  by  rapidly  repeated  induction-shocks  (60  per  second)  alter- 
nating with  equal  periods  of  rest ;  c,  strychnine  spasm  ('  natural ' 
contraction). 


has  proved  conclusively  that  this  discontinuity  is  so  to  speak 
an  accident,  and  an  expression  of  the  tendency  of  the  spinal 
cord  to  vhythmic  activity.  If  the  electrical  changes  of  a 
strychnine  spasm  l)e  photographed  by  means  of  the  capillary 
electrometer,  it  will  be  seen  that  each  individual  spasm  or 
twitch  is  not  a  simple  muscle  twitch,  but  a  prolonged  con- 


148  PHYSIOLOGY 

traction  similar  to  a  short  tetanus.  This  is  well  shown  in  the 
accompanying  photographs  (Fig.  66,  c).  The  period  elapsing 
between  the  beginning  and  the  culminating  point  of  the 
electrical  change  of  a  single  twitch  is  about  tx/Vo  of  a  second 
(Figs.  59  and  66,  a).  In  the  spasms  making  up  the  reflex 
(natural)  contraction,  the  duration  of  the  electrical  change 
is  more  than  yfifo  of  a  second  (Fig.  66,  c).  The  second 
photograph  (Fig.  66,  b)  shows  how  we  may  imitate  such  a 
series  of  changes,  not  by  a  series  of  single  stimuli,  but  by 
tetanising  for  a  succession  of  periods  of  0*2  second  alternated 
with  equal  periods  of  rest. 

It  seems  therefore  highly  probable  that  the  normal  reflex 
or  voluntary  contraction  is  not  a  tetanus  (a  condition  which 
is  a  product  of  the  physiological  laboratory),  but  a  prolonged 
single  contraction  caused  by  a  constant  stimulus  arriving  at 
the  muscle  from  the  central  nervous  system. 


THE    CONTRACTILE   TISSUES  149 


Section    9 
OTHER   FORMS   OF  CONTRACTILE  TISSUE 

Smooth  or  Unstriated  Muscle 

The  little  we  know  about  the  physiology  of  unstriated 
muscle  is  derived  chiefly  from  experiments  on  the  intestine, 
ureter,  l)ladder,  and  retractor  penis.'  This  tissue  ditiers  from 
voluntary  muscle  in  containing  numerous  plexuses  of  nerve - 
fibres  (non-medullated)  and  ganglion-cells,  so  that  in  all  our 
researches  it  is  difficult  to  be  certain  whether  the  results  are 
due  to  the  muscle-fibres  themselves,  or  to  the  nerves  and 
nerve-cells  which  are  so  intimately  connected  with  them  ; 
especially  as  we  have  as  yet  no  convenient  drug  like  curare, 
by  aid  of  which  we  might  discriminate  between  action  on 
muscle  and  action  on  nerve. 

The  differences  between  unstriated  and  voluntary  muscle, 
although  at  first  sight  very  pronounced,  on  further  investiga- 
tion prove  to  be  in  most  cases  differences  of  degree  only  ; 
qualities  and  reactions  which  are  marked  in  involuntary 
muscle  being  also  present  in  a  minor  degree  in  the  more 
highly  differentiated  tissue. 

The  contraction  of  smooth  muscle  is  so  sluggish  that  the 
various  stages  of  latent  period,  shortening,  and  relaxation  can 
be  easily  followed  with  the  eye.  The  latent  period  may  be 
from  0*2  to  0*8  second,  and  the  contraction  may  last  from 
half  to  three  minutes. 

In  consequence  of  this  lack  of  differentiation,  the  smooth 
muscle  preserves  many  of  the  properties  of  undifferentiated 
protoplasm,   especially   an   automatic   power   of   contraction, 

'  The  retractor  penis,  which  is  found  in  the  dog,  cat,  horse,  hedgehog  (but 
not  in  rabbit  or  man),  is  a  thin  band  of  longitudinally  arranged  unstriated 
muscle,  which  is  inserted  at  the  attachment  of  the  prepuce,  and  is  continued  back- 
wards in  a  sheath  of  connective  tissue  to  the  bulb,  where  it  divides  into  two 
slips  which  pass  on  either  side  of  the  anus.  It  is  innervated  from  two  sources, 
the  motor  fibres  being  derived  from  the  lumbar  sjnnpathetic  and  running  to  the 
muscle  in  the  internal  pudic  nerve,  while  the  inhibitory  fibres  run  in  the  pelvic 
visceral  nerves  (nervi  erigentes)  and  are  derived  from  the  second  and  third 
sacral  nerve-roots. 


150  PHYSIOLOGY 

which  is  regulated  by  the  condition  of  the  muscle.  Thus 
whereas  the  voluntary  muscle  is  intimately  dependent  on  its 
connection  with  the  central  nervous  system,  and  in  the 
absence  of  this  is  reduced  to  a  flabby  inert  tissue,  the  smooth 
muscle  isolated  from  all  its  nervous  connections  presents  in 
many  cases  rhythmic  contractions,  and  can  carry  out  a 
peripheral  adaptation  to  its  environment. 

These  rhythmic  contractions  are  almost  invariably  ob- 
served if  the  muscular  tissue  be  subjected  to  a  certain  amount 
of  tension,  after  separation  from  the  central  nervous  system. 
The  rhythm  of  the  contractions  may  vary  from  one  (spleen) 
to  twelve  (small  intestine)  contractions  in  the  minute. 

The  stimuli  for  smooth  muscle  are  essentially  the  same  as 
for  striated.  As  we  should  expect  however  from  the  sluggish 
response  of  this  kind  of  contractile  tissue,  the  optimum  rate 
of  change  of  current  which  excites  is  very  much  slower  than 
in  the  case  of  striated  muscle.  Thus  in  many  instances  a 
single  induction  shock,  even  if  very  strong,  is  powerless  to 
excite  contraction,  and  the  make-induction  shock  of  long 
duration  and  low  intensity  is  always  more  efficacious  than  the 
short  sharp  break-induction  current.  A  still  better  stimulus 
is  the  make  or  break  of  a  constant  current.  When  the  latter 
form  of  stimulation  is  used,  response  occurs  at  the  make 
sooner  than  at  the  break,  and,  just  as  in  voluntary  muscle, 
the  make  excitation  starts  from  the  kathode  and  the  break 
excitation  from  the  anode. 

An  apparent  exception  to  this  statement  is  afforded  by  the  behaviour  of 
certain  forms  of  involuntary  muscle.  In  the  intestine,  in  the  skin  of  worms, 
and  in  many  other  muscular  tubes,  the  smooth  muscle-fibres  are  arranged  in 
two  definite  sheets,  one  consisting  of  longitudinal,  the  other  of  circular  fibres. 
If  non-polarisable  electrodes,  connected  with  a  constant  source  of  current,  be 
applied  to  the  surface  of  the  small  intestine,  when  the  current  is  made  there 
will  be  apparently  a  strong  contraction  of  the  circular  coat  at  the  anode,  which 
spreads  up  and  down  the  intestine,  and  a  linear  contraction  of  the  longitudinal 
coat  at  the  kathode.  The  same  result  is  observed  in  the  earthworm  and  leech. 
But  careful  observation  shows  in  each  case  that  the  irregularity  is  really  only 
apparent,  and  that  in  the  immediate  neighbourhood  of  the  anode  there  is 
relaxation  of  both  coats,  with  a  contraction  of  the  circular  coat  on  each  side, 
and  that  at  the  kathode  there  is  a  contraction  of  both  coats.  The  accom- 
panying diagram  (Fig.  67)  will  serve  to  show  the  condition  of  the  circular  coat 
at  each  electrode. 

As  a  matter  of  fact,  in  consequence  of  the  arrangement  of  the  fibres,  we  have  in 
the  neighbourhood  of  the  anode  a  number  of  places  (virtual  kathodes)  where 
the  current  is  leaving  the  muscle  cells  to  enter  inert  conducting  tissues,  and  in 


THE   CONTRACTILE   TISSUES 


151 


the  same  way  there  will  be  in  the  neighbourhood  of  the  kathode  a  number  of 
virtual  anodes.  Thus  if  we  take  the  ureter  and  lead  a  current  through  it 
while  it  is  slung  up  in  thread  loops  serving  as  electrodes,  there  is  contraction  of 
both  coats  at  the  kathode  and  relaxation  of  both  at  the  anode.     If  however  the 

Fig.  67. 


At  the  kathode  k  there  is  a  small  line  of  constriction,  surrounded 
by  an  area  of  relaxation.  At  the  anode  itself  the  muscle  is  re- 
laxed, but  is  strongly  contracted  on  each  side  of  the  anode,  so 
that  on  rough  observation  it  would  be  thought  that  contraction 
occurred  at  the  anode  itself. 


ureter  be  packed  in  a  pulp  of  blotting  paper  moistened  with  normal  saline, 
thus  allowing  the  current  to  leave  the  contractile  tissues  anywhere  along  the 
ureter,  we  get  the  same  aberrant  results  of  stimulation  as  are  obtained  with  the 
intestine. 

Snmmation 

^Ye  have  already-  seen  that  if   two  stimuli  be  sent  into 
a  voluntary  muscle  within  a  short  interval  of  time,  there  is 

Fifi.  68. 


Diagram  to  show  the  spread  of  current  which  occurs  when  a  current 
is  led  through  a  tube  such  as  the  ureter  by  means  of  two  elec- 
trodes applied  to  its  surface.  It  will  be  noticed  that  while  +  E 
is  the  anode,  there  are  immediately  below  and  around  it  a  number 
of  kathodes  e^,  e^,,  e^,,,  e,,,,,  due  to  the  current  leaving  the  muscle 
to  flow  through  indift'erent  tissues.     (Biedermann.) 


a  summation  of  effect,  the  contraction  due  to  the  second 
stimulus  being  piled  so  to  speak  on  the  top  of  the  first  con- 
traction. That  a  maximal  twitch  is  not  as  high  as  a  tetanus, 
the  product  of  summation  of  many  twitches,  is  due   to  the 


152 


PHYSIOLOGY 


fact  that  the  relaxation  processes  of  a  muscle  begin  before 
it  has  time  to  overcome  the  inertia  of  the  mass  moved,  and 
so  accomplish  its  maximum  shortening.  If  therefore  we 
support  the  muscle  in  any  way,  whether  by  screwing  up  the 
lever  (after-loading)  or  by  sending  in  a  previous  stimulus,  the 
contraction  due  to  a  stimulus  will  be  more  pronounced,  until 
the  shortening  of  the  muscle  attains  that  observed  in  tetanus. 
For  the  same  reason  the  height  of  a  single  twitch  in  relation 
to  a  tetanus  of  the  same  muscle  increases  as  we  sloio  the 
contraction,  until,  with  a  prolongation  such  as  is  produced  by 
veratrin,  there  is  no  difference  at  all  l)etween  the  height  of 
a  maximal  single  contraction  and  the  height  of  a  tetanus 
(Fig.  G3,  p.  143). 

Fig.  r,9. 


Contractions  of  a  frog's  muscle.  Two  single  twitches  are  followed 
by  a  tetanus,  which  is  almost  twice  as  high  as  a  single  contrac- 
tion. After  two  more  single  twitches,  the  drum  was  made  to 
rotate  more  slowly,  and  single  shocks  employed,  at  the  same  time 
as  the  '  after-loading  '  was  continually  increased.  It  can  be  seen 
that  the  curve  obtained  in  this  way  is  as  high  as  the  original 
tetanus,     (v.  Frey.) 


These  considerations  would  lead  us  to  expect  no  trace  of 
any  process  analogous  to  summation  of  contraction  in  the 
slowly  moving  smooth  muscle.  In  the  heart  muscle  this  is 
the  case,  no  increase  in  the  height  of  a  contraction  being 
produced  by  sending  in  one,  two,  or  more  shocks  in  quick 
succession.  When  however  we  record  the  contractions  of  a 
muscle,  such  as  the  retractor  penis,  which  is  more  closely 
under  the  control  of  the  nervous  system,  and  excite  with  a 
series  of  induction  shocks,  we  get  results  which  at  first  sight 
are  exactly  analogous  to  the  summation  of  contraction  in  a 
voluntary  muscle.  It  may  be  noticed  however  that  the  first 
three  or  four  stimuli  are  ineffective,  and  that  there  is  in  this 
case  a  summation  before  any  contraction  has  occurred,  a 
summation  of  stimuli.     Each  stimulus  in  fact  alters  the  state 


THE   CONTRACTILE   TISSUES  153 

of  the  contractile  tissue  and  makes  it  more  ready  to  respond 
to  the  next  stimulus,  so  that  the  stimuli  become  more  and 
more  effective.  If  time  is  allowed  for  the  muscle  to  relax 
between  successive  stimuli,  this  summation  is  evidenced  by 
a  continually  increasing  height  of  contraction,  the  so-called 
'  staircase.'  It  will  be  remembered  that  the  same  initial 
increase  of  effect  was  observed  when  voluntary  muscle  was 
excited  by  continually  recurring  stimuli  (v.  Fig.  62,  p.  141). 

We  shall  meet  with  other  examples  of  this  summation  of 
stimuli  when  dealing  with  the  physiology  of  the  central 
nervous  system.  It  is  indeed  a  fundamental  phenomenon  in 
the  physiology  of  excitation. 

Chemical  Stimulation 

Strong  salt  solution  excites  contractions  just  as  in  the  case 
of  skeletal  muscle.  Many  drugs  such  as  physostigmin,  ergot, 
lead  salts,  digitalis,  may  act  directly  on  smooth  muscle  and 
cause  contraction.  As  one  would  expect  however  from  the 
greater  independence  of  the  smooth  muscle,  the  action  of 
these  drugs  varies  from  organ  to  organ,  muscle-fibres,  which 
apparently  are  histologically  identical,  reacting  diversely 
according  to  their  origin. 

Mechanical  Stimulation 

Smooth  muscle  may  react  to  a  local  pinch  or  blow  with  a 
local  or  a  general  (propagated)  contraction.  The  most  impor- 
tant form  of  mechanical  stimulation  is  that  produced  by  ten- 
sion. The  effect  of  increasing  the  tension  on  smooth  muscle 
may  be  twofold  :  causing  in  the  first  place  relaxation  and  in 
the  second  excitation  with  increased  contraction.  These  two 
effects  may  be  illustrated  by  taking  the  case  of  the  bladder. 
If  this  viscus  (which  is  surrounded  by  a  complete  coat  of 
smooth  muscle)  has  all  its  connections  with  the  central  nervous 
system  severed,  it  is  when  empty  in  a  state  of  tonic  contraction. 
If  fluid  be  injected  into  it  rapidly  there  is  a  great  rise  of 
pressure  in  its  cavity,  due  to  the  forcible  distension.  If  how- 
ever the  fluid  be  injected  slowly  the  bladder  muscle  relaxes  to 
make  room  for  it,  so  that  a  considerable  amount  of  fluid  may 
be  accommodated  in  the  bladder   without   any  great  rise  of 


154  PHYSIOLOGY 

pressure.  This  process  of  relaxation  has  however  its  limit. 
If  the  injection  of  fluid  be  continued,  the  walls  begin  to  be 
stretched  passively,  and  this  increased  tension  acts  as  a 
stimulus  causing  marked  rhythmic  contractions  of  the  whole 
bladder. 

In  the  same  way  the  response  of  a  smooth  muscle  to  an 
electrical  stimulus  is  much  increased  by  previous  increase  of 
the  tension  on  the  muscle-fibres. 


Proi^agation  of  the  Excitatory  State,  or  Wave  of 
Contraction 

In  the  case  of  voluntary  muscle  we  have  seen  that,  on 
stimulating  any  part  of  a  muscle-fibre,  a  wave  of  contraction 
is  started  which  travels  to  each  end  of  the  fibre,  but  no  further. 
There  is  no  propagation  from  muscle-fibre  to  muscle-fibre,  the 
synchronous  contraction  of  the  whole  muscle  being  brought 

Fig.  70. 


about  by  simultaneous  excitation  of  all  its  fibres.  This  isola- 
tion of  the  excitatory  state  is  not  found  in  smooth  muscle.  A 
stimulus  applied  to  any  part  of  a  sheet  of  smooth  fibres  may 
travel  all  over  the  sheet  just  as  if  it  were  a  single  fibre.  It 
seems  probable  indeed  that  there  is  protoplasmic  continuity 
by  means  of  fine  bridge-like  processes  between  adjacent  muscle- 
cells.  And  even  in  the  absence  of  such  bridges,  the  propaga- 
tion of  the  contraction  could  be  easily  accounted  for.  Although 
in  the  case  of  voluntary  muscle  the  rule  is  isolated  contraction, 
yet  a  very  small  change  in  the  muscle,  such  as  is  produced 
by  partial  drying  or  by  pressure,  is  sufficient  to  cause  the 
contraction  to  spread  from  one  fibre  to  another.  Indeed  by 
clamping  two  curarised  sartorius  muscles  together,  as  in  the 
diagram  (Fig.  70),  it  is  found  that  stimulation  of  the  muscle 
A  causes  contraction  of  the  muscle  b.  The  current  of  action 
of  A  in  this  case  has  served  to  excite  a  contraction  in  b. 


THE    CONTRACTILE   TISSUES  155 

It  must  be  remembered  that  in  all  nnstriated  muscle  the  fibres  are  sur- 
rounded by  a  network  of  non-medullated  nerve-fibres.  Some  physiologists 
are  inclined  to  ascribe  to  these  fibres  an  important  part  in  the  propagation 
of  the  contraction  wave.  In  the  case  of  the  heart  muscle  however,  it  can 
be  shown  almost  conclusively  that  the  propagation  takes  place  indepen- 
dently of  nerve-fibres,  and  probably  the  same  is  true  for  certain  involuntary 
muscles. 

Infiience  of  Temjyei'atttre 

Smooth  muscle  is  extremely  susceptible  to  changes  of 
temperature  ;  we  may  say  as  a  rule  that  warming  causes 
relaxation,  while  application  of  cold  causes  a  tonic  contraction. 
The  condition  of  the  muscle  at  any  given  time  does  not  depend 
only  on  its  actual  temperature  but  also  on  the  rapidity  with 
which  this  temperature  has  been  reached.  Thus  a  rapid 
cooling  of  the  retractor  penis  muscle  of  a  dog  from  35°  to  25° 
may  cause  a  contraction  as  extensive  as  would  be  produced 
by  a  slow  cooling  to  5°  C.  On  warming  a  muscle  from  30°  to 
50°  C.  it  lengthens  gradually  up  to  about  40°,  and  it  may  then 
undergo  a  marked  heat  contraction  (varying  in  degree  in 
different  muscles)  at  about  50"^  C.  which  may  pass  off  at  a 
somewhat  higher  temperature.  It  is  killed  somewhere  between 
40°  and  50°  C.  It  seems  very  doubtful  whether  any  true 
rigor  mortis  occurs  in  smooth  muscle.  The  hard  contracted 
appearance  of  the  smooth  muscle  in  a  recently  dead  animal 
is  chiefly  conditioned  by  the  fall  of  temperature.  On  excising 
the  muscle  and  warming  it  up  again  to  body  temperature,  it 
may  again  relax  and  show  signs  of  irritability  two  or  three 
days  after  the  death  of  the  animal.  Different  smooth  muscles 
however  vai\v  very  much  in  their  tenacity  of  life. 

Double  Innervation 

We  have  seen  that  voluntary  muscle  is  absolutely  depen- 
dent for  its  activity  on  the  central  nervous  system.  Cut  oft" 
from  this  it  is  flabby  and  motionless.  Its  sole  function  is  to 
contract  efiliciently  and  smartly  on  receipt  of  impulses  arriving 
along  its  nerve.  It  is  only  necessary  therefore  that  these 
impulses  should  be  of  one  character — motor,  and  we  know 
that  each  fibre  of  a  muscle,  such  as  the  sartorius,  receives  one 
efferent  nerve-fibre  terminating  in  an  end-plate. 

In  the  case  of  smooth  muscle  however  we  have  a  tissue 


156 


PHYSIOLOGY 


which  has  an  activity  and  reactive  power  of  its  own,  and  apart 
from  its  innervation  may  be  at  one  time  in  a  state  of  relaxation, 
at  another  in  a  state  of  tonic  contraction.  In  order  therefore 
that  the  central  nervous  system  should  have  efficient  control 
over  such  a  tissue,  it  must  be  able  to  influence  it  in  two  direc- 
tions :  it  must  be  able  to  induce  a  contraction  or  increase  a 
contraction  already  present,  and  it  must  also  be  able  to  put 
an  end  to  a  spontaneous  contraction,  i.e.  to  induce  relaxation. 
In  order  to  carry  out  these  two  effects,  smooth  muscle  receives 
nerve-fibres  of  two  kinds  from  the  central  nervous  system, 
one  kind  motor,  analogous  to  the  motor   nerves   of   skeletal 

Fig    71. 


Tracing  from  the  retractor  penis  muscle  of  the  dog,  showing  length- 
ening (inhibition)  on  stimulation  of  the  nervus  erigens,  and  a 
smart  contraction  on  stimulating  the  pudic  (motor)  nerve.  (Move- 
ments of  muscle  reduced  J.) 


muscle,  the  other  kind  inhibitory,  causing  relaxation  or  cessa- 
tion of  a  previous  contraction.  All  these  fibres  belong  to  the 
visceral  or  *  autonomic '  system.  They  are  connected  with 
ganglion-cells  in  their  course  outside  the  central  nervous 
system,  and  their  ultimate  ramifications  in  the  muscle  are 
always  non-medullated.  A  typical  tracing  of  the  opposite 
effects  of  these  two  sets  of  nerves  is  given  in  Fig.  71. 

In  the  invertebrata  many  '  voluntary  '  striated  muscles  probably  possess 
a  double  innervation.  Thus  in  the  crayfish,  the  adductor  muscle  of  the  claw 
consists  of  striated  muscular  fibres,  every  fibre  of  which  is  supplied  with 
two  kinds  of  nerve-fibres.  By  exciting  these  fibres  one  may  get,  according  to 
the  conditions  of  the  experiment,  either  contraction  of  a  relaxed  muscle  or 
relaxation  of  a  tonically  contracted  muscle  (Fig.  72). 


THE    CONTEACTLLE   TISSUES  157 

Amceboid  Movement 

We  have  already  described  amceboid  movement  as  seen 
in  the  amoeba  and  the  white  blood-corpuscles.  It  only 
remains  to  enumerate  the  chief  factors  that  influence  its 
activity. 

Amoeboid  movements  can  occur  only  within  certain  limits 
of  temperature  (about  0''  C.  to  40°)  ;  within  these  limits  it  is 
the  more  active  the  higher  the  temperature.  At  about  45° 
the  cell  goes  into  a  condition  resembling  heat  rigor. 

The  fluid  in  which  the  corpuscles  are  suspended  is  of 
great   importance.     Distilled   water,   almost   all   salts,    acids 

Fig.  72. 


Tracing  of  contraction  of  adductor  muscle  of  claw  of  crayfish, 
showing  inhibition  resulting  from  stimulation  of  its  nerve 
(at  b)  by  means  of  a  constant  current.  The  break  of  the 
current  causes  a  second  smaller  inhibition.     (Biedermann.) 

and  alkalies,  if  strong  enough,  stop  the  action  and  kill  the 
cell. 

The  movements  are  also  stopped  by  CO^  or  by  absence  of 
oxygen. 

Artificial  excitation,  whether  electrical,  chemical,  or  ther- 
mal, causes  universal  contraction  of  the  corpuscle,  which 
therefore  assumes  the  spherical  form. 

Ciliary  Movement 

Cilia  are  met  with  in  man  in  nearly  the  whole  of  the 
respiratoi^Y  passages  and  the  cavities  opening  into  them,  in 
the  generative  organs,  in  the  uterus  and  Fallopian  tubes  of 
the  female,  and  the  epididymis  of  the  male,  and  on  the 
ependyma  of  the  central  canal  of  the  spinal  cord  and  its 
continuation  into  the  cerebral  ventricles. 


158 


PHYSIOLOGY 


The  cilia  (Fig.  78)  are  delicate  tapering  filaments  which 
project  from  the  hyaline  border  of  the  epithelial  cells.  There 
are  about  twenty  or  thirty  to  each  cell.  The  hyaline  border 
is  really  made  up  of  the  enlarged  basal  portions  of  the  cilia. 

In  action  the  cilia  bend  suddenly  down  into  a  hook  or 
sickle  form,  and  then  return  more  slowly  to  the  erect  position. 
This  movement  is  repeated  many  (twelve  to  twenty)  times 
a  second,  and  thus  serves  to  move  forward  mucus,  dust,  or 
an  ovum,  as  the  case  may  be.  The  movement  seems  to  be 
entirely  automatic,  and  it  is  quite  unaffected  by  nerves,  at 
any  rate  in  all  the  higher  animals. 

Fig.  73. 


Ciliated  columnar  epithelium  from  the  trachea  of  a  rabbit : 
]ii\  m',  vi',  mucus-secreting  cells.     (Schafer.) 


There  is  however  a  functional  connection  between  all  the 
cells  of  a  ciliated  epithelial  surface,  so  that  movement  of 
the  cilia,  started  in  one  cell,  spreads  forward  as  a  wave,  just 
as,  when  the  wind  blows,  waves  of  bending  pass  over  a  field 
of  corn. 

The  conditions  of  ciliary  action  are  exactly  the  same  as 
those  for  amoeboid  movement  of  naked  cells. 

The  minuteness  of  the  object  has,  up  to  now,  prevented 
us  from  deciding  whether  the  cilium  is  itself  .actively  con- 
tractile, or  whether  it  is  simply  passively  moved  by  the 
action  of  the  basal  part  situated  in  the  hyaline  border  of  the 
cell. 


159 


CHAPTER  V 

NERVE-FIBRES  (CONDUCTING  TISSUES) 

Section  1 
GENEEAL  PEOPEETIES   OP   NEEVE-FIBEES 

In  the  last  chapter  we  studied  incidentally  some  of  the 
functions  of  nerve-fibres.  We  found  that,  when  we  stimu- 
lated the  nerve  of  a  nerve-muscle  preparation  at  any  part 
by  electrical,  thermal,  or  mechanical  means,  the  stimulus  was 
followed,  after  a  very  short  interval,  by  a  contraction  of  the 
muscle.  This  observation  illustrates  the  two  functions  of 
nerve-fibres,  irritability  and  conductivity, — that  is  to  say, 
a  suitable  stimulus  can  set  up  changes  in  any  part  of  the 
nerve,  which  are  transmitted  down  the  nerve  without  any 
visible  eliects  occurring  in  it ;  and  it  is  not  until  this  nervous 
change  has  reached  the  muscle  that  a  visible  effect  takes 
place  in  the  shape  of  a  contraction.  Now  in  the  human 
body  a  direct  excitation  of  the  nerve-fibre  in  its  course  never 
takes  place  under  normal  circumstances.  The  only  function 
the  nerve-fibre  has  to  perform  is  that  of  conducting  impulses 
from  the  sense  organs  at  the  periphery  to  the  central 
nervous  system,  and  efferent  impulses  from  this  to  the 
muscles  and  others  of  its  servants.  Hence  it  is  absolutely 
essential  that  there  should  be  vital  continuity  along  the  whole 
length  of  the  fibre.  Damage  to  any  part  of  it,  such  as  by 
crushing,  heat,  or  any  other  injurious  condition,  infallibly 
causes  a  block  to  any  impulse. 

It  was  pointed  out  in  the  introductory  chapter  that  a 
nerve-fibre  is  essentially  a  long  process  or  arm  of  a  nerve- 
cell.  The  cells  may  either  be  situated  on  the  surface  of  the 
body  or,  as  in  most  cases  in  the  higher  animals,  they  may  be 
withdrawn  from  the  surface  to  form  special  collections  of 
cells  such  as  the  posterior  root  ganglia,  or  a  whole  mass  of 


160 


PHYSIOLOGY 


cells  and  interlacing  processes  making  up  a  central  nervous 
system.  Although  all  nerves  are  alike  in  possessing  as  their 
conducting  part  the  continuous  strand  of  protoplasm  produced 
from  the  nerve-cell,  they  develop  in  the  course  of  growth 
certain  histological  differences,  which  appear  to  bear  some 
relation  to  the  nature  of  the  processes  they  conduct  or  to  the 
character  of  their  parent-cell.  Thus  all  the  fibres  which  are 
given  off  from  and  which  enter  the  central  nervous  system, 
i.e.  the  brain  and  spinal  cord,  belong  to  the  class  known  as 
medullated.     In  this  type  the  conducting  core  or  axis -cylinder 

Fig.  74. 


Three  meduUitteJ  and  two  nuu-medullated  nerve-fibres ;  n,  node 
of  Ranvier  (from  Yeo). 


is  surrounded  with  a  layer  of  apparently  insulating  material 
known  as  myelin,  forming  the  medullary  sheath,  or  the  sheath 
of  Schwann.  This  sheath  consists  of  a  fatty  material  com- 
posed largely  of  lecithin,  and  staining  black  with  osmic  acid, 
supported  apparently  in  the  interstices  of  a  network  formed 
of  a  horny  substance  known  as  neurokeratin.  The  medul- 
lary sheath  is  surrounded  by  a  structureless  membrane,  the 
primitive  sheath  or  neurilemma.  At  regular  intervals  a  break 
occurs  in  the  medullary  sheath,  the  neurilemma  coming  in 
close  contact  with  the  axis- cylinder.  This  break  is  the  node 
of  Ranvier,  the  intervening  portions  of  medullated  nerve  being 
the  internodes.  In  each  internode,  lying  closely  under  the 
neurilemma,  is  an  oval  nucleus  embedded  in  a  little  granular 
protoplasm.     The  medullated  nerve-fibres  vary  considerably 


NERVE-FIBKES  (CONDUCTING  TISSUES)  161 

in  diameter,  the  largest  fibres  being  distributed  to  the  muscles 
and  skin,  the  smallest  carrying  impulses  from  the  central 
nervous  system  to  the  viscera.  The  latter  class  all  come  to 
an  end  in  some  collection  of  ganglion  cells  of  the  sympathetic 
chain  or  peripheral  ganglia,  the  impulses  being  carried  on 
to  their  destination  by  a  fresh  relay  of  non-meduUated  nerve- 
fibres. 

The  non-medullated  fibres  differ  from  the  medullated 
simply  in  the  absence  of  a  medullary  sheath.  They  possess, 
in  many  cases  at  any  rate,  a  primitive  sheath,  under  which  we 
find  nuclei  lying  closely  on  the  side  of  the  fibre  and  bulging 
out  the  sheath.  In  their  ultimate  ramifications  they  tend  to 
form  close  networks  or  plexuses  and  appear  to  lose  the  last 
traces  of  a  sheath. 

The  medullated  nerves  are  bound  together  by  connective 
tissue  (endoneurium)  into  small  bundles,  which  are  again 
united  by  tougher  connective  tissue  into  larger  nerve-trunks. 
These  fibres  as  a  rule  branch  only  when  in  close  proximity  to 
their  destination,  and  then  the  branching  always  occurs  at 
a  node  of  Ranvier. 


11 


162 


PHYSIOLOGY 


Section    2 
PROPx\GATION    ALONG   NERVE-FIBRES 

The  velocity  of  propagation  along  a  nerve-fibre  may  be 
measured,  although  in  early  times  it  was  thought  to  be  as 
instantaneous  as  the  lightning  flash.  To  measure  the  velocity 
of  propagation  in  a  motor  nerve,  a  frog's  gastrocnemius  is 
prepared,  Avith  a  long  piece  of  sciatic  nerve  attached.  The 
muscle  is  arranged  (Fig.  75)  so  that  its  contraction  may  be 

Fig.  75. 


Diagram  of  arrangement  of  experiment  for  the  determination  of  the 
velocity  of  transmission  of  a  motor  impulse  down  a  nerve.  The 
battery  current  passes  through  the  primary  coil  of  the  inductorium 
C,  and  a  '  kick  over  '  key  k.  By  means  of  the  switch  S,  the  break 
shock  in  the  secondary  circuit  can  be  sent  through  the  nerve  n,  either 
at  b,  or  at  a.  The  muscle  in  is  arranged  to  write  on  the  blackened 
surface  of  a  trigger  or  pendulum  myograph,  and  is  excited  during 
the  passage  of  the  recording  surface  by  the  automatic  opening  of 
the  key  h.     (The  time-marker  is  not  shown.) 


recorded  on  a  rapidly  moving  surface,  on  which  are  also  re- 
corded, by  means  of  electro-magnetic  signals,  the  moment  at 
which  the  stimulus  is  sent  into  the  nerve,  and  also  a  time- 
marking  showing  2-^  sec.  Tracings  are  now  taken  of  the 
contraction  of  the  muscle  :  first,  when  the  nerve  is  stimulated 
at  its  extreme  upper  end ;  secondly,  as  close  as  possible  to 
the  muscle.  It  will  be  found  that  the  latent  period,  which 
elapses  between  the  point  at  which  the  stimulus  is  sent  into 
the  nerve  and  the  point  at  which  the  lever  begins  to  rise,  is 
rather  longer  in  the  first  case  than  in  the  second.  The 
difference  in  the  two  latent  periods  gives  the  time  that  it  has 


NERVE-FIBEES   (CONDUCTING   TISSUES)  163 

taken  for  the  nervous  impulse  to  travel  down  the  length  of 
nerve  between  the  two  stimulated  points.  Calculated  in  this 
way,  the  velocity  of  propagation  in  frog's  nerve  is  about  28 
metres  per  second. 

The  velocity  of  propagation  in  sensory  nerves  is  more 
difficult  to  determine,  owing  to  the  fact  that  a  sensory  impulse, 
on  arrival  at  the  receiving  organ — i.e.  some  part  of  the  central 
nervous  system — does  not  at  once  give  rise  to  some  definite 
recordable  mechanical  change,  such  as  a  muscular  contraction. 
There  is  another  method  of  determining  the  velocity  of  conduc- 
tion, which  may  be  used  also  with  sensory  fibres.  The  pas- 
sage of  a  nerve-impulse  down  a  nerve,  just  as  the  passage  of 
a  wave  of  contraction  along  a  muscle-fibre,  is  immediatel}' 
preceded  or  accompanied  by  an  electrical  change,  which  also 
travels  along  the  nerve  as  a  wave  of  '  negativity.'  The  velocity 
of  propagation  of  this  wave  may  be  measured,  and  is  found 
to  give  the  same  numbers  as  the  velocity  determined  by  the 
preceding  method. 

The  existence  of  this  electrical  change  enables  us  to  show 
that  a  nerve-impulse,  excited  at  any  point  in  the  course  of  a 
nerve-fibre,  travels  in  both  dircctiuns  along  the  fibre.  The 
power  of  nerves  to  transmit  impulses  in  either  direction  is 
shown  further  by  the  experiment  known  as  Kiihne's  gracilis 
experiment.  The  gracilis  muscle  of  the  frog  is  separated  into 
two  portions  by  a  tendinous  intersection,  so  that  there  is  no 
muscular  continuity  between  the  two  halves.  The  nerve  to  the 
muscle  divides  into  two  branches,  one  to  each  half,  and  at 
the  point  of  junction  there  is  diuisLO)i  uf  the  axis-cylinders 
themselves.  It  is  found  that  if  the  section  a  in  the  diagram 
(Fig.  76),  which  is  quite  isolated  from  the  rest  of  the  muscle, 
be  stimulated,  as  by  snipping  it  with  scissors,  the  whole 
muscle  contracts.  If  however  the  portion  of  the  muscle  which 
is  free  from  nerve-fibres  be  stimulated  in  the  same  way,  the 
contraction  is  limited  to  the  fibres  directly  stimulated,  show- 
ing that  in  the  first  case  the  stimulus  excited  nerve -fibres 
which  transmitted  the  impulse  iqj  the  nerve  to  the  point  of 
division  and  then  down  again  to  the  other  half  of  the  muscle. 
Since  nerves  have  this  power  of  conduction  in  ])oth  directions, 
it  might  be  thought  that  a  single  set  of  nerve-fibres  might 
very  well  subserve  both  afferent  and  efl:erent  functions,  at 
one  time  conducting  sensory  impulses  fi-om  periphery  to  cord, 


164 


PHYSIOLOGY 


at  another  time  motor  impulses  from  cord  to  muscles.  But 
this  is  not  the  case.  As  a  matter  of  fact  we  find  in  the  body 
a  marked  differentiation  of  function  between  various  nerve- 
fibres. 

We  have  already  mentioned  the  division  of  nerves  into 
afferent  and  efferent,  and  we  find  further  that  each  of  these 
afferent  or  eiferent  fibres  has  its  own  proper  impulses  to  con- 
duct, and  conducts  only  these  impulses.  Thus  some  fibres  in 
the  anterior  spinal  roots  conduct  ordinary  motor  impulses  to 
muscles,  others  impulses  causing  contraction  or  relaxation 
of  the  muscular  walls  of  arteries,  or  which  quicken  or  slow 

Fig.  76. 


Kiihne's  gracilis  experiment. 


the  contractions  of  the  heart.  We  shall  return  to  this  ques- 
tion of  restricted  function  of  nerve-fibres  under  the  heading 
of  *  Miiller's  law  of  specific  irritability  '  when  we  come  to  treat 
of  the  special  senses. 

The  rate  of  propagation  of  a  nervous  impulse  is  quickened 
by  heat  (up  to  about  38°  C.)  and  slowed  by  cold.  At  a  little 
over  0"  C.  the  rate  in  frog's  nerve  is  only  about  one  metre  per 
second. 


Events  accompanying  the  Passage  oj  a  Nervous  Impidse 

In  muscle  we  saw  that  the  passage  of  an  excitatory  wave 
was  accompanied  or  followed  by  electrical  changes,  production 
of  heat,  and  mechanical  change,  all  pointing  to  an  evolution 


NERVE-FIBRES  (CONDUCTING  TISSUES)  165 

of  energy  from  the   explosive   breaking  down  of  contractile 
material. 

In  nerve  however,  which  serves  merely  as  a  conducting 
medium,  we  should  not  expect  so  much  expenditure  of  energy, 
or  in  fact  any  expenditure  at  all.  All  that  is  necessary  is 
that  each  section  of  the  nerve  should  transmit  to  the  next 
section  just  so  much  kinetic  energy  as  it  has  received  from 
the  section  above  it.  And  experiment  bears  out  this  con- 
clusion. The  most  refined  methods  have  failed  to  detect  the 
slightest  development  of  heat  in  a  nerve  during  the  passage  of 
an  excitatory  process  ;  and  we  know  already  that  there  is  no 
mechanical  change  in  the  nerve.  The  only  physical  change 
in  a  nerve  under  these  circumstances  is  the  development  of 
a  current  of  action.  A  nerve  becomes,  when  excited  at  any 
point,  negative  at  this  point  to  all  other  parts  of  the  nerve, 
and,  just  as  in  muscle,  this  'negativity'  is  propagated  in  the 
form  of  a  wave  in  both  directions  along  the  nerve. 

That  the  excitatory  process  in  nerves  is  probably  accom- 
panied by  certain  small  chemical  changes  is  indicated  by  the 
facts  that,  in  the  complete  absence  of  oxj^gen,  the  nerve  fibres 
lose  their  irritability,  and  that  this  loss  of  irritability  is 
hastened  by  repeated  stimulation  of  the  nerve.  When  the 
irritability  has  been  abolished  by  stimulation  in  the  absence 
of  oxygen,  it  may  be  restored  within  a  few  minutes  by  re- 
administration  of  oxygen  to  the  nerve. 

If  we  connect  a  galvanometer  to  two  points  of  an  uninjured 
nerve,  no  current  is  observed,  all  points  of  a  living  nerve  at 
rest  being  iso-electric.  On  making  a  cross-section  of  the 
nerve  at  one  leading-off  point,  a  current  is  at  once  set  up, 
which  passes  from  the  surface  through  the  galvanometer  to 
the  cross-section.  This  is  a  demarcation  current,  set  up  at 
the  junction  between  living  and  dying  nerve.  It  will  be 
noticed  that  this  current  rapidly  diminishes  in  strength  and 
finally  disappears,  owing  partly  to  the  fact  that  the  dying  pro- 
cess started  in  the  nerve  by  the  section  extends  only  as  far  as 
the  next  node  of  Ranvier  and  there  ceases,  so  that  after  a 
short  time  the  electrode  applied  to  the  cross-section  is  simply 
leading  off  an  intact  living  axis-cylinder  through  the  dead 
portion  of  the  nerve,  which  acts  as  an  ordinary  moist  con- 
ductor. On  making  a  fresh  section  just  above  the  previous 
one,  the  process  of  dying  is  again  set  up,  and  the  demarcation 


166  PHYSIOLOGY 

cuiTent  is  restored  to  its  original  strength.  If,  while  the 
demarcation  current  is  at  its  height,  we  stimulate  the  other 
end  of  the  nerve  with  an  interrupted  current,  the  needle  of 
the  galvanometer  swings  back  towards  zero,  i.e.  there  is  a 
negative  variation  of  the  resting  current. 

In  order  to  demonstrate  the  wave-like  progression  of  the 
electrical  change  from  the  excited  spot  along  the  nerve,  it  ia 
necessary,  as  in  the  case  of  muscle,  to  make  use  of  the  repeat- 
ing rheotome  or  of  a  very  sensitive  capillary  electrometer.  It 
is  then  found  that  the  change  progresses  along  the  nerve  at 
the  same  rate  as  the  nervous  impulse,  i.e.  twenty-eight  to 
thirty-three  metres  per  second.  But  it  lasts  only  an  extremely 
short  interval  of  time  at  each  spot,  viz.  six  to  eight  ten-thou- 
sandths of  a  second.  Thus  the  length  of  the  excitatory  wave 
in  nerve  is  about  18  mm. 


NERVE-FIBRES   (CONDUCTING  TISSUES)  167 


Section  3 
EXCITATION   OF   NERVES 

We  must  now  study  more  fully  the  changes  which  take 
place  in  a  nerve  during  the  passage  of  a  current  through  it, 
and  the  manner  in  which  these  changes  are  ahle  to  generate  a 
nervous  impulse.  Under  normal  circumstances,  if  a  constant 
current  he  passed  through  the  nerve  of  a  nerve-muscle  pre- 
paration for  a  short  time,  the  muscle  responds  only  at  the 
make  and  the  hreak  of  the  current,  remaining  perfectly 
quiescent  all  the  time  the  current  is  passing.  If  the  nerve  he 
in  a  very  excitahle  condition,  it  is  possible  that  the  muscle 
may  be  thrown  into  a  tetanus  or  continued  contraction  during 
the  whole  time  that  the  current  is  passing  (closing  tetanus). 
On  the  other  hand,  if  a  strong  ascending  current  be  passed 
through  the  nerve  for  a  considerable  time,  the  muscle  when 
the  current  is  broken  may  go  into  continued  contraction, 
which  may  last  some  time.  Normally  however  the  muscle 
simply  responds  with  a  single  twitch  at  the  make  and  break 
of  the  current ;  although,  on  investigating  the  condition  of 
the  nerve  during  the  passage  of  the  current,  we  find  that  it  is 
considerably  modified. 

This  modification  in  the  condition  of  the  nerve  is  spoken 
of  as  eJectrotonus,  and  includes  changes  in  its  irritability  and 
its  electrical  condition. 

To  investigate  these  changes  the  following  apparatus  is 
necessary : — two  constant  batteries,  induction  coil,  a  reverser 
and  keys,  a  pair  of  non-polarisable  electrodes,  and  a  pair  of 
ordinary  platinum  electrodes.  Fig.  77  represents  roughly 
the  arrangement  of  the  experiment.  A  constant  current 
from  the  battery  is  led  through  a  part  of  the  nerve  by  means 
of  non-polarisable  electrodes,  which  are  about  one  inch  apart. 
In  this  circuit  we  put  a  reverser,  b}^  means  of  which  the 
direction  of  the  current  of  the  nerve  may  be  changed  at  will, 
and  a  key  to  make  or  break  the  current.  This  is  the  polaris- 
ing cu'cuit.  The  other  battery  is  arranged  in  the  primary 
circuit  of  the  coil,  a  key  being  interposed,  so  that  we  may  use 
make   or   break   induction-shocks,  which  are   applied  to  the 


168  PHYSIOLOGY 

nerve  by  means  of  the  small  platinum  electrodes.  The 
tendon  of  the  muscle  is  connected  by  a  thread  with  a  lever, 
which  is  arranged  to  write  on  a  smoked  surface,  so  that  the 
height  of  the  contraction  can  be  recorded. 

We  first  find  the  position  of  the  secondary  coil,  at  which 
the  break  induction-shock  is  a  submaximal  stimulus,  and  we 
employ  this  strength  of  stimulus  throughout  the  experiment. 
The  make  induction-shock  is  prevented  from  acting  on  the 
nerve  by  closing  a  short-circuiting  key  in  the  circuit  of  the 
secondary  coil.  The  nerve  is  now  stimulated  at  various 
points  with  a  single  break  induction-shock,  and  the  con- 
tractions recorded.     The  heights  of  these  contractions  serve 


Arrangement  of  api^aratus  for  showing  eleetrotonic  changes  in 
irritability,  e.  Exciting  current,  p.  Polarising  circuit,  r.  Pohl's 
reverser. 


to  indicate  the  irritability  of  the  nerve  at  the  point  stimu- 
lated. We  now  throw  the  polarising  current  into  the  nerve. 
At  the  make  of  this  current  the  muscle  will  probably  respond 
with  a  twitch  which  is  not  recorded.  We  then  test  once 
more  the  irritability  of  different  points  of  the  nerve,  and  we 
find  that,  when  the  stimulus  is  applied  near  (a)  the  point 
where  the  current  enters  the  nerve  (anode),  the  stimulus, 
which  before  gave  a  moderately  large  contraction  of  the 
muscle,  now  has  either  no  effect  or  else  produces  a  very 
weak  contraction.  On  the  other  hand,  in  the  region  of  the 
kathode  the  stimulus,  which  before  was  submaximal,  has 
now  become  maximal,  as  is  shown  by  the  increase  in  the 
height  of  the  contraction  evoked  by  the  induction-shock. 

We  now  reverse  the  direction  of  the  polarising  current, 
so  that  the  curx-ent  of  the  nerve  runs  from  (k)  to  (a).     With 


NERVE-FIBRES  (CONDUCTING  TISSUES)  169 

this  reversal  of  current  there  is  also  a  reversal  of  the  changes 
in  the  nerve ;  that  is  to  say,  the  normally  submaximal 
stimulus  is  maximal  when  applied  near  (a),  and  minimal 
when  applied  near  (k).  On  break  of  the  polarising  current 
the  condition  of  the  nerve  returns  to  normal,  and  the  sub- 
maximal  stimulus  is  once  more  submaximal  throughout. 

This  return  to  normal  conditions  however  is  not  immediate,  since  the  first 
effect  of  breaking  tlie  current  is  a  swing  back,  so  to  speak,  past  the  normal, 
the  diminished  irritabilitj'  at  the  anode  giving  place  to  an  increased  irritability, 
which  only  gradually  subsides.  In  the  same  way,  immediately  after  the 
polarising  current  has  ceased  to  flow,  the  neighbourhood  of  the  kathode 
acquires  a  condition  of  diminished  irritability,  and  this  only  gradually  gives 
place  to  a  normal  condition. 

This  experiment  teaches  us  that,  when  a  constant  current 
is  passed  through  a  nerve,  there  is  increase  in  the  irritability 
in  the  nerve  near  the  kathode,  and  a  diminution  in  irrita- 
bility near  the  anode.  These  conditions  of  increased  and 
diminished  irritability  are  spoken  of  as  katelectrotonus  and 
anelectrotonus  respectively.  In  the  previous  chapter  we 
learnt  that  a  make  contraction  always  starts  from  the 
kathode,  and  a  break  contraction  from  the  anode.  Now 
the  event  that  takes  place  at  the  kathode  on  make  and  at 
the  anode  on  break  of  a  constant  current  is,  as  the  last 
experiment  shows  us,  a  rise  in  irritability,  in  the  former 
case  from  normal  to  above  normal,  in  the  latter  from  sub- 
normal to  normal.  Hence  we  may  sa}''  that  the  excitation 
is  caused  by  a  sudden  rise  of  irritability,  which  may  be  due 
either  to  a  sudden  appearance  of  katelectrotonus,  or  a  sudden 
disappearance  of  anelectrotonus.  I  have  said  sudden,  be- 
cause the  steepness  of  the  rise  of  irritability  is  a  necessary 
factor  in  causing  excitation.  If  the  polarising  current  passing 
through  a  nerve  be  slowly  and  gradually  increased  to  con- 
siderable strength,  it  will  give  rise  to  no  contraction.  The 
degree  of  suddenness  of  the  rise,  which  is  most  beneficial 
in  causing  contraction,  varies  with  the  nature  of  the  tissue 
stimulated.  Thus  it  is  more  rapid  in  nerve  than  in  muscle, 
and  in  pale  muscle  than  in  red  muscle,  and  in  voluntary 
muscle  than  in  unstriated  muscle. 

It  is  evident  that  there  must  be,  somewhere  between  the 
anode  and  kathode,  an  indifferent  point — that  is  to  say,  a  re- 
gion where  the  irritability  is  neither  increased  nor  diminished. 


170 


PHYSIOLOGY 


We  find  experimentally  that  this  indijfferent  point  is  nearer 
the  anode  when  the  polarising  current  is  weak,  and  gets 
nearer  to  the  kathode  as  the  current  is  strengtliened,  so 
that  with  very  strong  currents  nearly  the  whole  intrapolar 


Fir,.  78. 


Diagram  to  show  the  variations  of  irritability  in  a  nerve  during  the 
passage  of  polarising  currents  of  different  strengths.  The  degree 
of  change  is  represented  by  the  distance  of  the  curves  from  the 
base  line  ;  the  part  of  the  curve  below  the  line  signifying  den-ease, 
that  above  the  line  increase  of  irritability.  A,  anode;  B,  kathode; 
7/1,  effect  of  weak  current ;  ?/.„  medium  current ;  y.^,  strong  current. 
It  will  be  noticed  that  the  indifferent  point,  ,r,  where  the  curve 
crosses  the  horizontal  line,  approaches  nearer  and  nearer  the 
kathode  as  the  current  is  increased  in  strength  (from  Foster,  after 
Pfiiiger). 

length  is  in  a  condition  of  anelectrotonus  (Fig.  78).  When  a 
strong  polarising  current  is  used,  the  depression  of  irritahility 
at  the  anode  is  so  marked  that  no  impulse  can  pass  this 
region.     Thus    if   we   send  a  very  strong  ascending  current 

Fig.  79. 
ascendinq  current 


kath. 


an. 


make  excitation  blocked 
at  anode. 


kath. 


break  excitation  at  anode 
blocked  at  kathode. 


Diagram  to  show  the  blocking  effect  of  a  strong   constant   current 
passed  through  the  nerve  of  a  nerve-muscle  preparation. 


through  the  nerve,  there  is  no  contraction  at  make.  This 
is  owing  to  the  fact  that  the  impulse  started  at  the  kathode 
on  make  of  the  current  cannot  reach  the  muscle,  its  passage 
down  the  nerve  heing  hlocked  in  the  region  of  the  anode 
(Fig.  79,  A). 


NERVE-FIBRES   (CONDUCTING   TISSUES) 


171 


The  results  of  stimulating  motor  nerves  by  means  of  constant  currents 
were  stuclied  by  Pfliiger  and,  embodied  in  a  table,  make  up  what  is  known  as 
Pfliiger's  law.  The  result  of  stimulation  varies  with  the  strength  of  a 
current. 

Laiv  of  Contracticm 


Strenglib  of  current. 

Ascending. 

Descending. 

Make.       Break. 

Make.       Break. 

Weak  .... 

0              0 

0 

Medium 

c              c 

C               c 

Strong 

0          C  or  T 

C  or  T          0 

c  =  contraction.    C  =  strong  contraction.    T  =  tetanus.    0  =  no  effect. 

With  the  weakest  currents,  excitation  occurs  only  at  make,  since  a  make- 
stimulus,  i.e.  the  rise  of  katelectrotonus,  is  always  more  effectual  than  a  break- 
stimulus,  i.e.  the  disappearance  of  anelectrotonus.  With  currents  of  moderate 
strength,  excitation  occurs  both  at  make  and  break,  being  better  marked  at 
make,  especially  in  the  case  of  descending  currents.  With  very  strong  currents, 
we  get  a  contraction  at  make  only  when  the  current  is  descending,  since,  when 

Fig.  80. 


Arrangement  of  experiment  to  demonstrate  Pfliiger's  law 
of  contraction. 


the  current  is  ascending,  the  excitation  started  at  the  kathode  cannot  pass  the 
block  at  the  anode.  For  the  same  reason  a  break  contraction  is  obtained  only 
with  an  ascending  current,  since  at  the  break  of  a  descending  current  there 
is  a  swing-back  of  the  nerve  at  the  kathode  to  a  condition  of  diminished 
irritability,  which  effectually  blocks  the  excitation  started  higher  up  the  nerve 
at  the  anode. 

The  aiTangement  of  the  experiment  for  demonstrating  Pfliiger's  law  is 
shown  in  Fig.  80.  The  strength  of  the  current  is  graduated  by  means  of  the 
rheochord,  the  current  being  led  into  the  nerve  by  means  of  non-polarisable 
electrodes.  It  is  extremely  important  in  these  experiments  to  avoid  any 
injury  or  drying  of  the  nerves  at  either  of  the  two  electrodes,  since  the  excita- 
tory efi'ect  either  at  make  or  break  would  be  abolished  by  local  injury. 


172  PHYSIOLOGY 

These  results,  worked  out  chiefly  on  motor  nerves,  have 
been  confirmed  as  far  as  possible  experimentally  on  sensory 
nerves,  and  on  muscle  and  contractile  tissues  generally,  and 
probably  hold  good  for  all  irritable  living  tissues. 

It  is  said  that  an  anelectrotonus  takes  some  time  to  attain 
its  full  height,  and  a  katelectrotonus  reaches  its  maximum 
almost  directly  after  the  current  is  made.  Hence  a  current  of 
very  short  duration  probably  excites  only  at  the  make,  the  break 
occurring  before  the  anelectrotonus  is  developed  enough  for 
its  disappearance  to  cause  a  stimulus.  Thus  induction-shocks 
(both  make  and  break)  may  be  looked  upon  as  make-excitations, 
the  excitation  however  being  stronger  in  the  case  of  the  break 
induction-shock  than  in  that  of  the  make. 

Other  things  being  equal,  a  current  of  given  strength  causes 
a  stronger  excitation  the  greater  the  length  of  nerve  that 
it  flows  through.  It  must  be  remembered  however  that  the 
nerve  offers  considerable  resistance  to  the  passage  of  the 
current,  and  so,  to  keep  the  current  constant  while  increasing 
the  length  of  intrapolar  nerve,  we  must  largely  increase  the 
electromotive  force  employed. 

A  very  convenient  method  of  showing  the  effect  of  the  length  of  intrapolar 
nerve  on  excitation  has  been  suggested  by  Gotch.  The  two  sciatic  nerves  of 
a  frog  are  dissected  out,  one  of  them  being  in  connection  with  the  gastrocnemius. 
These  are  first  arranged  as  in  Fig.  81  a  (p.  173).  a,  b,  and  c  are  three  non-polaris- 
able  electrodes,  the  terminals  of  a  constant  battery  being  connected  to  a  and  c. 
The  position  of  the  rider  on  the  rheochord  is  then  ascertained  at  which  make 
of  the  current  just  excites  contraction  in  the  muscle  of  nerve  2,  the  current  in 
this  case  passing  from  a  to  b  along  nerve  1,  and  from  6  to  c  along  a  small 
piece  of  nerve  2.  We  will  suppose  that  eleven  units  of  current  are  necessary 
to  produce  excitation,  b  is  then  withdrawn  and  the  nerve  2  laid  on  a  (Fig  81,  b). 
so  that  the  current  can  now  pass  from  ft  to  c  entirely  through  a  long  stretch 
of  nerve  2.  On  again  seeking  the  minimal  stimulus,  it  will  be  found  that  a 
smaller  current  is  sufficient  to  excite,  contraction  being  obtained  with  seven 
units.  Since  the  length  of  nerve  traversed,  and  therefore  the  resistance  to  the 
current,  aie  the  same  in  both  cases,  it  is  evident  that  a  current  is  more  effective 
the  greater  the  length  of  excited  nerve  that  it  traverses. 

A  nerve  cannot  be  excited  by  currents  passed  transversely 
across  it,  since  in  such  cases  the  anode  and  kathode  lie  so 
close  to  one  another  in  a  nerve-fibril,  as  it  is  traversed  by 
a  current,  that  their  effects  counteract  one  another. 


NERVE-FIBRES   (CONDUCTING   TISSUES) 


173 


Electrical  Stimuli  as  aiyplied  to  Human  Nerves 

When  we  attempt  to  apply  the  results  gained  on  frog's 
nerves  to  man,  we  are  met  at  once  by  the  difficulty  that  we 
cannot  dissect  out  the  nerves  and  apply  stimuli  to  them  directly. 
So  usually  unipolar  excitation  is  used,  one  electrode,  either 
anode  or  kathode,  being  applied  to  the  nerve  to  be  stimulated, 
and  the  other  to  some  indifferent  point,  such  as  the  back.  It 
is  evident  under  these  circumstances  that  the  current  is  con- 
centrated as  it  leaves  the  anode  and  reaches  the  kathode,  and 


Diagram  of  anangcment  of  experiment  for  investigating  the 
influence  of  the  intrapolar  length  of  nerve  on  the  excitatory 
effect  of  the  current. 


diffuses  widely  in  the  body,  seeking  the  lines  of  least  resistance. 
Thus  it  is  impossible  to  get  pure  anodic  or  kathodic  effects. 
If  the  anode  be  applied  over  the  nerve,  the  current  enters  by 
a  series  of  points  (the  polar  zone),  and  leaves  by  a  second 
series  (the  peripolar  zone).  The  polar  zone  will  thus  be  in  the 
condition  of  anelectrotonus,  and  the  peripolar  zone  in  that 
of  katelectrotonus.  The  current  however  will  be  more  con- 
centrated at  the  polar  than  at  the  peripolar  zone,  and  so 
the  former  effect  will  predominate.     These  restrictions  in  the 


174 


PHYSIOLOGY 


application  of  the  current  cause  slight  apparent  irregularities 
in  the  law  of  contraction  as  tested  on  man. 


Other  Methods  of  Stimulation 

1,  Thermal.  If  the  temperature  of  a  nerve  be  gradually 
raised,  no  effect  is  noticed  till  about  40'  C.  is  reached,  when 
the  muscle  may  enter  into  weak  quivering  contractions. 
Sudden  warming  of  the  nerve  always  gives  rise  to  excitation. 
At  about  45°  C.  the  nerve  loses  its  irritability  and  dies. 

A  nerve  may  be  rapidly  cooled  without  any  excitation 
taking  place.     At  about  0°  C.  the  conductivity  of  mammalian 

Fig.  82. 


Electrodes  applied  to  the  skin  over  a  nerve-trunk.  In  a  the  polar  area 
is  anelectrotonic  and  the  peripolar  katelectrotonic.  The  former 
condition  therefore  preponderates,  since  the  current  here  is  more 
concentrated.  In  b  the  conditions  are  reversed,  the  polar  zone 
corresponding  in  this  case  to  the  kathode.     (Waller.) 


nerve-fibres  is  absolutely  abolished,  and  hence  this  method  of 
cooling  is  of  great  value  when  it  is  required  to  divide  a  nerve 
physiologically  without  exciting  it. 

2.  Mechanical.  A  nerve  may  be  excited  by  crushing  or 
cutting.  These  methods  however  destroy  the  nerve.  It  is 
possible  to  excite  a  nerve  mechanically,  without  any  serious 
injury  to  it,  by  carefully  graduated  taps,  and  this  method  has 
been  used  in  investigating  the  phenomena  of  electrotonus. 

3.  Chemical.     All  chemical  stimuli  applied  to  the  nerve 


NERVE-FIBRES   (CONDUCTING   TISSUES) 


175 


have  a  speedy  effect  in  destroying  its  irritability.  The  che- 
mical stimuli  most  used  are  strong  salt  solutions,  glycerin,  or 
weak  acids.  If  any  one  of  these  be  applied  to  a  motor  nerve, 
the  muscle  enters  into  an  irregular  tetanus,  which  lasts  till 
the  irritability  of  the  nerve  is  destroyed  at  the  part  stimulated. 
It  is  thus  evident  that  we  are  justified  in  our  choice  of  electrical 
stimuli  in  all  ordinary  experiments  on  nerves. 

The  Energy  involved  in  tlic  Excitatiun  of  a  Nerve 

We  have  hitherto  only  considered  electrical  stimulation  by  means  of  a  con- 
stant current  or  an  induced  current.  It  is  possible  however  to  store  up  a 
certain  (juantity  of  electricity  in  a  condenser  and  then  to  excite  a  nerve  by  dis- 
charging the  condenser  through  it.'  The  arrangement  of  such  an  experiment 
is  shown  in  Fig.  83.     By  means  of  the  switch  S,  the  condenser  can  be  put  into 

Fig.  83. 


Arrangement  of  apparatus  for  the  excitation  of  a  nerve  by  means  of 
condenser  discharges.  B,  battery;  R,  rheochord ;  c,  rider  of  rheo- 
chord ;  S,  switch  (Pohl's  reverser  without  cross  wires) ;  C,  conden- 
ser ;  n,  nerve  ;  m,  muscle  ;  e,  non-polarisable  electrodes. 


connection  either  with  the  battery  from  which  it  receives  its  charge  or  with  the 
nerve  through  which  it  can  discharge.  By  knowing  the  capacity  of  the  con- 
denser and  the  electromotive  force  by  which  it  is  charged,  we  can  estimate  the 
energy  of  the  charge  sent  through  the  nerve. 

E  (energy  in  ergs)  -  =  5  F V- 
(F  ^  capacity  in  microfarads  ;  V  =  electromotive  force  in  volts). 


'  For  explanation  of  condenser  see  Appendix. 

-  An  erg  is  the  amount  of  work  produced  or  energy  expended  by  the  action 
of  one  dyne  through  one  centimetre. 

A  dyne  is  the  force  which  will  give  to  a  mass  of  one  gram  an  acceleration  of 
one  centimetre  per  second  per  second. 


176  PHYSIOLOGY 

In  this  way  it  has  been  found  that  the  energy  of  a  minimal  effective  stimulus 
for  frog's  nerve  is  about  j~^^  of  an  erg. 

From  what  we  have  previously  said  about  the  relation  of  the  rate  of  change 
of  current  to  its  excitatory  effect,  it  is  evident  that  the  amount  of  energy  neces- 
sary to  excite  the  nerve  will  vary  with  the  rate  at  which  the  condenser  is  allowed 
to  discharge  through  the  nerve.  This  rate  can  be  modified  by  altering  the  resist- 
ance in  the  discharging  circuit  or  by  altering  the  electromotive  force  of  the 
charge  ;  and  this  method  has  been  adopted  by  Waller  in  determining  the  so 
called  '  characteristic  '  of  nerve,  viz.  the  rate  of  change  at  which  excitation  is 
obtained  with  a  minimal  expenditure  of  energy. 


NERVE-FIBBES  (CONDUCTING  TISSUES)  177 


Section  4 

THE   CONDITIONS   AFFECTING   THE   ACTIVITY 
OF  NEEVE 

The  hifiuence  of  Fatigue 

In  the  description  of  the  phenomena  of  muscular  fatigue 
given  in  the  last  chapter  it  was  assumed  that  the  muscle 
was  being  excited  directly.  The  same  phenomena  are  how- 
ever observed  when  the  muscle  is  excited  through  its  nerve, 
though  in  this  case  fatigue  comes  on  much  more  quickly.  If, 
after  the  muscle  has  been  excited  in  this  way  until  exhausted, 
it  be  excited  directly,  it  will  respond  with  a  contraction 
nearly  as  high  as  at  the  beginning  of  the  experiment.  We 
see  therefore  that  the  nervous  structures  are  more  susceptible 
to  the  influences  causing  fatigue  than  the  muscle  itself,  and 
it  can  be  shown  that  the  weak  point  in  the  nerve-muscle 
preparation  is  not  the  nerve,  but  the  end-plates.  In  fact  it 
is  not  possible  to  demonstrate  any  phenomena  of  fatigue  in 
the  nerve-trunk.  This  fact  can  be  shown  in  mammals  by 
poisoning  the  animal  with  curare,  and  then  stimulating  a 
motor  nerve  continuously  while  the  animal  is  kept  alive  by 
means  of  artificial  respiration.  As  the  effect  of  the  curare  on 
the  end-plates  begins  to  wear  off  in  consequence  of  its  excre- 
tion, the  muscles  supplied  by  the  stimulated  nerve  enter  into 
tetanus. 

The  same  fact  may  be  shown  on  the  excised  nerve-muscle 
preparation  of  the  frog.  The  gastrocnemii  of  the  two  sides 
with  the  sciatic  nerves  are  dissected  out,  and  an  exciting  circuit 
is  so  arranged  that  the  interrupted  secondary  currents  pass 
through  the  upper  ends  of  both  nerves  in  series.  At  the  same 
time  a  constant  cell  is  connected  with  two  non-polarisable 
electrodes  {np,  np)  placed  on  the  nerve  of  B,  so  that  a  current 
runs  in  the  nerve  in  an  ascending  direction.  The  effect  of 
passing  a  constant  current  through  a  nerve  is  to  block  the 
passage  of  impulses  through  the  part  traversed  by  the  current. 
When  the  constant  polarising  current  is  made,  the  muscle  B 
may  give  a  single  twitch,  and  then  remains  quiescent.  The 
exciting  current  is  then  sent   through    both    nerves  by  the 

12 


178 


PHYSIOLOGY 


electrodes  e,  and  e,.  The  muscle  A  enters  into  tetanus,  which 
gradually  subsides  owing  to  '  fatigue.'  When  A  no  longer 
responds  to  the  stimulation,  the  constant  current  through  the 
nerve  of  B  is  broken.  B  at  once  enters  into  tetanus,  which 
lasts  as  long  as  the  contraction  did  in  the  case  of  A,  and 
gradually  subsides  as  fatigue  comes  on.  Since  both  nerves 
have  been  excited  throughout,  it  is  evident  that  the  fatigue 
does  not  affect  the  nerve-trunk.  We  have  already  seen  that 
a  muscle  will  respond  well  to  direct  stimulation  when  stimula- 
tion of  its  nerve  is  without  effect,  and  must  therefore  conclude 

Fig.  84. 


A  B 

Arrangement  of  experiment  for  demonstrating  the  absence  of  fatigue 
in  medullated  nerve-fibres,  ec,  exciting  circuit ;  cp,  polarising 
circuit. 

that  the  first  seat  of  fatigue  is    the  junction   of   nerve   and 
muscle,  i.e.  the  end-plates. 

In  the  normal  intact  animal,  the  break  in  the  neuro- 
muscular chain  which  is  the  expression  of  fatigue  occurs  still 
higher  up,  i.e.  in  the  central  nervous  system,  and  is  probably 
due  to  some  reflex  mhibition  of  the  central  motor  apparatus 
from  the  muscle  itself.  Thus  after  complete  fatigue  has  been 
produced  in  a  muscle  so  far  as  regards  voluntary  efforts,  direct 
stimulation  of  the  muscle  itself  or  its  nerve  will  produce 
a  contraction  as  great  as  would  have  been  the  case  at  the 
beginning  of  the  experiment. 


NERVE-FIBRES   (CONDUCTING   TISSUES)  179 

The  E^ffect  of  Injury —The  Bitter-VaUi  Lato 

The  irritability  of  the  nerve  of  a  muscle-nerve  preparation 
is  not  equal  in  all  parts  of  its  course,  but  is  greater  at  the 
upper  end,  probably  in  consequence  of  the  proximity  of  the 
cross-section. 

Some  time  after  a  motor  nerve  is  divided,  the  increased 
irritability  at  the  upper  end  gives  way  to  a  decreased  irrita- 
bility, and  this  decrease  goes  on  till  the  nerve  is  no  longer 
excitable.     The  diminution   in  excitability  gradually  extends 

Fio.  85. 


Tracing  of  muscle  contractions  to  show  effect  of  cooling  a  nerve  on  ity 
excitability.  The  lower  line  indicates  the  changes  in  temperature  of 
the  excited  part  of  the  nerve.  The  muscle  responded  only  when  the 
nerve  was  cooled,  the  stimulus  becoming  ineffectual  when  the  nerve 
was  warmed  (Gotch). 

down  the  nerve-fibre,  so  that  the  part  of  the  nerve  nearest 
the  muscle  remains  excitable  the  longest.  This  progressive 
change  in  the  irritability  of  a  nerve  after  section  is  spoken  of 
as  the  Eitter-Valli  law.  It  is  soon  followed  by  definite  histo- 
logical changes  in  the  nerve,  which  we  shall  describe  later  on 
(see  Chap.  XIV.). 

The  Influence  of  Temperature 

The  excitability  of  a  nerve  is,  within  certain  limits,  in- 
creased by  cooling  the  nerve,  and  diminished  by  raising  its 
temperature  (Fig.  85). ^    Thus,  if  a  frog  be  cooled  to  2°  or  3°  C. 

'  This  is  true  for  all  stimuli  except  induction-shocks  and  extremely  short 
galvanic  currents  (less  than  0-005  sec),  with  which  the  irritability  of  nerve  is 
increased  by  warming  and  diminished  by  cooling.  The  same  appears  to  be 
true  for  ventricular  cardiac  muscle,  but  in  the  case  of  voluntary  muscle  the 
excitability  for  all  forms  of  stimuli  is  increased  by  cooling.     (Gotch.) 


180  PHYSIOLOGY 

for  a  day,  it  will  be  found  that  simple  section  of  its  sciatic 
nerve  may  suffice  to  send  the  gastrocnemius  into  continued 
contraction,  and  under  these  circumstances  *  closing  tetanus  ' 
may  be  obtained  with  the  greatest  ease.  The  opposite  effects 
of  cooling  on  the  conductivity  and  excitability  of  nerves  show 
that  these  properties  are  possibly  independent,  and  are  not 
affected  by  the  same  conditions. 

The  Inflitence  of  Drugs 

The  most  important  drugs  with  an  influence  on  nerve- 
fibres  are  those  belonging  to  the  class  of  anaesthetics.  Of 
these  we  may  mention  carbon  dioxide,  ether,  chloroform,  and 
alcohol. 

The  action  of  any  of  these  substances  on  the  excitability  and  conductivity 
of  a  nerve  may  be  studied  by  means  of  the  simple  apparatus  represented  in 
Fig.  80.   The  nerve  of  a  nerve-muscle  preparation  is  passed  through  a  glass  tube 

Fig.  86. 


which  is  made  air-tight  by  plugs  of  normal  saline  clay  surrounding  the  nerve 
at  the  two  ends  of  the  tube.  By  means  of  two  lateral  tubulures  a  current  of 
CO2,  or  air  charged  with  vapour  of  ether  or  other  narcotic,  can  be  passed  through 
the  tube.  The  nerve  is  armed  with  two  pairs  of  electrodes  which  are  stimu- 
lated alternately,  the  pair  within  the  tube  serving  to  test  the  action  of  the  drug 
on  the  excitotiZi^y,  while  the  pair  outside  the  tube  shows  the  presence  or  absence 
of  any  effect  on  the  conducting  iMwer  of  the  nerve  below  it. 

Of  the  gases  and  vapours  mentioned  above,  CO.,  and 
ether  both  diminish  and  finally  abolish  the  excitability  and 
conductivity  of  the  nerve-fibres.  The  conductivity  however 
persists  after  all  trace  of  excitability  has  disappeared,  before 
in  its  turn  being  also  abolished,      On  removing  the  gas   or 


NERVE-FIBRES  (CONDUCTING  TISSUES)  181 

vapour  by  blowing  air  over  the  nerve,  the  conductivity  and 
excitability  gradually  return  in  the  reverse  order  to  their 
disappearance  (Fig.  87). 

Alcohol   is   said   to   increase  the  excitability  or   leave   it 
unaffected,  while  diminishing  the  conductivity  of  the  nerve. 

Fig.  87. 


Tracing  to  show  the  effect  of  ether  on  excitability  and  conductivity  of 
nerve.  Nerve  excited  by  single  induction-shocks  alternately  within 
and  above  ether  chamber.  The  vertical  lines  indicate  contractions 
of  the  muscle  (gastrocnemius).  The  lower  line  indicates  the  periods 
during  which  the  nerve  was  exposed  to  the  action  of  ether,  a,  dis- 
appearance of  excitability;  b,  reappearance  of  excitability;  c,  dis- 
appearance of  conductivity ;  v,  reappearance  of  conductivity.  (From 
a  tracing  kindly  lent  by  Prof.  Gotch.) 

Chloroform  rapidly  abolishes  both  excitability  and  con- 
ductivity. It  is  a  much  more  severe  poison  than  the  drugs 
just  mentioned,  so  that  in  many  cases  its  effects  are  per- 
manent, and  no,  or  only  a  very  partial,  recovery  of  the  nerve 
is  obtained  on  removal  of  the  chloroform  vapour  from  the 
apparatus. 


182 


PHYSIOT,OGY 


Section  5 
POLAEISATION   PHENOMENA   IN    NEEVE 

Electrotonic  current.— In  describing  the  effects  of  the 
passage  of  a  constant  current  through  a  nerve,  we  only 
mentioned  alterations  of  irritability.  There  are  however 
various  other  phenomena  which  are  more  purely  physical  in 
their  nature. 

If  a  constant  current  be  passed  through  a  nerve-fibre 
through  the  electrodes  (x)  and  (y) — (x)  being  the  anode  and 
(y)  the  kathode — and  the  extrapolar  portions  of   the  nerve 

Fio.  S8. 


^  -SV-^ 


Gi  G2 

Diagram  showing  electrotonic  currents.     P,  polarising  circuit; 
G'  G'-,  galvanometers. 


(ab,  cd)  be  connected  with  galvanometers,  it  is  found  that  the 
needles  of  both  are  deflected,  and  the  direction  of  the  deflection 
shows  the  existence  of  a  current  in  the  extrapolar  portions  of 
the  nerve  (a)  to  (b),  and  from  (c)  to  (d). 

The  galvanometers  will  indicate,  before  the  passage  of  the  polarising  current, 
the  ordinary  demarcation  current  of  the  nerve  resulting  from  the  cross-section 
at  the  upper  end.  This  current  flows,  in  the  outer  circuit,  from  equator  to  cut 
end,  and  therefore  in  the  nerve-fibre  from  (a)  to  (b),  and  from  (d)  to  (c).  The 
effect  of  closing  the  polarising  current  will  be  to  increase  the  current  of  rest 
between  (a)  and  (b),  and  to  diminish  that  between  (c)  and  (d). 

We  thus  see  that  the  passage  of  a  current  through  a  part 
of  a  nerve  gives  rise  to  a  current  flowing  through  a  consider- 
able portion  of  the  nerve-fibre  on  each  side  of  the  polarising 


NERVE-FIBRES   (CONDUCTING   TISSUES)  183 

current  and  in  the  same  direction.  This  current  is  called 
the  electrotonic  current.  It  must  not  be  confounded  with 
the  current  of  action,  which  originates  at  one  of  the  poles, 
only  at  make  or  break  of  the  current,  and  is  transmitted 
thence  in  the  form  of  a  wave  with  a  measurable  velocity 
of  about  30  metres  per  second.  The  electrotonic  current 
is  developed  instantaneously,  and  lasts  the  whole  time  that 
the  current  is  flowing  through  the  nerve.  Its  production 
is  dependent  on  the  occurrence  of  polarisation  between  the 
sheathing  and  conducting  part  of  the  nerve-fibre  and  may  be 
exactly  reproduced  on  a  model  consisting  of  a  core  of  zinc  or 
platinum  wire  in  a  casing  of  cotton  soaked  with  ordinary  salt 
solution.  Although  thus  physical  in  origin,  its  production  is 
dependent  on  the  vitality  of  the  nerve,  and  so  is  not  to  be 
confounded  with  the  simple  spread  of  current. 


Glass  tube 

containing   0-6%  Na  Cl. 

Pt.wire 


Apparatus  for  iruitating  the  polarisation  phenomena  in  medullated 
nerve  ('  Kcrnlciter  '  model). 

The  polarisation  phenomena  resulting  from  the  passage  of 
a  constant  current  through  a  medullated  nerve  can  be  studied 
on  a  model  made  up  of  a  glass  tube  filled  with  normal  salt 
solution,  containing  a  platinum  or  zinc  wire  stretched  through 
it  (Fig.  89).  On  leading  a  current  through  a  and  b,  and 
connecting  c  and  d  with  a  galvanometer,  a  current  will  be 
observed  in  the  extrapolar  portion  of  the  model  in  the  same 
direction  as  in  the  intrapolar.  That  this  spread  of  current  is 
due  to  polarisation  is  shown  by  the  fact  that,  if  the  model 
be  made  of  zinc  wire  immersed  in  saturated  zinc  sulphate 
solution,  where  no  polarisation  can  occur,  the  spread  of 
current  to  the  extrapolar  area  is  also  wanting.  If  we  ex- 
mine  the  phenomena  taking  place  at  the  anode,  we  see  that  a 
current  passes  here  through  an  electrolyte  to  the  conducting 
core.  Every  passage  of  a  current  through  an  electrolyte 
must  be  accompanied  by  dissociation,  the  current  being  carried 


184  PHYSIOLOGY 

by  the  ions.  We  get  therefore  a  movement  of  negative  ions 
up  into  the  electrode,  and  a  deposition  of  electro-positive 
ions  on  the  core  (Fig.  90,  a).  In  the  same  way  at  the  kathode 
there  will  be  a  deposit  of  electro-negative  ions  on  the  core 
(Fig.  90,  d),  so  we  may  say  that  the  core  is  positively 
polarised  at  the  anode  and  negatively  polarised  at  the  kathode. 

Vm.  90. 


♦  -+ -  *  -  +  +  .  +    +  - 

c         b        a  d        e        f 


Diagram  to  show  polarisation  at  the  surface  between  conducting 
core  and  electrolyte  sheath  in  a  '  Kcrnleiter.' 

This  polarisation,  while  opposing  the  primary  current,  will 
set  up  currents  in  the  surrounding  electrolytic  sheath,  as 
shown  by  the  arrows  in  Fig.  91,  the  current  passing  from 
a  to  6  and  from  6  to  c  in  the  electrolyte,  returning  towards  a  in 
the  core.  Hence  if  we  lead  off  from  the  sheath  in  the  neigh- 
bourhood of  the  anode  from  a  and  c,  a  current  will  pass  in  the 
galvanometer  from  a  to  c,  that  is,  along  the  core  in  the  same 

Fig.  91. 


y^KX 


d     e     ( 


Diagram  to  show  polarisation  currents  in  a  ^Kcrnleiter,'  or  in  a 
medullated  nerve. 

direction  as  the  intrapolar  current.  The  same  factors  will 
cause  an  extrapolar  current  in  the  kathodic  area,  the  katelec- 
trotonic  current. 

It  is  evident  that  this  polarisation  will  not  disappear  at 
once  on  breaking  the  polarising  current.  The  nerve  or  nerve- 
model  will  still  be  positively  polarised  at  the  anode  and 
negatively  polarised  at  the  kathode.     On  connecting  therefore 


NERVE-FIBRES  (CONDUCTlNa  TISSUES)  185 

these  two  points  with  the  galvanometer,  we  shall  get  a  current 
in  the  opposite  direction  to  the  previous  polarising  current, 
viz.  from  anode  to  kathode  (Fig.  92).  This  is  the  so-called 
negative  polarisation  of  nerve.  Similarly  in  the  extrapolar 
regions  of  the  nerve,  we  shall  have  currents  in  the  same 
direction  as  the  previous  polarising  current,  as  shown  by 
the  arrows.      So  far  then  the  nerve  behaves  exactly  like  the 


/         \ 


Fig.  92, 
Polarising 


\V^  ^    //  Neqative  polarisation. 
Diagram  to  show  direction  of  the  negative  polarisation  current. 

mechanical  model.  If  however  we  pass  a  very  strong  current 
through  a  nerve,  and  then  quickly  switch  the  nerve  on  to 
a  galvanometer,  we  find  a  momentary  current  through  the 
galvanometer  in  the  same  direction  as  the  previous  polarising 
current.  This  is  known  as  positive  polarisation  of  nerve. 
It  is  absolutely  dependent  on  the  living  condition  of  the 
nerve,  and  is  in  fact  an  excitatory  phenomenon  due  to  the 
strong  excitation  which  occurs  at  break  of  the  current  at  the 

Fig.  93. 
/^I  rx      Polarising 

a/ ' -^  V 

I  "Mini  -I 


'-/Ty-'^^    Positive  polarisation 

Diagram  to  show  direction  of  the  positive  polarisation  current, 
due  to  a  break  excitation  at  the  anode. 

anode.  Thus  in  the  diagram  (Fig.  93)  a  strong  current  is 
passing  through  the  nerve  from  a  to  k.  When  this  current 
is  broken,  excitation  occurs,  as  we  have  already  learnt,  at  the 
anode,  and  this  excitatory  state  may,  if  the  previous  currents 
were  strong,  last  two  or  three  seconds.  An  excited  tissue  is 
however  always  negative  towards  adjacent  unexcited  tissue, 
and  therefore  if  we  connect  a  to  k,  there  must  be  a  current 
outside  the  nerve  from  k  to  a,  and  in  the  nerve  from  a  to  k, 


im  PHYSIOLOGY 

viz.  in  the  same  direction  as  the  polarising  current.  We  see 
therefore  that  negative  polarisation  is  due  to  polarisation 
occurring  between  an  electrolytic  sheath  and  a  conducting 
core,  whereas  positive  polarisation  is  hardly  a  polarisation 
effect  at  all  but  is  a  current  of  action. 

Paradoxical  contraction.  —If  the  sciatic  nerve  of  a  frog 
be  dissected  out,  and  one  of  the  two  branches  into  which  it 


Diagram  of  arrangement  for  showing  paradoxical  contraction. 

divides  be  cut,  and  the  central  end  of  this  branch  stimulated, 
the  muscles  supplied  by  the  other  half  of  the  nerve  contract 
to  each  stimulus.  Ligature  or  crushing  of  the  nerve  (x) 
between  the  points  stimulated  and  the  point  which  joins  the 
main  trunk  puts  a  stop  to  this  effect,  showing  that  it  is  not 
due  to  a  mere  spread  of  current.  The  fibres  passing  down 
(n)  are  in  fact  stimulated  by  the  electrotonic  current  developed 
in  (x)  during  the  passage  of  the  exciting  current. 


NERVE-FIBRES  (CONDUCTING  TISSUES)  187 


Section  6 
THE   NATUKE   OF  THE   NEEVOUS   IMPULSE 

It  has  been  a  subject  of  much  debate  what  the  exact 
nature  of  the  nervous  impulse  really  is,  and  how  it  is  trans- 
mitted down  the  nerve.  We  have  no  other  evidence  of  the 
passage  of  an  impulse  down  a  nerve  than  the  current  of  action 
accompanying  the  impulse,  and  the  result  of  the  arrival  of 
the  impulse  at  the  terminal  organ,  whether  it  be  contraction 
of  muscle,  secretion  of  gland,  or  production  of  sensation  in 
the  central  nervous  apparatus.  Since  all  these  effects  may  be 
produced  by  electrical  stimulation  of  the  end-organs  them- 
selves, it  has  been  thought  that  the  wave  of  '  negativity  '  is 
the  nervous  impulse  ;  that  is  to  say,  that  the  current  pro- 
duced in  any  given  section  of  the  nerve-fibre,  when  it  is 
stimulated  and  so  becomes  negative,  excites  the  adjacent 
segments  or  molecules,  causing  them  to  become  negative,  and 
thus  setting  up  another  current  of  action,  which  in  its  turn 
excites  the  molecules  next  in  order  ;  and  that  in  this  way 
the  excitatory  process  travels  the  whole  length  of  the  nerve. 
A  natural  corollary  of  this  view  would  be  that  the  normal 
excitation  of  nerve-fibre  in  the  brain  or  at  the  periphery 
is  also  brought  about  by  electrical  means.  There  are  how- 
ever considerable  difficulties  in  the  way  of  accepting  this 
hypothesis.  Although  we  can  to  a  great  extent  imitate  the 
results  of  the  natural  excitation  of  nerve-fibres  by  artificial 
electrical  stimulation,  yet  the  phenomena  in  the  two  cases  do 
not  run  exactly  parallel,  and  many  differences  are  found  to 
exist  between  the  two  forms  of  stimulation. 

One  of  the  most  striking  examples  of  this  difference  is 
the  varying  nature  of  the  muscular  contraction  as  produced  by 
electrical  stimulation  or  by  natural  (voluntary)  stimulation. 
In  dealing  with  the  physiology  of  muscle,  we  saw  reason  to 
conclude  that  the  natural  contraction  of  muscle  is  a  con- 
tinuous one,  although  our  only  certain  way  of  producing  an 
approximate  resemblance  of  such  a  contraction  was  by  the 
use  of  discontinuous  electrical  stimulation. 


188  PHYSIOLOGY 

An  attempt  has  been  I'ecently  made  by  Boruttau  to  explain  the  nerve- 
process,  not  as  a  wave  of  electrical  change  affecting  the  substance  of  the 
axis-cylinder  itself,  but  as  a  propagated  katelectrotonic  current.  This 
observer  found  that  by  working  with  a  'platinum  core  model'  {'  Kernleiter') 
(Fig.  89)  of  considerable  length,  the  katelectrotonic  current  was  developed 
at  one  end  of  the  model  some  appreciable  time  after  a  current  had  been  sent  in 
at  the  other  end,  thus  resembling  a  current  of  action. 

It  is  however  impossible  to  explain  all  the  electrical  phenomena  of  nerve 
as  due  simply  to  polarisation.  We  might  go  so  far  as  to  assume  that  the 
excitatory  effect  at  the  kathode  is  due  to  negative  polarisation,  and  that  ex- 
citation at  break,  i.e.  at  the  anode,  is  caused  by  the  sudden  coming  into 
existence  of  a  negative  polarisation  current ;  but  then  it  would  be  impossible 
to  understand  how  the  excitation,  so  produced  at  the  anode,  should  give 
rise  to  a  current  so  much  exceeding  the  current  which  produced  it,  that 
it  would  appear  in  our  external  circuit  as  a  current  of  positive  polarisation. 
We  must  in  fact  conclude  that  the  axis-cylinder  of  the  nerve  is  endowed 
with  an  energy  of  its  own,  which  is  let  loose,  so  to  speak,  under  the  influence 
of  chemical  or  electrical  changes,  just  as  the  energy  of  a  contracting  muscle 
is  set  free  by  the  exertion  of  an  infinitesimal  force  applied  as  a  stimulus. 
The  nerve  does  not  simply  transmit  the  energy  which  is  imparted  to  it,  like 
a  telegraph  wire,  but  itself  furnishes  the  energy  of  the  descending  nerve- 
process. 

Against  this  view  might  be  urged  the  absence  of  phenomena  of  fatigue  in 
nerve,  as  showing  that  nervous  activity  is  not  accompanied  by  any  expenditure 
of  energy  or  using  up  of  material.  But  it  must  be  remembered  that  this 
absence  of  fatigue  holds  good  only  for  meduUated  nerve-fibres  and  is  not 
found  in  non-medullated  nerves,  and  even  in  meduUated  nerves  the  persistence 
of  irritability  is  dependent  on  the  continual  supply  of  a  certain  small  amount 
of  oxygen.  It  may  therefore  possibly  be  explained  by  a  continual  process  of 
restitution  taking  place  at  the  expense  of  the  sheath.  Fatigue  is  absent,  not 
because  nothing  is  used  up,  but  because  the  assimilative  changes  exactly 
balance  and  make  good  the  dissimilation  involved  in  the  propagation  of  a 
nervous  impulse. 


X89 


CHAPTER  VI 
THE  VASCULAR  MECHANISM 

Section   1 

THE   MECHANICAL  PRINCIPLES   OF  THE 
CIRCULATION 

Structure  and  Properties  of  the  Blood-vessels 

We  have  now  to  study  more  fully  the  manner  in  which 
the  continuous  circulation  of  the  hlood  through  the  body  is 
carried  on. 

It  will  be  remembered  that  the  vascular  system  consists 
of  a  closed  system  of  tubes  divided  into  two  circuits,  known 
respectively  as  the  greater  or  systemic  circulation,  and  the 
lesser  or  pulmonary  circulation,  both  connected  with  a  cen- 
tral pumping  organ,  the  heart,  in  the  thorax.  By  means  of 
the  pumping  action  of  the  heart  a  continuous  stream  of  blood 
is  kept  up  from  the  right  side,  through  the  lungs,  back  to 
the  left  side  of  the  heart,  whence  the  blood  is  sent  by  the 
contraction  of  the  left  ventricle  through  all  the  rest  of  the 
body,  and  is  returned  by  means  of  the  superior  and  inferior 
venae  cavse  to  the  right  side  of  the  heart,  so  completing  the 
double  circuit.  The  blood-vessels  vary  considerably  in  struc- 
ture according  to  their  situation  in  the  circuit.  The  vessels 
which  carry  the  blood  from  the  heart  to  the  tissues,  the 
arteries,  are  thick-walled,  and  contain  an  abundance  of 
muscular  and  elastic  elements  in  their  walls.  The  typical 
medium-sized  artery  is  described  as  consisting  of  three  coats 
(Fig.  95) :  an  intima  lined  by  a  continuous  layer  of  flattened 
endothelial  cells,  which  rest  on  a  well-marked  lamina  of 
yellow  elastic  tissue  ;  a  media  composed  of  unstriated  mus- 
cular fibres  arranged  longitudinally  and  circularly  ;  and  an 
external  coat  or  adventitia  of  fibrous  tissue,  with  a  number 


190  -  PHYSIOLOGY 

of  longitudinal  elastic  Hbres.  Near  the  heart,  in  the  great 
vessels  such  as  the  aorta  and  its  larger  branches,  there  is  a 
preponderance  of  elastic  tissue  as  compared  with  the  muscular  ; 
and  we  find  in  the  media  alternate  layers  of  muscle-fibres 
and  fenestrated  elastic  membranes.  In  the  smallest  arteries, 
on  the  other  hand,  the  arterioles,  the  elastic  element  entirely 
disappears,  so  that  the  wall  consists  of  muscle-fibres,  chiefly 
circular,  lined  by  the  endothelium.  It  is  evident  that  in 
these  latter  vessels  a  contraction  of  their  walls  may  result  in 
an  entire  obliteration  of  their  lumen.  Judging  simply  from 
the  structure  of  these  vessels,  one  might  guess  that  the  chief 
function  of  the  large  arteries  is  to  serve  as  elastic  conduits, 
whereas  that  of  the  arterioles  is  to  regulate  the  amount  of 
blood  flowing  through  them,  and  therefore  the  vascular  supply 

Fig.  95. 


Transverse  section  of  part  of  the  wall  of  the  posterior  tibial  artery 
( X  75).  a,  endothelial  and  subendothelial  layers  of  intima ; 
6,  lamina  of  elastic  tissue ;  c,  media  consisting  of  muscle-fibi'es ; 
d,  adventitia.     (Schiifer.) 

to  the  tissues  beyond.  As  the  arteries  branch  there  is  a 
gradual  enlargement  of  the  total  cross-section  of  the  system 
of  tubes,  although  the  individual  tubes  become  progressively 
smaller. 

Although  the  muscle-fibres  in  the  coats  of  the  aorta  and  large  arteries  cannot 
by  their  contraction  alter  appreciably  the  flow  of  blood  through  these  tubes, 
they  have  nevertheless  an  important  function.  Like  all  other  kinds  of  un- 
striated  muscle,  they  are  extremely  susceptible  to  changes  of  tension.  Any 
sudden  increase  in  the  pressure  within  the  arteries  will  therefore  excite  these 
fibres  to  contract,  so  that  the  muscular  tissue  is  able  to  support  and  reinforce 
the  inert  elastic  tissue  of  the  arterial  wall  in  withstanding  any  sudden  strain 
caused  by  a  great  rise  of  arterial  pressure.  The  muscle-fibres  in  the  arteries 
have  in  fact  the  same  protecting  function  on  the  connective  tissue  elements  that 
is  exercised  (but  by  a  more  complex  mechanism)  by  the  skeletal  muscles  which 
send  their  tendons  across  a  joint.  In  the  latter  case,  as  we  shall  see  later,  any 
sudden  stretching  of  the  tendon  excites  the  corresponding  muscle  to  contract, 
so  taking  the  strain  of  the  joint  movement  off  the  ligamentous  structures  which 
surround  it. 


THE   VASCULAR   MECHANISM 


191 


In  the  tissues  themselves  the  blood  flows  through  a  close 
network  of  canals — the  capillaries.  In  these  all  the  elements 
of  the  vascular  wall  have  disappeared,  except  the  endo- 
thelium, so  that  the  blood  may  come  in  closest  possible  con- 
nection with  the  tissues,  without  actual  extravasation  into 
the  tissue-spaces.  Owing  to  the  great  number  of  these 
capillaries,  the  vascular  bed  undergoes  a  sudden  enormous 
enlargement  as  we  proceed  from  arterioles  to  capillaries. 

The  blood  is  collected  from  the  capillaries  into  small 
venous  radicles,  which  unite  to  form  larger  and  larger  veins. 


Capacity  iu  c.c. 


Fig.  96. 


\,  /- — I 1 1 1 1 . 

-74—1 n ^-^^'' 


120       130       140        ISO        160 


Curves  of  distensibility  of  an  artery  (thick  line)  and  of  a  vein  (thin 
line).  The  figures  at  the  left  side  of  the  diagram  represent  the 
capacity  of  a  section  of  the  vessel  when  distended  under  a  certain 
pressure,  expressed  by  the  figures  on  the  base  line  in  mm.  Hg. 
(Constructed  from  figures  given  by  Koy.) 

Although  in  the  vein  we  can  distinguish  by  dissection  the 
same  three  coats  as  in  the  typical  artery,  the  thickness  of 
the  wall  of  the  vein  is  very  much  less  in  proportion  to  its 
lumen  than  is  the  case  with  the  arteries.  There  is  moreover 
a  diminution  of  the  muscular  and  elastic,  and  a  relative 
increase  of  the  fibrous  tissue-elements  of  the  wall.  A 
vein  therefore  collapses  altogether,  unless  distended  by  some 
internal  pressure. 

This  histological  difference  between  veins  and  arteries 
is  of  considerable  importance  for  the  understanding  of  the 
distribution  of  pressures  in   the   vascular  system,  since  the 


192  PHYSIOLOGY 

distensibility  and  reaction  to  pressure  of  these  vessels  are 
conditioned  by  their  structure.  In  Fig.  96  is  represented  the 
extensibility,  i.e.  the  increase  in  capacity  of  an  artery  and 
a  vein  under  gradually  increasing  internal  pressure.  It  will 
be  seen  that  an  artery  which  has  a  certain  capacity  at  zero 
pressure,  gradually  distends  with  increasing  pressure.  The 
increase  in  capacity  is  however  small  at  first,  and  becomes 
most  rapid  between  90  and  110  mm.  Hg.  After  this 
point  every  increment  of  pressure  brings  about  a  gradually 
diminishing  increment  of  capacity.  Thus  a  change  of  in- 
ternal pressure  causes  the  greatest  change  in  capacity  when 
the  pressure  in  the  artery  corresponds,  as  we  shall  see,  to 
the  average  arterial  pressure  in  the  normal  animal. 

In  the  vein,  on  the  other  hand,  the  capacity,  which  is 
nothing  at  zero  pressure,  becomes  considerable  on  raising  the 
pressure  to  1  mm.  Hg.  A  further  rise  of  pressure  to  10  mm. 
Hg.  causes  a  considerable  increase  in  volume,  but  from  this 
point  the  increments  of  volume  with  rising  pressure  rapidly 
diminish.  Whereas  therefore  the  artery  is  most  distensible 
at  about  100  mm.  Hg.,  the  vein  has  its  limits  of  optimum 
distensibility  between  0  and  10  mm.  Hg. 

The  Blood-pressure 

If  an  artery  in  the  living  animal  be  cut  across,  blood 
spurts  from  it  to  a  considerable  height,  escaping  in  jerks 
corresponding  to  every  heart-beat.  This  fact,  which  shows 
the  existence  of  a  certain  amount  of  pressure  on  the  blood  in 
the  artery,  may  be  illustrated  in  another  way.  If  a  vertical 
glass  tube  be  connected  with  the  central  cut  end  of  the  carotid 
artery,  the  blood  will  rise  in  it  to  a  height  of  three  feet  (in 
the  rabbit),  or  still  higher  in  the  case  of  the  dog,  and  remain 
about  this  height,  rising  a  little  with  every  heart-beat,  and 
falling  again  between  the  heart-beats. 

We  thus  see  that  the  blood  in  the  artery  is  under  a  con- 
stant pressure,  which  varies  to  a  slight  extent  with  the  heart- 
beats, rising  with  and  sinking  between  the  beats,  but  never 
approaching  the  line  of  no  pressure. 

This  blood-pressure  may  be  more  conveniently  measured  and  its  variations 
studied  by  means  of  an  instrument  called  a  manometer.  If  we  simply 
measure  the  pressure  by  inserting  a  vevtical  tube  into  the  cut  central  end  of  aa 


THE   VASCULAR   MECHANISM 


193 


artery,  the  animal  is  injured  by  the  loss  of  the  blood  which  is  necessary  to  fill 
the  tube,  and  the  experiment  is  soon  stopped  by  the  clotting  of  the  blood  in 
the  tube. 

There  are  many  different  forms  of  manometer.  The  best  for  investigating 
changes  in  the  mean  arterial  blood-pressure  is  Ludwig's  mercurial  manometer. 
This  instrument  consists  essentially  of  a  U-tube  with  two  vertical  limbs  about 
eighteen  inches  long.  This  is  half  filled  with  clean  mercury.  On  the  surface 
of  the  mercury  in  one  limb  is  a  float,  from  which  a  stiff  fine  rod  (of  steel  or 
glass)  rises,  bearing  on  its  upper  end  a  writing  point.  This  point  is  adjusted 
so  as  to  record  its  movements  by  scratching  a  white  line  on  the  smoked  paper 
of   a    kymograph.      (A   kymograph    is   merely  an    arrangement   of   revolving 

Fig.  97. 


Arrangement  of  apparatus  for  taking  blood-pressure  tracing,  a, 
artery  (carotid) ;  c,  cannula ;  d,  three-way  cock ;  m,  mercurial 
manometer  ;  b,  drum  covered  with  smoked  paper. 


cylinders,  moved  by  clockwork  or  other  means,  arranged  to  carry  a  roll  of 
smoked  paper.)  Instead  of  using  the  smoked  paper,  a  pen  may  be  fitted  to  the 
end  of  the  vertical  rod,  and  its  excursions  recorded  in  ink  on  a  moving  band  of 
white  paper. 

The  other  limb  of  the  manometer  is  connected  by  a  flexible  rigid  tube  (such 
as  lead)  with  a  small  tube  or  cminula,  which  is  tied  into  the  central  end  of  the 
artery.  While  the  cannula  is  being  tied,  a  clip  is  placed  on  the  artery  at  (a), 
so  as  to  prevent  the  blood  escaping.  At  (d)  there  is  a  three-way  cock.  This  is 
first  turned  so  as  to  put  the  tube  (x)  into  connection  with  the  tube  to  the 
cannula,  and  the  whole  is  then  filled  with  a  half-saturated  solution  of  sodium 
sulphate,  or  a  25  per  cent,  solution  of  magnesium  sulphate.  The  cock  is  then 
turned  and  the  tube  leading  to  the  manometer  filled  in  the  same  way 

Both  tubes  being  full,  the  solution  is  injected  forcibly  into  (x),  so  as  to 
raise  the  column  of  mercury  about  150  mm.     The  cock   is  then  turned  so 

u 


194  PHYSIOLOGY 

that  the  manometer  is  put  into  connection  with  the  artery.  The  clip  is  then 
talien  oE  tlie  artery.  Tlie  column  of  mercury  drops  to  about  120  mm.  (if  the 
carotid  of  the  dog  is  the  artery  used),  and  stops  at  about  that  level,  rising  and 
falling  slightly  with  every  heart-beat.  The  object  of  using  sodium  sulphate  or 
magnesium  sulphate  solution  is  to  delay  clotting  in  the  cannula. 

On  investigating  in  a  similar  manner  the  condition  of  the 
veins,  we  find  quite  a  different  state  of  things.  If  a  vein  be 
ligatured  in  any  part  of  its  course,  it  swells  up  on  the  distal 
side  and  shrinks  on  the  side  towards  the  heart.  If  it  be  cut 
across,  the  bleeding  that  occurs  takes  place  nearly  entirely 
from  the  distal  end.  The  haemorrhage  moreover  is  of  a 
different  character  from  that  which  occurs  when  an  artery  is 

Fig.  98, 


Blood-pressure  tracing  taken  with  mercurial  manometer  (from 
carotid  of  rabbit).     A,  abscissa  or  line  of  no  pressure. 

divided.  The  blood,  instead  of  spui'ting  out  to  a  distance, 
wells  up  and  is  not  increased  or  in  any  way  affected  by  the 
heart-beat.  If  we  connect  a  manometer  with  a  vein,  we  find 
that  the  pressure  amounts  to  a  few  mm.  of  mercury.  Thus 
we  see  that  the  blood,  which  in  the  arteries  is  under  high 
pressure  and  has  an  intermittent  flow,  by  the  time  it  has 
reached  the  veins  is  at  a  low  pressure,  and  the  flow  has  lost 
its  intermittent  character. 

What  is  the  cause  of  this  change  in  the  character  of  the 
flow?  The  blood  in  passing  from  arteries  to  veins  has  to 
traverse  the  arterioles  and  capillaries  and  in  so  doing  meets 
with  considerable  resistance  owing  to  the  friction  between  the 
blood  and  the  vessel-walls.  Every  time  an  artery  divides, 
although  each  separate  branch  is  smaller  than  the  original 
branch  from  which  it  springs,  the  united  sectional  area  of  the 


THE   VASCULAK  MECHANISM  195 

two  branches  is  greater,  so  that  the  sectional  area  of  the 
capillaries  exceeds  by  many  hundred  times  that  of  the  aorta. 
If  this  increase  in  the  sectional  area  took  place  without 
division,  its  effect  would  be  to  lower  resistance  to  the  flow  of 
blood.  But  if  we  consider  for  a  moment  the  condition  of  the 
circulation  of  the  capillaries,  we  see  that  the  friction-lowering 
efBfect  of  increased  area  is  much  more  than  compensated  for 
by  the  increased  surface  and  therefore  increased  friction. 
Many  of  the  capillaries  are  no  wider  than  a  single  blood- 
corpuscle.  The  resistance  of  such  a  capillary  system  would 
be  very  large  even  to  a  stream  of  water,  much  more  so  then 
to  a  fluid  which  is  somewhat  viscid,  and  has  suspended  in  it 
a  number  of  solid  particles. 

Another  factor  however  is  involved  in  this  question  of 
resistance.  The  friction  between  the  blood  and  the  walls  of 
the  vessels  depends  not  only  on  the  extent  of  surface-con- 
tact, but  also  on  the  velocity  of  the  blood.  Owing  to  the 
great  increase  in  area  on  passing  from  arterioles  to  capillaries, 
there  is  a  sudden  slowing-down  of  the  blood-stream.  The 
arterioles  may  be  compared  to  small  inlets  into  a  wide  lake. 
On  this  account  it  seems  that  the  main  resistance  to  the 
passage  of  blood  through  the  tissues  is  situated,  not  in  the 
capillaries  but  in  the  arterioles,  and  may  be  varied  within 
very  wide  limits  by  alterations  in  the  calibre  of  these  vessels. 

The  main  factors  in  converting  the  intermittent  flow  of 
the  arteries  into  a  constant  flow  through  capillaries  and  veins 
and  in  maintaining  a  mean  blood-pressure  in  the  arteries 
are  — 

1.  The  contraction  of  the  heart. 

2.  The  peripheral  resistance. 

3.  The  elastic  reaction  of  the  arterial  walls. 

Since  this  is  a  purely  mechanical  question,  it  will  be  more 
easily  understood  by  a  simple  illustration.  The  heart  may 
be  regarded  as  a  pump,  forcing  a  certain  amount  of  blood 
(about  3  oz.)  into  the  circulation  at  each  stroke.  If  a  pump 
be  connected  with  a  rigid  tube,  every  time  that  a  certain 
amount  is  forced  into  the  beginning  of  the  tube,  an  exactly 
equal  quantity  will  be  forced  out  at  the  other  end  at  the 
same  time.  Increasing  the  peripheral  resistance  by  partial 
closure  of  the  end  of  the  tube  will  not  afiect  the  intermittent 
character  of  the  flow,  but  will  merely  serve  to  diminish  the 


196  PHYSIOLOGY 

quantity  thrown  in,  as  well  as  the  quantity  which  escapes  at 
the  other  end  of  the  tube,  supposing  that  the  work  done  by 
the  pump  is  equal  in  both  cases.  If,  instead  of  a  rigid  tube, 
we  employ  an  elastic  tube,  and  the  end  be  left  open  so  that 
no  resistance  is  offered  to  the  outflow  of  the  fluid,  the  effect 
will  be  the  same  as  when  we  used  the  rigid  tube  ;  the  outflow 
will  correspond  exactly  to  the  inflow,  and  will  be  just  as  inter- 
mittent. But  now,  if  the  end  of  the  elastic  tube  be  partially 
clamped,  so  as  to  increase  the  resistance  to  the  outflow,  there 
will  be  a  marked  difference  between  this  effect  and  that  pro- 
duced by  the  rigid  tube.  Each  stroke  of  the  pump  forces 
a  certain  amount  of  fluid  into  the  tube.  Owing  to  the 
peripheral  resistance,  this  cannot  all  escape  at  once,  and  so  part 
of  the  force  of  the  pump  is  spent  in  distending  the  walls  of  the 
tube,  and  part  of  the  fluid  that  was  forced  in  remains  in  the 
tube.  The  distended  elastic  tube  however  tends  to  empty 
itself,  and  forces  out  the  fluid  which  over-distends  it  before 
the  next  stroke  of  the  pump  occurs.  So  now  the  outflow  may 
be  divided  into  two  parts, — one  part  which  is  forced  out  by 
the  immediate  effect  of  the  stroke  of  the  pump,  and  another 
part  which  is  forced  out  by  the  elastic  reaction  of  the  tube 
between  the  strokes.  If  the  strokes  be  rapidly  repeated 
before  the  tube  has  time  to  thoroughly  empty  itself,  it  will 
get  more  and  more  distended.  Greater  distension  means 
stronger  elastic  reaction,  and  therefore  stronger  outflow  of 
fluid  between  the  beats.  This  distension  goes  on  increasing 
till  the  fluid  forced  out  between  the  strokes  by  the  elastic  re- 
action of  the  wall  of  the  tube  is  exactly  equal  to  that  entering 
at  each  stroke,  and  the  outflow  thus  becomes  continuous. 

The  same  thing  occurs  in  the  living  body.  A  man's 
heart  at  each  beat  or  contraction  forces  about  three  ounces 
of  blood  into  the  already  distended  aorta.  The  first  eftect  of 
this  is  to  distend  the  aorta  still  further.  The  elastic  reaction 
of  the  walls  drives  on  another  portion  of  blood,  which  distends 
the  next  segment  of  the  arterial  wall,  and  so  the  wave  of 
distension  is  transmitted  with  gradually  decreasing  force 
along  the  arteries.  This  wave  of  distension  is  what  we  feel 
on  the  radial  artery,  or  any  exposed  artery,  as  the  pulse. 
After  each  heart-beat  the  arteries  tend  to  return  to  their 
original  size,  and  drive  the  blood  on  through  the  arterioles 
(the  peripheral  resistance)  into  the  capillaries  and  so  into  the 


THE   VASCULAE   MECHANISM 


197 


veins.  By  the  time  the  blood  has  reached  the  veins,  all 
trace  of  the  heart-beat  has  disappeared,  and  the  pressure  has 
fallen  to  a  few  mm.  of  mercury. 

The  accompanying  diagram  represents  roughly  the  dis- 
tribution of  pressure  along  the  vascular  system.  The  blood- 
pressure  falls  only  slowly  in  the  great  arteries,  as  is  shown  by 
the  line  bp  in  the  first  part  of  section  a.  Towards  the  end  of 
this  section  there  is  a  sudden  and  extensive  fall  of  pressure 
caused  by  the  increase  of  resistance  in  the  arterioles.  In  the 
capillaries  (c)  and  in  the  veins  (v)  the  blood -pressure  once 
more  falls  gradually  until,  in  the  big  veins  near  the  heart,  it 
may  be  negative. 

Fig.  99. 


Scheme  of  blood-pressure  in — a,  the  arteries  ;  c,  capillaries ;  and 
V,  veins,  oo.  Line  of  no  pressure,  lv.  Left  ventricle,  ra. 
Eight  auricle,     bp.  Height  of  blood-pressure. 


The  following  table  may  serve  to  give  some  idea  of  the 
probable  average  height  of  the  blood-pressure  at  different 
parts  of  the  vascular  system  in  man.  It  must  be  remembered 
however  that  these  pressures  are  all  subject  to  considerable 
variations  according  to  the  physiological  condition  of  the 
various  parts  and  organs  of  the  body. 


Large  arteries  {e.g.  carotid) 

Medium  arteries  {e.g.  radial) 

Capillaries 

Small  veins  of  arm  . 

Portal  vein 

Inferior  vena  cava  . 

Large  veins  of  neck 


+  140  mm.  mercury. 
+  110  mm.         „ 

about         -r     15  to  -f  20  mm.  mercury. 

+       9  mm.  „ 

+     10  mm.  „ 

-1-       3  mm.  „ 

from  0  to  -       8  mm.  „ 


These  main  facts  of  the  circulation  can  be  well  illustrated 
on  a  model  made  of  indiarubber  tubing,  such  as  the  artificial 
schema  represented  in  Fig.  100.  (b)  is  a  basin  of  water,  (e) 
an  enema  syringe,  which  can  be  used  to  force  on  water,  and 
represents  the  heart.     This  is  connected  by   an  indiarubber 


198 


PHYSIOLOGY 


tube  (a)  with  a  tube  (c-)  ^vhich  is  packed  with  sponges  to 
represent  the  peripheral  resistance  in  the  capillaries.  From 
the  distal  end  of  (c-)  a  tube  (v)  serves  to  conduct  the  lluid 


Fig.  100. 


Diagram  of  artificial  circulation  schema. 

back  to  the  basin.  To  side  branches  of  (a)  and  (v)  two 
mercurial  manometers  (m'  and  m-)  are  connected,  and  these 
are  arranged  to  write  one  below  the  other  on  the  smoked 
surface  of  a  kymograph.  Another  route  for  the  fluid  from 
(a)  to  (v)  is  aflbrded  by  the  tube  (c'),  which  may  be  clamped 

Fig.  101. 


Tracing  taken  from  artificial  schema  with  slight  peripheral 
resistance  (Foster),     a,  arterial ;  v,  venous  manometer. 

at  will.  Tracings  are  first  taken  of  the  pressures  on  the 
arterial  and  venous  sides  with  the  tube  (c')  open,  while  the 
fluid  is  forced  through  the  system  by  rhythmical  compression 


THE   A'ASCULAR   MECHANISM 


3  99 


of  the  bulb  of  the  enema.  The  tiuid  in  passmg  from  (a)  to  (v) 
has  now  practically  no  resistance  to  overcome,  and  accordingly 
we  find  the  pressm-e-tracings  of  the  two  manometers  (Fig.  101) 
are  almost  identical,  the  fluid  escaping  from  the  end  of  (v)  at 
each  stroke  of  the  pump. 

c'  is  now  clamped  so  that  all  the  fluid  must  pass  through 
the  tube  (c'-)  with  a  high  resistance.  Tracings  are  again 
taken  (Fig.  102),  and  they  show  that  the  pressure  on  the 
arterial  side  at  first  rises  with  every  beat  till  it  has  attained  a 

Fig.  102. 


Tracing  from  artificial  schema  with  considerable  peripheral 
resistance,     a,  arterial ;  v,  venous  manometer. 


certain  height,  where  it  remains  stationary,  merely  oscillating 
with  every  stroke  of  the  pump.  The  venous  manometer,  on 
the  other  hand,  shows  scarcely  any  rise  of  pressure,  and  its 
oscillations  become  less  and  less  till  they  disappear,  and  the 
flow  becomes  continuous. 


There  is  one  feature  in  the  circulation  which  is  not  represented  in  the 
above  schema.  In  the  latter  the  system  of  tubes  is  open  at  both  ends,  and 
the  amount  of  fluid  supplied  to  the  contracting  heart-model  is  constant.  In 
this  model  therefore,  so  long  as  the  resistance  in  the  circuit  is  constant,  the 
pressure  in  all  parts  of  the  circuit  will  vary  directly  with  the  force  of  the 
heart-beat,  and  a  rise  of  arterial  pressure  will  be  attended  with  a  smaller  rise 
of  venous  pressure,  as  shown  in  Fig.  102.  In  the  body  however  the  blood- 
vessels form  a  closed  circuit,  containing  a  certain  invariable  quantity  of  fluid. 
We  may  imitate  this  condition  in  the  schema  by  connecting  the  venous  end  of 
the  tube  with  the  supply-tube  of  the  enema  syi'inge,  having  previously  over- 


200  PHYSIOLOGY 

filled  the  system  to  a  slight  extent.  Under  these  circumstances,  so  long  as 
the  heart  is  not  beating,  the  pressure  in  all  parts  of  the  system  will  be  the 
same,  and  this  pressure,  which  may  be  called  the  mean  general  blood-pressure, 
amounts  in  a  large  dog  to  about  10  mm.  Hg.  It  will  be  seen  at  once  that  the 
pump  or  heart  cannot  alter  this  mean  general  pressure,  but  can  only  give 
rise  to  an  unequal  distribution  of  the  pressure.  Thus  it  may  diminish  the 
pressure  in  the  veins  and  increase  the  pressure  in  the  arteries  by  pumping 
the  fluid  from  the  veins  into  the  arteries.  We  may  take  Fig.  103  to  repre- 
sent the  vascular  system,  which  has  a  definite  capacity  and  contains  a 
definite  quantity  of  blood.  If  the  heart  (ii)  is  not  acting,  and  the  fluid  is 
motionless,  the  pressures  at  all  parts  of  the  system  will  be  the  same  (10  mm. 
Hg.).  If  now  the  heart  begins  to  act,  it  pumps  blood  from  the  veins  into  the 
arteries,  so  that  the  latter  become  distended  at  the  expense  of  the  former,  and 


the  arterial  pressure  rises  above,  and  the  venous  pressure  sinks  below,  the 
mean  pressure  of  the  system.  It  must  be  remembered  that  in  the  body  the 
tubes  forming  the  veins  are  much  more  distensible  at  low  pressures  than  are 
the  tubes  on  the  arterial  side.  Hence,  when  the  heart  begins  to  beat,  we  get 
a  large  rise  of  pressure  on  the  arterial  side  (from  10  mm.  to  120  mm.)  and 
only  a  small  fall  of  pressure  on  the  venous  side  (from  10  mm.  to  5  mm., 
0  mm.,  or  near  the  heart  about  —5  mm.).  It  is  evident  that  in  such  a  system, 
while  the  resistance  remains  constant,  the  venous  pressure  will  vary  inversely 
with  the  arterial  pressure,  and  not  directly  with  the  latter,  as  is  the  case  with 
an  open  circuit. 

There  is  one  other  important  fact  arising  from  the  closed  condition  of  the 
circulatory  system.  In  the  body,  changes  in  the  peripheral  resistance  are 
effected  by  contractions  of  the  smaller  arteries  and  of  some  of  the  veins.  Now 
the  contractions  of  these  vessels  not  only  increase  the  peripheral  resistance, 
but  also  diminish  the  total  capacity  of  the  system,  so  that  now  we  have  the 
same  amount  of  fluid  as  before  enclosed  in  a  smaller  cavity.  This  must  mean 
an  increased  elastic  distension  of  the  walls  of  the  cavity  and  a  rise  of  the  mean 
general  pressure,  so  that  a  general  contraction  of  the  arterioles,  the  heart-beat 
being  unchanged,  may  cause  a  rise  of  pressure  both  on  arterial  and  venous 
sides. 

It  must  therefore  be  remembered  that,  whenever  a  rise  in  arterial  pressure 
is  produced  by  active  contraction  of  the  smaller  arteries,  it  is  due  to  the 
coincidence  of  two  factors— (a)  increased  peripheral  resistance,  (b)  diminished 
capacity  of  vascular  system. 


THE   VASCULAR   MECHANISM  201 

In  the  living  body  there  are  two  aids  to  the  circulation  on 
the  venous  side,  which  are  not  represented  in  our  schema. 

Firstly,  nearly  all  the  veins  in  the  body  possess  valves, 
formed  by  reduplication  of  their  lining  membrane.  These 
valves  are  so  placed  that  they  allow  the  passage  of  the  blood 
only  in  one  direction,  viz.  towards  the  heart.  Thus  any 
muscular  contraction  pressing  on  the  veins  can  squeeze  their 
blood  only  in  one  direction,  and  in  this  manner  it  assists  the 
onward  flow  of  blood. 

Secondly,  as  we  shall  see  in  treating  of  respiration,  the 
movement  of  the  chest-walls  at  every  inspiration  causes  a 
suction  of  the  blood  towards  the  thorax,  and  it  is  this  aspira- 
tion of  the  thorax  which  gives  rise  to  the  negative  pressure 
found  in  the  big  veins  near  the  heart. 

Velocity  of  the  Blood-jioiv  in  the  Vessels 

Since  the  area  of  the  blood-vessels  increases  progressively 
from  aorta  to  capillaries,  the  rate  of  flow  must  decrease  in  like 
proportion.  There  are  several  methods  by  which  the  rate  of 
flow  in  the  larger  arteries  may  be  measured. 

Fig.  104. 


't 
b 
Diagram  of  Ludwig's  '  Stromiihr.^ 

One  of  the  most  important  historically  is  that  devised  by 
Ludwig,  in  which  the  '  Stromuhr  '  is  used.  This  (Fig.  104) 
consists  of  two  oval  glass  vessels  (a)  and  (b),  connected  above 
by  means  of  a  glass  tube.  The  capacity  of  these  vessels  is 
accurately  known.    They  are  fixed  at  their  lower  ends  in  a  metal 


202  PHYSIOLOGY 

disc  (c),  which  is  fitted  on  to  another  disc.  The  upper  disc  can 
be  turned  round  on  the  lower  disc.  Through  the  latter  run 
two  tu))es  (t)  and  (f),  the  upper  ends  of  which  are  continuous 
with  the  vessels  (a)  and  (b),  and  the  lower  ends,  which  are  bent 
outwards  in  a  horizontal  direction,  can  be  connected  with  the 
central  and  peripheral  ends  of  a  cut  artery  such  as  the  carotid. 
In  using  this  instrument  the  vessel  (a),  the  connecting  tube 
as  far  as  the  mark  (x),  and  the  two  tubes  (t)  and  (f)  are  filled 
with  defibrinated  blood.  The  vessel  (b)  is  filled  up  to  the 
mark  (x)  with  oil.  The  tube  (t)  is  now  fixed  into  the  central 
and  the  other  tube  (t)  into  the  peripheral  end  of  a  cut  artery. 
As  soon  as  the  clips  on  the  artery  are  released,  blood  flows 
from  the  artery  through  (f)  into  (b),  displacing  the  defibri- 
nated blood  in  (a),  which  flows  into  the  peripheral  end  of  the 
artery.  As  soon  as  the  inflowing  blood  has  reached  the 
mark  (x),  the  whole  upper  part  of  the  instrument  is  turned  sud- 
denly round  180°,  so  that  (b)  is  now  in  communication  with  the 
tube  (t).  The  blood,  which  is  still  flowing  steadily  into  (f), 
now  rises  into  (a),  driving  the  oil  back  into  (b),  and  the  blood 
in  (b)  onwards  into  the  peripheral  parts  of  the  artery.  As  soon 
as  the  oil  reaches  its  own  level,  the  instrument  is  turned  round 
again  into  its  previous  position,  and  so  on.  From  the  number 
of  times  that  the  '  Stromulir''  has  been  turned  round,  we  can 
reckon  the  amount  of  blood  that  has  flowed  through  in  a  given 
time,  and  from  this  number,  knowing  the  calibre  of  the  artery, 
it  is  easy  to  compute  the  velocity  of  the  blood  in  the  artery.' 

The  earliest  direct  determinations  of  the  velocity  of  the  blood  in  the  larger 
vessels  were  made  by  Volckmann  by  means  of  an  instrument  called  the 
licemodromonieter.  This  consists  simply  of  a  U-tube  (Fig.  105)  of  approxi- 
mately the  same  size  as  the  vessel  in  which  the  velocity  is  to  be  measured. 
After  being  filled  with  normal  salt  solution  it  is  connected  by  its  two  arms, 
a  and  c,  with  the  proximal  and  distal  ends  of  the  cut  vessel.  By  means  of  the 
taps  at  a  and  c,  the  blood  can  either  flow  directly  from  a  to  c,  or  can  be 
directed  so  as  to  flow  entirely  through  the  U-tube.  In  this  way  the  velocity  of 
the  blood  is  measured  by  seeing  how  long  it  takes  for  the  blood  to  flow  round 
the  U-tube  from  a  to  c. 

Both  these  methods  give  only  the  average  velocity  of  the  blood  in  the  larger 
vessels.  In  order  to  determine  the  rapid  changes  in  velocity  which  occur 
during  each  pulse,  several  instruments  have  been  devised. 

The  liannodromograph  of  Marey  consists  of  a  wide  tube  with  a  window  at 


'  If  we  take  v  for  velocity,  vi  for  volume  of  blood  that  has  flowed  through 

in  a  unit  of  time,  and  a  for  the  sectional  area  of  the  artery,  then  v  =  ~, 

a 


THE   VASCUIiAR  MECHANISM 


203 


one  side  covered  in  by  a  piece  of  elastic  membrane.  Through  this  membrane 
passes  a  writing-point  which  has  an  oar-shaped  enlargement  within  the  tube. 
On  placing  the  tube  in  the  course  of  a  blood-vessel  so  as  to  allow  blood  to  flow 

Fig.  105. 


'<^ 


Hsemodromometer  of  Volckmann. 


through  it,  the  blood-current  causes  a  deflection  of  the  flattened  extremity  of 
the  lever  which,  communicated  to  the  arm  outside  the  tube,  can  be  recorded 
on  a  blackened  surface. 

Fig.  106. 


Diagram  to  show  principle  of  construction  of  Cybulski's  photo- 
hsematachometer. 

A  much  better  instrument  for  this  purpose  is  the  plwtoluTmatachometer  of 
Cybulski   (Fig.   106).      If   a   current  of   blood  be   directed  along  the  tube  ab 


204 


PHYSIOLOGY 


possessing  two  vertical  side  tubes  c  and  d,  the  pressure  at  c  will  be  greater 
than  that  at  d,  since  at  c  the  momentum  of  the  moving  mass  of  blood  is  added 
to  the  lateral  pressure  of  the  fluid.  A  tube  of  this  shape  is  connected  with  an 
artery,  such  as  the  carotid,  and  the  tubes  h  and  h'  are  attached  at  the  points 
c  and  d.  These  two  tubes  are  united  at  their  upper  extremities.  In  this  case, 
so  long  as  the  blood  flows  from  a  to  b,  the  fluid  in  h  will  rise  higher  than  in  h', 
and  the  difference  in  height  of  the  fluid  in  the  two  tubes  will  be  proportional 
to  the  velocity  of  the  blood.  A  graphic  record  of  this  difference  of  pressure  is 
obtained  by  allowing  a  narrow  beam  of  light  to  throw  an  image  of  the  menisci 
of  the  two  columns  of  fluid  through  a  slit  on  to  a  moving  photographic  plate. 
Such  a  record  is  given  in  Fig.  107.  The  width  of  the  black  space  at  any  point 
is  proportional  to  the  velocity  of  the  blood  at  the  moment  at  which  this  part  of 
the  record  was  being  taken.  Of  course  this  instrument  has  to  be  calibrated 
if  we  wish  to  determine  the  velocity  of  the  blood  in  absolute  measure.     In 

Fig.  107. 


Eecord  of  blood-velocity  in  the  carotid  artery  of  the  rabbit. 
(Cybulski.) 


Fig.  107  the  velocity  at  the  point  1  and  1',  corresponding  to  the  cardiac  systole, 
was  248  mm.  per  second.  At  2  and  2',  corresponding  to  the  dicrotic  elevation, 
the  velocity  was  also  248  mm.  At  3  and  3',  towards  the  end  of  diastole,  the 
velocity  sank  to  127  mm. 


The  rate  of  flow  in  the  capillaries  may  be  measured  by 
direct  observation  of  the  rate  at  which  a  blood-corpuscle  moves 
along  a  capillary  of  the  frog's  web.  It  probably  varies  from 
^  to  1  mm.  per  second.  The  area  of  the  large  veins  near  the 
heart  (under  normal  pressure)  is  equal  to  about  twice  that  of 
the  arteries,  and  this  relationship  between  veins  and  arteries 
holds  good  for  the  entire  system.  Hence,  since  the  same 
amount  of  blood  enters  the  heart  as  leaves  it  at  each  beat,  the 
rate  of  flow  in  the  veins  must  be  about  half  that  in  the  corre- 
sponding arteries. 


THE   VASCULAR  MECHANISM  205 

It  is  also  possible  to  measure  the  time  taken  up  by  the 
blood  in  traversing  the  whole  of  the  circulation  once,  the  '  total 
circulation  time.'  To  this  end  a  solution  of  sodium  ferrocyanide 
is  injected  into  the  jugular  vein  on  one  side,  and  the  time  in 
seconds  is  noted  that  elapses  before  the  salt  can  be  detected  in 
the  samples  of  blood  taken  from  the  jugular  vein  of  the  other 
side.  This  time  in  the  horse  is  thirty-one  seconds,  and  in  the 
dog  seventeen  seconds.  In  man  it  has  been  computed  to  be 
about  twenty-three  seconds. 

Many  other  methods  have  been  devised  for  the  same  end.  Thus  we  may 
inject  a  strong  solution  of  a  colouring  matter  such  as  methylene  blue  into  the 
jugular  vein,  and  determine  by  actual  inspection  through  the  wall  of  the  artery 
the  moment  at  which  the  darkly  coloured  blood  arrives  at  the  point  under 
observation.  Or  we  may  use  the  electrical  method  ]3roposed  by  Stewart :  a 
compensated  current  is  sent  through  a  small  section  of  artery  and  a  galvano- 
meter. A  little  strong  salt  solution  is  then  injected  into  the  jugular  vein  on 
the  opposite  side.  As  soon  as  the  blood  containing  the  stronger  salt  solution 
arrives  at  the  artery,  the  resistance  between  the  two  electrodes  is  diminished, 
the  compensation  of  the  current  is  therefore  at  once  destroyed,  and  the  needle 
of  the  galvanometer  is  deflected. 

It  must  be  noted  that  the  '  total  circulation  time  '  does  not  represent  the 
time  it  would  take  for  the  whole  mass  of  blood  to  pass  round  the  entire  circula- 
tion once.  For  in  every  tube  the  particles  at  the  centre  will  move  much  more 
quickly  than  those  in  contact  with  the  walls.  In  the  living  animal  one  blood 
corpuscle  may  pass  rapidly  from  the  aorta  through  the  widely  dilated  arterioles 
of,  e.g.,  the  thyroid  gland,  while  another  corpuscle  may  have  to  pass  slowly 
through  the  constricted  vessels  of  the  skin  of  the  foot  before  returning  to  the 
heart.  The  circulation  time  measured  by  the  above  methods  is  merely  the 
shortest  possible  time  in  which  a  blood  corpuscle,  taking  all  the  short  cuts, 
can  pass  from  right  ventricle  to  left  ventricle,  and  from  left  ventricle  back  to 
right  ventricle.  The  average  circulation  time,  i.e.  the  time  necessary  for  the 
whole  mass  of  the  animal's  blood  to  pass  once  through  the  heart,  is  probably 
two  or  three  times  as  great  as  the  times  given  above. 


206 


PHYSIOLOGY 


Section  2 

THE   CHANGES   OCCUEEING   IN   THE   HEAET   AT 
EACH   CONTEACTION 

Anatomical  Arrangements  of  tJie  Heart-jmnij) 

We  have  already  seen  that  the  heart  consists  of  four 
cavities,  two  auricles  and  two  ventricles,  and  that  each  side  of 
the  heart,  consisting  of  one  auricle  and  ventricle,  represents  a 
pump  which  has  for  its  function  the  driving  of  blood  through 
the  pulmonary  or  the  systemic  circulation.  The  muscular 
fibres  forming  the  wall  of  the  heart  are  so  arranged  that  the 
contraction  of  the  fibres  causes  a  diminution,  and  the  relaxa- 

FiG.  lOB. 


Muscular  fibres  from  the  mammalian  heart  (  x  425).     (Schafer.) 


tion  an  enlargement  of  the  heart-cavities.  The  contractile 
tissue  resembles  voluntary  muscle  in  presenting  a  cross  and 
longitudinal  striation,  but  differs  from  it  in  the  fact  that  each 
contractile  unit  is  not  a  fibre  but  a  quadrilateral  cell,  having 
a  single  nucleus  at  its  centre.  These  cells  are  arranged  end 
to  end  so  as  to  form  fibres  and  possess  no  sarcolemma,  the 
muscle-cells  being  apparently  functionally  continuous  by 
means  of  fine  intercellular  protoplasmic  bridges.     In  both  its 


THE   VASCULAR  MECHANISM  207 

histology  and  mode  of  contraction,  cardiac  muscle  takes  its 
place  midway  between  unstriated  and  skeletal  muscle. 

In  the  mammal  the  auricles  are  separated  from  the 
ventricles  by  a  fibrous  ring,  from  which  most  of  the  muscle- 
fibres  of  the  two  cavities  take  their  origin.  There  is  one  band 
of  fibres  however,  which  are  continuous  across  the  auriculo- 
ventricular  junction,  taking  a  course  down  the  intraventricular 
septum.  It  is  this  bundle  which  is  responsible  in  the  mammal 
for  the  propagation  of  the  contraction  from  the  auricles  to 
the  ventricles.  In  the  ventricles  the  fibres  have  for  the  most 
part  an  oblique  course,  running  from  above  downwards  and 
to  the  left,  sinking  deeply  into  the  substance  of  the  ventricle 
towards  its  apex,  where  they  make  a  spiral  turn  and  are  con- 
tinued up  again  on  the  inner  surface  of  the  ventricle,  ending 
in  many  cases  in  one  of  the  papillary  muscles.  Besides  these 
oblique  fibres,  there  are  a  number  of  longitudinal  and  circular 
fibres,  of  which  many  are  continuous  over  the  two  ventricles. 

Corresponding  with  the  greater  amount  of  work  thrown 
on  the  left  ventricle,  its  wall  is  about  twice  as  thick  as  that 
of  the  right  ventricle  ;  and  on  cutting  a  section  through  the 
two  ventricles  in  a  contracted  condition,  we  see  that  the  thin 
wall  of  the  right  ventricle  lies  in  the  form  of  a  crescent  round 
the  circular  left  ventricle.  The  capacity  of  both  ventricles 
is  approximately  equal,  amounting  in  each  case  to  about  140 
c.c.  (in  complete  relaxation). 

Whereas  the  ventricles  have  to  pump  their  contents  into 
the  arteries  against  a  considerable  pressure,  the  sole  function 
of  the  auricles  is  to  empty  themselves  into  the  relaxed  and 
flaccid  ventricles.  We  therefore  find  that  the  walls  of  the 
auricles  are  considerably  thinner  than  those  of  the  ventricles. 
Their  muscle-fibres  run  both  in  a  circular  and  a  longitudinal 
direction,  the  circular  fibres  being  continued  round  both 
auricles,  special  rmgs  of  circular  fibres  surrounding  the 
openings  of  the  great  veins. 

The  endocardium  which  lines  the  heart-cavities  is  covered 
by  a  continuous  layer  of  endothelium  resting  on  a  little 
fibrillated  connective  tissue  and  similar  to  that  which  lines 
the  vascular  system  generally.  The  auriculo-ventricular 
orifices  as  well  as  the  openings  of  the  aorta  and  the 
pulmonary  artery  are  guarded  by  valves  permitting  the 
passage   of   the  blood  only  in  one  direction.     The  auriculo- 


208 


PHYSIOLOGY 


ventricular  valves  are  thin  flaps  of  fibrous  and  elastic  tissue 
covered  on  each  side  with  endocardium  and  projecting  down- 
wards into  the  cavities  of  the  ventricles.  Their  sail-like 
margins  are  connected  by  thin  tendinous  cords  with  nipple- 
shaped  projections  of  the  muscular  walls  of  the  ventricles,  the 

Fig.  109. 


Left  auricle  and  ventricle,  with  outer  side  cut  away  to  show  chief 
points  in  anatomy  of  heart  (Testut).  1,  aorta ;  2,  pulmonary 
artery  ;  3,  ant.  coronary  vessels ;  5,  5',  puhiionary  veins ;  C,  left 
auricle;  7,  auricular  appendage;  10,  cavity  of  left  ventricle;  11, 
12,  mitral  valves  ;  13,  14,  papillary  muscles ;  16,  arrow  pointing  to 
aortic  orifice. 

so-called  papillary  muscles.  By  these  attachments  the  edges 
of  the  valves  are  kept  close  together  and  prevented  from  ever- 
sion  under  the  strong  pressure  exerted  by  the  contracting 
ventricles.  These  valves  are  two  in  number  on  the  left  side 
of  the  heart,  forming  the  mitral  valves  ;  while  on  the  right 


THE   VASCULAR   MECHANISM  209 

side  of  the  heart,  the  auriculo -ventricular  valves,  though  other- 
wise exactly  similar  to  the  mitral  valves,  are  three  in  number 
and  are  called  the  tricuspid  valves. 

From  a  purely  mechanical  standpoint,  the  valves  guarding 
the  arterial  orifices  are  much  more  perfect  than  those  just 
described,  which  depend  for  their  efficiency  on  the  proper 
contraction  of  the  ventricular  wall  and  musculi  papillares. 
Each  orifice  is  guarded  by  three  valves,  which  are  semilunai 
in  shape,  are  attached  by  their  convex  borders  to  the  arterial 
wall,  and  present  in  the  middle  of  their  free  border  a  small 
fibro-cartilaginous  nodule,  the  corpus  Arantii,  from  which 
fine  elastic  fibres  pass  to  all  parts  of  the  valve.  The  extreme 
margin  of  each  valve,  the  lunula,  on  each  side  of  the  corpus 
Arantii,  is  extremely  thin,  being  formed  of  little  more  than 
the  endocardium.  Whenever  the  pressure  in  the  arteries  is 
greater  than  that  in  the  ventricles,  these  valves  are  closed, 
and  the  thin  margins  come  in  contact  with  similar  portions  of 
adjacent  valves,  so  preventing  the  reflux  of  a  single  drop  of 
blood.  The  borders  of  the  valves  under  these  circumstances 
come  together  in  the  form  of  a  star  composed  of  three  lines  at 
angles  of  120^,  the  three  corpora  Arantii  being  pressed  together 
at  the  centre  of  the  star. 

No  valves  are  found  at  the  orifices  of  the  great  veins  into 
the  auricles,  a  reflux  of  blood  in  this  situation  during  contrac- 
tion of  the  heart  being  prevented  by  the  contraction  of  the 
muscular  rings  round  the  veins,  which  always  precedes  the 
auricular  contraction. 

The  heart,  as  well  as  the  roots  of  the  great  vessels,  lies 
almost  free  in  a  special  serous  cavity,  the  wall  of  which  is 
formed  by  a  tough  fibrous  membrane,  the  pericardium.  This 
is  attached  below  to  the  central  tendon  of  the  diaphragm  and 
above  to  the  arterial  trunks.  It  is  lined  by  a  layer  of  endo- 
thelium continuous  with  a  similar  layer  covering  the  surface 
of  the  heart.  These  two  surfaces  are  kept  continually  moist 
by  means  of  lymph,  forming  the  pericardial  fluid,  so  that  the 
heart  can  move  freely  within  the  pericardium  without  friction. 
One  of  the  chief  functions  of  the  pericardium  appears  to  be  to 
check  an  excessive  dilatation  of  the  heart  during  conditions 
attended  by  a  rise  of  venous  pressure. 


14 


210  PHYSIOLOGY 


The  Phenomena  of  the  Normal  Beat 

If  we  expose  the  heart  of  a  mammal,  as  the  rabbit,  by 
opening  the  chest,  the  animal  being  kept  alive  by  artificial 
respiration,  it  is  possible  to  observe  the  sequence  of  events 
which  happen  at  each  contraction.  The  first  thing  to  be 
noticed  is  a  contraction  of  the  great  veins  near  the  heart, 
which  occurs  simultaneously  on  the  two  sides.  When  the 
wave  of  contraction  reaches  the  auricles,  these  contract 
shortly  and  sharply  towards  the  ventricles,  dragging  down 
with  them  the  auricular  appendages,  which  also  take  part 
in  the  contraction  and  become  pale  and  bloodless.  This  is 
followed  almost  immediately  by  the  contraction  of  the  ven- 
tricles, which  is  more  prolonged  and  forcible.  Contraction 
of  both  ventricles  occurs  synchronously.  As  we  shall  show 
later,  contraction  of  the  ventricles  begins  at  the  base  and 
extends  thence  to  the  apex,  but  the  propagation  of  this  con- 
traction-wave occurs  so  rapidly  that  it  is  impossible  to  follow 
it  with  the  eye,  all  parts  of  the  ventricle  appearing  to  con- 
tract simultaneously.  During  contraction,  the  ventricle 
undergoes  changes  in  shape,  size,  and  position,  becoming 
shorter  from  above  downwards  and  changing  in  cross-section 
from  an  elliptical  to  a  circular  form,  i.e.  the  heart  becomes 
more  conical.  There  is  at  the  same  time  a  twisting  of 
the  inferior  parts  of  the  ventricle,  owing  to  the  oblique  course 
of  the  muscular  fibres.  If  the  pericardium  is  opened,  this 
torsion  causes  a  tilting  of  the  apex  forward  and  to  the  right 
with  each  contraction.  In  the  normal  condition,  when  the 
pericardium  is  intact,  the  apex  remains  almost  stationary 
during  contraction.  This  is  owing  to  the  fact  that  the  peri- 
cardium is  fixed  externally,  and  the  apex  could  not  rise 
without  causing  a  vacuum  between  it  and  the  pericardium. 
The  shortening  of  the  long  axis  of  the  heart  is  rendered 
possible  by  a  change  in  the  position  of  the  auriculo-ven- 
tricular  groove,  which  descends  with  every  contraction,  the 
large  arteries  elongating  at  the  same  time  as  they  are 
stretched  by  the  amount  of  blood  thrown  into  them. 

The  contraction  or  systole  of  the  ventricles  is  followed  by 
a  rapid  relaxation,  and  they  remain  for  some  time  at  rest, 
being  simply  passively  filled  with  the  blood  flowing  in  through 


THE   VASCULAR  MECHANISM  211 

the  great  veins  and  auricles.      This  period  of  relaxation  and 
rest  is  called  the  diastole. 

The  Valvular  Mechanisms  of  the  Heart 

We  may  now  consider  the  effect  of  these  cardiac  events 
on  the  contained  blood.  During  diastole  the  blood  is  flowing 
in  a  steady  stream  from  the  inferior  and  superior  vense  cavae 
into  the  right  auricle  and  thence  into  the  right  ventricle, 
the  propelling  force  being  supplied  by  the  systemic  blood- 
pressure  and  the  auxiliary  forces  already  mentioned,  i.e. 
muscular  movement  and  aspiration  of  the  thorax.  As  the 
great  veins  contract  they  simply  hurry  on  their  contained 
blood  into  the  auricle,  which  immediately  contracts  on  its 
contents,  driving  them  through  the  open  tricuspid  valves 
into  the  right  ventricle.  It  must  be  remembered  that  there 
are  no  valves  at  the  mouths  of  the  great  veins  (except  in 
the  great  coronary  sinus)  to  prevent  reflux  of  blood  into 
them  during  the  auricular  contraction.  They  are  indeed 
unnecessary,  the  exclusive  flow  of  blood  into  the  ventricles 
being  determined  by  the  way  in  which  the  auricles  contract 
towards  the  ventricles,  and  the  low  pressure  in  the  ventricles 
during  diastole.  The  ventricles  during  the  whole  of  diastole 
have  been  getting  more  and  more  distended  by  the  gradual 
inflow  of  blood.  This  distension  is  suddenly  increased  bj^ 
the  auricular  contraction,  and  then  follows  almost  immediately 
the  contraction  of  the  ventricles.  As  the  blood  is  flowing 
from  auricles  to  ventricles,  reflux  currents  or  eddies  must  be 
formed  on  the  ventricular  side  of  the  tricuspid  valves, 
tending  to  keep  these  from  being  widely  opened.  Directly 
the  pressure  rises  in  the  ventricles,  in  consequence  of  the 
contraction  of  their  walls,  the  valves  are  forced  back  till 
their  edges  come  in  contact  and  effectually  prevent  any 
reflux  of  blood  into  the  auricles.  The  close  apposition  of 
the  edges  of  the  valves  is  further  provided  for  by  the 
attachment  of  the  chordae  tendineae  of  the  papillary  muscle  to 
the  adjacent  valves.  As  the  ventricle  shortens,  the  papillary 
muscles  contract,  thus  preventing  the  valves  from  being 
forced  backwards  and  rendered  incompetent.  The  eflect  of 
this  contraction  of  the  papillary  muscles  is  that  the  auriculo- 
ventricular   valves,  which   during  the  diastolic   period    form 


212  PHYSIOLOGY 

a  truncated  cone  open  towards  the  ventricle,  are  dragged  still 
nearer  to  the  ventricular  wall,  so  that  the  blood  is,  as  it  were, 
expressed  between  two  cones  (heart-wall  and  valves).  At  the 
same  time  the  contraction  of  the  circular  fibres  at  the  base 
of  the  heart  constricts  the  auriculo-ventricular  orifices,  so 
bringing  the  valves  at  their  origin  close  together,  and  enabling 
their  inner  surfaces  as  well  as  their  free  edges  to  be  apposed 
for  a  considerable  extent  {v.  Fig.  110).  The  valves  being 
thus  closed,  the  pressure  rises  higher  and  higher  in  the 
ventricle,  till  it  exceeds  that  in  the  pulmonary  artery. 
Directly  this  is  the  case,  the  semilunar  valves  open  and  allow 
the  ventricle  to  discharge  its  contents.  The  flow  of  blood 
from  ventricle  to  artery  goes  on  during  the  whole  systole  of 

Fia.  110  A.  Fig.  110  r. 


Diagram  to  show  position  of  mitral  valves  in  diastole  (110  a)  and 
systole  (110  b).     A.  Auricle.     V.  Ventricle.     Ao.  Aorta. 

the  ventricle ;  during  this  time  the  semilunar  valves  are 
pressed  outwards,  but  not  close  to  the  arterial  wall,  since  they 
are  probably  kept  in  an  intermediate  position  by  the  reflux 
currents  or  eddies  set  up  in  the  blood  on  their  arterial  side. 
They  thus  form  an  orifice,  triangular  in  shape  with  curved 
sides,  presenting  but  little  resistance  to  the  onward  flow  of 
blood. 

Directly  the  ventricular  contraction  ceases  and  the  blood 
stops  flowing,  these  reflux  currents  tend  themselves  to  bring 
the  valves  together.  At  the  same  time  the  pressure  in  the 
right  ventricle  falls  quickly  to  nothing,  and  the  sudden 
difference  in  the  pressures  on  the  two  sides  of  the  valves 
causes  them  to  shut  tightly  and  sharply,  giving  rise  to  a 
click  which  is  distinctly  audible  on  listening  with  one's  ear 


THE   VASCULAR   MECHANISM  213 

closely  applied  to  the  chest-wall,  and  represents  the  second 
heart-sound.  The  lunulae  of  two  adjacent  valves  are  closely 
pressed  together,  thus  preventing  the  possibility  of  the  leak- 
age of  even  a  single  drop  of  blood  back  into  the  ventricle. 

While  these  events  are  occurring  on  the  right  side  of  the 
heart,  an  exactly  similar  series  is  taking  place  on  the  left. 
During  the  diastole  blood  flows  from  the  pulmonary  veins 
to  the  left  auricle  and  ventricle.  The  left  auricle  then 
contracts,  and  this  is  followed  by  the  contraction  of  the 
left  ventricle.  The  only  difference  between  right  and  left 
sides  consists  in  the  fact  that  the  pressure  which  has  to  be 
overcome  in  forcing  blood  into  the  aorta  is  much  greater 
than  that  in  the  pulmonary  artery,  and  so  the  left  ventricle, 
having  much  more  work  to  do,  is  much  thicker,  and  contracts 
more  forcibly,  than  the  right.  The  closure  and  opening  of  the 
mitral  and  aortic  valves  occur  in  just  the  same  way  as  the 
corresponding  events  affecting  the  tricuspid  and  pulmonary 
valves. 

The  Heart-sounds 

If  we  apply  our  ear  to  the  front  of  a  person's  chest  (it  is 
more  convenient  to  use  the  stethoscope  for  the  purpose), 
we  hear  two  distinct  sounds  accompanying  each  beat  of  the 
heart,  followed  by  a  pause  corresponding  to  the  diastole. 
The  sounds  are  compared  to  the  syllables  luhh,  dup,  the  first 
sound  being  low-pitched  and  prolonged,  the  second  sound  high 
and  sharp. 

Thus  the  heart-sounds  may  be  represented  : 

lubb,  dup  (pause),  lubb,  dup  (pause). 

The  causation  of  the  second  sound  is  very  simple,  and 
may  be  considered  first.  It  is  heard  just  over  the  second 
right  costal  cartilage,  i.e.  the  place  where  the  aorta  lies 
nearest  the  surface. 

It  comes  at  the  end  of  the  systole,  as  determined  by  the 
hardening  of  the  apex  of  the  heart,  felt  as  the  apex-beat, 
and  can  be  shown  to  be  synchronous  with  the  closure  of  the 
aortic  valves.  It  is  in  fact  caused  by  the  sudden  shutting 
and  stretching  of  these  valves  that  occur  directly  the  heart 
ceases  to  contract  and  to  force  blood  into  the  aorta.  If  the 
valves  be  hooked  back  (by  means  of  a  wire  passed  down  a 


214 


PHYSIOLOGY 


carotid  artery)  in  an  animal,  the  second  sound  disappears,  and 
is  replaced  by  a  murmur  caused  by  the  blood  rushing  back 
into  the  ventricle  at  the  end  of  the  systole.  The  same  dis- 
appearance of  the  normal  second  sound  is  often  observed  in 
cases  where  the  valves  are  prevented  from  closing  by  diseased 
conditions. 

The  pulmonary  and  aortic  valves  generally  close  simul- 
taneously. In  some  cases  however  the  aortic  may  close 
slightly  before  the  pulmonary,  giving  rise  to  a  '  reduplicated 
second  sound.'  The  pulmonary  element  of  this  sound  is  best 
heard  over  the  second  left  cartilage. 

Fig.  111. 


>9 


^ 

Dia 

stolfi 

o 

o 

Blood     lowing 

>ystoU 

i — 

] 

in 

0  auri 

cles  and 

of 

Sj 

stole 

of 

vent: 

icles 

Aur- 

V 

mtric 

es 

Dia 

4ole. 

from 

veins. 

icles. 

01       Oa       03      04       0  5       OG       0  7      0  8       0  0       10  Bees. 

Heart  Sounds 


■  dup  Liilib diip 

Diagram  of  events  constituting  a  cardiac  cycle. 


The  first  sound  has  probably  a  twofold  origin,  viz.  from 
the  sudden  closure  of  the  auriculo-ventricular  valves,  and 
from  the  contraction  of  the  thick  muscular  wall  of  the 
ventricle. 

If  the  veins  going  to  the  heart  be  clamped,  so  that  the 
heart  can  no  longer  be  distended  with  blood  nor  the  valves 
put  on  the  stretch,  the  sound  is  altered  in  character  l)ut  not 
abolished.  The  first  sound  may  indeed  be  heard  on  listening 
with  a  stethoscope  to  the  beat  of  an  excised  heart.  It  is  said 
that  two  notes  maybe  detected  in  the  first  sound — a  high  note 
of  short  duration  due  to  closure  of  the  valves,  and  a  long  low- 
pitched  note  due  to  the  muscular  contraction.  This  muscular 
element  of  the  first  sound  has  the  same  pitch  as  the  sound 


THE    AVASCULAR   MECHANISM  215 

produced  by  voluntary  contracted  muscle,  and  therefore  as 
the  resonance-tone  of  the  ear. 

This  consideration  prevents  our  arguing  from  the  tone 
that  a  cardiac  contraction  is  a  tetanus.  As  we  shall  show 
later  on,  each  ventricular  contraction  is  analogous  to  a  simple 
muscle-twitch,  and  not  to  a  tetanus. 

By  means  of  these  sounds  we  are  able  to  determine  to  a 
certain  extent  the  amount  of  time  taken  up  by  each  phase  of 
the  cardiac  cycle.  In  a  healthy  man  the  heart  beats  about 
seventy-two  times  per  minute.  So  we  may  say  that  each 
systole  with  its  corresponding  diastole  (cardiac  cycle)  is 
completed  in  about  ^-^  of  a  second. 

This  time  is  divided  up  in  the  following  way  : — Systole  of 
auricle  j-L-  sec,  systole  of  ventricle  y\  sec,  diastole  y\  sec. 

The  relationship  between  the  phases  and  heart-sounds  is 
represented  by  Fig.  111. 

2'he  Pressure  in  the  Heart  Cavities  during  a  Cardiac 

Cycle 

We  arrive  at  a  clear  idea  of  the  events  occurring  during 
the  ventricular  systole  by  a  study  of  the  endocardiac  pressure- 
curve. 

There  are  several  methods  by  which  the  endocardiac 
pressure  may  be  recorded.     In  one  (Chauveau  and  Marey)  a 

Fig.  112. 


Diagram  of  Maiey's  cardiac  '  sound,'  consisting  of  a  long  tube 
a  b,  terminating  at  one  end  in  the  ampulla  ni,  which  is  covered 
with  an  elastic  membrane.  The  side-piece  c  serves  to  indicate 
the  position  of  the  ampulla  after  it  has  been  introduced  into 
the  vessels. 

cardiac  '  sound  '  is  put  down  the  jugular  vein  into  the  right 
auricle  or  ventricle,  or  down  the  carotid  into  the  left  ventricle. 
The  cardiac  sound  is  a  stiff  tube,  having  an  elastic  bag  or 
*  ampulla '  at  the  end  that  is  to  be  inserted  into  the  heart 
(Fig.  112).  The  upper  end  of  the  tube  is  connected  with  a 
tambour,  which  is  a  small  round  metal  tray  covered  with 
delicate  elastic  membrane.     To  the  top  of   the  membrane  a 


216 


PHYSIOLOGY 


writing  lever  is  attached  (Fig.  118).  Any  change  of  pressure 
on  the  ampulla  causes  a  corresponding  movement  of  the  lever 
of  tlie  tamhoiir,  which  may  be  recorded  on  a  moving  smoked 
sm'face. 

Since  this  instrument  is  very  easily  set  into  vibrations,  it 
is  often  difficult  to  know  whether  a  given  rise  or  depression  on 

Fig.  113. 


Marey's  tambour,  a,  axis  of  lever  ;  b,  metal  tray  covered  with  rubber 
membrane,  and  communicating  by  tube  /  with  free  end  of  cardiac 
sound. 


the  tracing  is  to  be  taken  as  of  cardiac  or  instrumental  origin. 
Hence  it  is  better  to  make  the  tambour  very  small,  with  thick 
rubber  so  as  to  limit  the  movements,  and  to  fill  it  with  saline 
fluid,  which  is  also  used  to  fill  the  tube  connecting  it  to 
the  heart.  This  is  the  principle  of  Hiirthle's  manometer 
(Fig.  114).  It  is  evident  that  the  mercurial  manometer  would 
be  no  good  for  this  purpose,  since  the  mercury  column  has  far 

Fig.  114. 


Diagram  to  show  construction  of  Hiirthle's  membrane 
manometer. 


too  much  inertia  to  follow  the  rapid  changes  of  pressure  in 
the  ventricles. 

By  the  introduction  of  a  valve  in  the  tube  leading  from 
the  manometer  to  the  heart,  it  may  be  used  as  a  maximum 
or  minimum  manometer.  If  the  valve  permits  fluid  to  go  only 
towards  the  heart,  the  manometer  will  indicate  the  minimum 
pressure  ever  attained  during  the  cardiac  cycle.  If  it  be 
turned  the  other  way,  it  will  indicate  the  maximum  pressure 


THE   VASCULAR   MECHANISM 


217 


(Fig.  115).  In  the  dog  the  maximum  pressure  in  the  left 
ventricle  may  be  140  mm.,  in  the  right  ventricle  60  mm.,  and 
in  the  right  auricle  about  20  mm.  Hg.  The  use  of  the  mini- 
mum manometer  reveals  the  striking  fact  that,  at  some  period 
of  the  cardiac  cycle,  there  is  a  negative  pressure  in  the 
ventricle ;  that  is  to  say,  the  mercury  is  sucked  up  in  the 
limbs  of  the  manometer  towards  the  heart.  This  negative 
pressure  may  amount  to  30  or  40  mm.  Hg.  in  the  left  ventricle, 
to  15  mm.  in  the  right  ventricle,  and  to  7  or  8  in  the  right 
auricle. 

If  however  we  register  the  variations  of  endocardiac  pres- 
sure by  means  of  a  manometer  which  is  sufficiently  accurate 

Fig.  115. 

to  manometer 


max.  valve 


mm.  valve 


to  heart 


.  Frank's  valve.  This  is  placed  in  the  course  of  the  tube  between 
heart  and  manometer,  so  that  the  latter  may  be  used  as  a  maximum, 
minimum,  or  ordinary  manometer  according;-  to  the  tap  which  is 
left  open. 


to  record  the  quick  changes  in  pressure  that  occur  in  the 
ventricle  with  each  heart-beat,  we  get  a  curve  like  Fig.  116. 
By  registering  this  curve  simultaneously  with  that  of  the 
blood-pressure  in  the  aorta,  we  may  determine  what  events 
are  occurring  during  each  phase  of  the  curve.  The  auricular 
systole  in  some  tracings  causes  a  small  rise  of  endocardiac 
pressure,  represented  by  an  elevation  on  the  curve  which 
would  occur  before  the  ordinate  0.  It  generally  lasts  about 
0*05  second.  It  is  not  represented  in  the  curve  reproduced 
in  Fig.  116.     This  is  immediately  followed  by  the  ventricular 


218  PHySIOLOGY 

coiiiraction,  which  lasts  from  o  to  2.  From  o  to  i  the  ven- 
tricle is  getting  up  pressure,  so  that  at  i  the  intraventricular 
pressure  is  equal  to  the  aortic  pressure.  This  process  takes 
from  0*02  to  0'04  second.  Directly  the  intraventricular  pressure 
rises  above  this  point  the  aortic  valves  open  and  blood  is 
driven  into  the  aorta.  The  outflow  of  blood  lasts  from  i  to  2, 
about  0'2  second.  At  2  the  ventricle  suddenly  relaxes,  the 
period  of  relaxation  occupying  about  0*05  second.  The  flat 
part  of  the  curve  is  often  spoken  of  as  the  systolic  plateau, 
and  on  an  average  occupies  about  0'18  second.  According  to 
the  condition  of  the  heart  and  peripheral  resistance,  this 
plateau  may  present  a  gradual  ascent  or  descent  {v.  Fig.  124). 

Fig.  116. 


Curve  of  intraventricular  pressure,  v,  compared  with  pressure 
in  aorta,  a.  Each  vibration  of  time-marker  =  jjy^  sec. 
(Hiirthle.) 

Almost  immediately  after  relaxation  commences  the  intraven- 
tricular pressure  falls  below  the  aortic,  so  that  the  aortic 
valves  close  somewhere  near  the  upper  part  of  the  descent 
(at  3). 

Negative  Pressure. — It  will  be  noticed  in  the  curve  of  endo- 
cardiac  pressure  that  the  line  drawn  by  the  lever  descends 
slightly  below  the  base  line  at  the  end  of  systole.  This  is  the 
period  at  which  the  negative  pressure  occurs.  This  is  however 
of  such  short  duration  that  the  most  delicate  manometers  fail 
to  show  its  maximum  value.  Several  explanations  have  been 
suggested  for  the  production  of  this  negative  pressure.  "When 
the  flow  of  fluid  through  a  tube  is  suddenly  interrupted,  the 
column  of  fluid,  which  has  a  certain  degree  of  inertia,  tends 
to  go  on,  so  that  a  negative  pressure  is  produced  in  its  rear. 
If  however  the  negative  pressure  in  the  ventricle  were  due  to 


THE   VASCULAR  MECHANISM  219 

the  sudden  cessation  of  flow  through  the  first  part  of  the  aorta, 
we  ought  to  obtain  with  the  minimum  manometer  a  negative 
pressure  at  the  root  of  the  aorta  equal  to  that  found  in  the 
ventricle.  But  this  is  not  the  case,  so  that  the  cause  of  the 
negative  pressure  must  be  sought  m  the  ventricle  itself.  It  is 
probably  due  to  the  fact  that  during  ventricular  contraction 
the  base  of  the  heart,  including  the  orifices  of  the  pulmonary 
artery  and  aorta,  is  constricted.  Directly  the  ventricle  relaxes, 
the  pressure  of  blood  in  these  two  trunks  causes  a  dilatation 
of  their  bases,  and  therefore  of  the  base  of  the  heart.  This 
dilatation  of  the  base  of  the  heart  increases  its  capacity,  and 
so  creates  a  negative  pressure  in  the  ventricular  cavities. 
This  mode  of  production  of  the  negative  pressure  may  be 
illustrated  experimentally  by  connecting  a  manometer  with  the 
interior  of  either  of  the  ventricles  of  an  excised  heart,  that  has 
ceased  beating,  and  then  forcing  fluid  into  the  aortic  and 
pulmonary  arteries.  With  each  distension  of  the  arteries  so 
produced  the  mercury  in  the  manometer  sinks,  showing  the 
production  of  a  negative  pressure  in  the  ventricles. 

It  is  possible  also  that  the  ventricles  exert  some  expanding 
force  as  they  return  from  a  contracted  to  an  uncontracted 
condition. 

The  Cardiac  Impulse  {Apex-heat) 

The  movement  of  the  heart  at  each  contraction  is  com- 
municated to  the  chest-wall,  over  a  limited  area  of  which  it 
may  be  felt  and  seen,  except  in  fat  individuals.  The  region 
whence  the  pulsation  of  the  chest-wall  is  most  marked  lies  in 
the  fifth  intercostal  space,  a  little  to  the  median  side  of  the 
left  nipple.  The  pulsation  is  often  spoken  of  as  the  apex-beat, 
and  was  formerly  thought  to  be  due  to  the  twisting  forward  of 
the  apex  at  each  systole,  but  really  lies  considerably  above  the 
apex  of  the  heart ;  and  as  we  have  already  seen,  the  apex,  so 
long  as  the  pericardium  is  intact,  is  relatively  motionless. 

During  diastole  the  ventricles  form  a  flabby  flattened 
cone  lying  against  the  chest  wall  and  slightly  deformed  by 
the  latter.  In  systole  the  ventricles  contract  forcibly  on  the 
contained  fluid,  and  become  hard  and  rigid,  assuming  the 
form  of  a  rounded  cone.  This  sudden  recovery  of  shape  and 
hardening  of  the  ventricular  walls  pushes  out  the  part  of  the 


220  PHYSIOLOGY 

chest -wall  in  immediate  proximity  to  the  ventricles,  and  so 
gives  rise  to  the  '  apex-beat.' 

The  cardiac  impulse  may  be  registered  by  means  of  a 
cardiograph.  In  nearly  all  forms  of  this  instrument  a 
button,  resting  on  the  chest-wall,  transmits  the  movements  of 
the  latter  to  a  tambour,  which  again  is  connected  by  a  tube 
to  a  registering  tambour.  One  such  instrument  is  shown  in 
Fig.  117. 

The  curves  so  obtained,  which  are  known  as  cardiograms, 
may  vary  considerably  in  the  same  subject  according  to  the 
pressure  employed  and  the  exact  spot  at  which  the  tambour 

Fig.  117. 


A  cardiograph.  This  is  strapped  round  the  chest,  the  central 
button  is  applied  to  the  '  apex-beat '  and  its  pressure  on  the 
chest-wall  regulated  by  means  of  the  three  screws  at  the 
sides.  The  tube  at  the  upper  part  of  the  instrument  serves 
to  connect  the  drum  of  the  cardiograph  with  a  registering 
tambour  such  as  that  shown  in  Fig.  113. 

is  applied.  Their  interpretation  often  presents  considerable 
difficulties,  owing  to  the  fact  that  their  form  is  conditioned  by 
two  factors,  viz.  : 

(1)  The  actual  size  (antero-pogterior  diameter)  of  the 
ventricles. 

(2)  The  resistance  to  distortion  {i.e.  the  tension)  of  the 
ventricular  wall.  This  factor  will  increase  in  importance 
with  increasing  pressure  of  the  cardiograph  button  on  the 
chest  wall. 

Fig.  118  represents  a  cardiographic  tracing  or  cardiogram, 
which  may  be  spoken  of  as  typical.  In  order  to  interpret 
this  curve,  we  must  record  at  the  same  time  either  the  intra- 


THE  VASCULAR  MECHANISM 


221 


ventricular  pressure  in  animals,  or  the  heart-sounds  in  man.' 
Applying  the  latter  method  we  may  obtain  the  curve  shown 
in  Fig.  119.     In  this  curve  it  will  be  seen  that  the  first  heart- 


Cardiogram  (Hiirthle 


sound,  corresponding  to  the  ventricular  systole,  begins,  not 
at  the  commencement  of  the  rise  of  the  cardiogram,  but  at 
the  notch  near  the  top  of  the  ascent.     From  this  fact  we  may 

Fig.  119. 


1                           ■ 1 

ifV.  _       ,\r.  -  _  _ 

__P=^^ 



Cardiogram  (r.)  with  simultaneous  record  of  heart-sounds  (a) 
(Hiirthle).  1.  Position  of  first  heart-sound.  2.  Position  of 
second  heart-sound. 


conclude  that  the  first  part  of  the  ascent  is  caused  by  the 
auricular  systole  forcing  blood  into  the  ventricle,  the  ventri- 

'  This  mechanical  record  of  the  heart-sounds  has  been  successfully 
accomplished  by  Hiirthle.  His  method  consists  in  an  application  of  the 
microphone.  A  special  form  of  stethoscope  is  so  arranged  that  by  its  means 
the  vibrations  corresponding  to  the  heart- sounds  are  transmitted  to  a  contact 
between  silver  and  carbon.  Through  this  contact  a  strong  current  is  passing. 
This  also  passes  through  an  electro-magnet,  which  attracts  an  iron  disc 
attached  to  the  membrane  of  a  Marey's  tambour.  Any  vibration  transmitted 
to  the  carbon-silver  contact  alters  its  resistance,  and  so  the  strength  of  the 
current  passing  through  the  electro-magnet.  In  this  way  the  heart-sounds  can 
affect  the  pull  exerted  by  the  electro-magnet  on  the  membrane  of  the  tambour, 
and  the  change  in  the  volume  of  the  contained  air  is  recorded  by  means  of  an 
ordinary  registering  tambour. 


222  PHYStOLOGY 

cular  systole  1)eing  marked  by  the  notch  near  the  top  of  the 
curve.  In  Fig.  118,  taken  from  the  dog,  the  auricular  curve 
is  more  distinct  as  a  slight  elevation  preceding  the  rise  due 
to  the  contraction  of  the  ventricles.  Other  forms  of  curve 
however  are  often  obtained,  which  show  considerable  differ- 
ences from  the  endocardiac  pressure,  and  are  spoken  of  as 
atypical.  They  are  often  conditioned  by  a  faulty  position 
of  the  cardiograph. 


THE   VASCULAR  MECHANISM  223 


Section  8 
THE  PULSE 

If  the  finger  be  placed  on  some  artery,  such  as  the  radial, 
we  feel  an  expansion  of  it  occurring  at  regular  intervals 
corresponding  to  the  heart-beats.  On  the  tracing  of  a  mer- 
curial manometer  we  saw  a  similar  rise  and  fall  produced 
by  each  heart-beat.  So  the  pulse  may  be  defined  as  the 
expansion  of  the  artery  under  the  increased  blood-pressure 
caused  by  each  ventricular  systole.  Just  as  the  blood-pres- 
sure diminishes  from  heart  to  periphery,  so  the  pulse 
diminishes  in  size  as  we  get  farther  away  from  the  heart. 
If  the  arterial  system  were  perfectly  rigid,  the  increased 
pressure  due  to  the  forcing  of  blood  into  it  by  each  ventri- 
cular systole  would  occur  almost  simultaneously  over  every 
point  of  it.  In  the  elastic  distensible  arteries  however,  the 
first  effect  of  the  inflow  of  blood  into  the  aorta  is  to  distend 
a  section  of  the  aorta  nearest  to  the  heart.  The  elastic  re- 
action of  this  forces  a  portion  of  blood  on  into  the  next 
section,  distending  it  in  its  turn.  And  so  the  increased 
pressure  is  transmitted  from  segment  to  segment  of  the 
arteries,  in  the  form  of  a  wave,  at  the  velocity  of  about  five 
metres  per  second.  We  must  be  careful  not  to  confuse  the 
velocity  of  the  pulse- wave  with  that  of  the  blood.  The 
velocity  of  the  blood  in  the  aorta  is  not  more  than  half  a 
metre  per  second,  and  gets  less  and  less  towards  the  periphery. 
The  pulse-wave  may  be  compared  to  a  wave  produced  by  the 
wind  travelling  rapidly  down  a  sluggishly  flowing  river. 

We  will  make  this  difference  clearer  by  an  illustration.  If 
the  hindmost  of  a  row  of  billiard  balls  be  struck  sharply  with 
a  cue,  the  foremost  ball  flies  off  and  the  others  stop  still.  In 
this  case  the  energy  imparted  to  the  first  ball  by  the  stroke 
has  been  transmitted  from  ball  to  ball,  just  as  the  effect  of  the 
ventricular  contraction  is  transmitted  from  section  to  section 
of  the  arterial  blood-stream.  If  the  balls  are  struck  so  that 
the  cue  continues  pressing  on  the  hindmost  after  the  stroke  is 
delivered,  the  front  ball  flies  off",  while  the  others  move  slowly 
along  in  the  direction   of  the  stroke.     In  the  arteries  this 


224  PHYSIOLOGY 

continuous  pressure  is  furnished  by  the  elastic  reaction  of  the 
arterial  wall,  and  we  see  how  the  impact  of  the  blood  may 
travel  quickly  along  as  a  wave  of  increased  pressure,  while  the 
blood  itself  is  moving  slowly  along,  impelled  by  the  elastic 
reaction  of  the  arterial  wall. 

To  study  the  pulse  more  fully  it  is  necessary  to  obtain 
a  graphic  record  of  the  expansion  of  the  arteries,  or,  what 
comes  to  the  same  thing,  of  the  exact  changes  in  pressure 
which  produce  this  expansion.  The  curve  obtained  with  the 
mercurial  manometer  shows  elevations  corresponding  to  the 
pulse  ;  but  the  instrument  is  far  too  sluggish  to  record  the 
finer  variations  of  pressure.  For  this  purpose  a  manometer 
such  as  Hiirthle's,  which  has  very  little  inertia,  must  be 
used. 

The  expansion  of  the  artery  is  registered  by  means  of  a 
lever  which  may  be  made  to  rest  more  or  less  heavily  upon 

Fig.  120. 


the  artery,  and  the  movements  of  which  are  recorded  on  a 
blackened  surface.  Such  an  instrument  is  called  a  sphygmo- 
grapli.  Of  the  many  forms  of  sphygmographs,  Marey's  or 
Dudgeon's  is  perhaps  the  most  convenient  for  clinical  pur- 
poses (Fig.  120). 

The  principle  of  Marey's  sphygmograpli  is  shown  in  Fig.  120.  The  button 
(b)  is  adjusted  so  as  to  press  on  the  radial  artery.  Its  movements  are  trans- 
mitted to  a  lever  (m).  The  screw  on  this  works  on  a  small  cogged  wheel  at  (o), 
which  is  also  the  axis  of  the  writing  lever  (1).  The  movements  of  the  button 
(b),  thus  transmitted  to  a  point  near  the  axis  of  (1),  are  reproduced  by  this 
lever  highly  magnified,  and  as  such  are  recorded  on  a  blackened  surface.  The 
pressure  on  the  artery  can  be  adjusted  by  means  of  the  screw  (s). 

Dudgeon's  sphygmograph  (Fig.  121)  is  rather  easier  to  use  than  Marey's, 
and  is  therefore  largely  employed  for  clinical  purposes.  It  is  provided  with 
a  dial  by  which  the  pressure  on  the  artery  can  be  graduated,  and  has  a  small 
clockwork  arrangement  for  moving  along  the  slip  of  smoked  paper  on  which 
the  records  are  taken.  The  arrangement  of  the  levers  in  this  form  of  sphyg- 
mograph is  shown  in  Fig.  122,  where  f  is  the  (adjustable)  spring  bearing  by  its 
button  p  on  the  artery.  The  up  and  down  movements  of  p  are  transmitted 
to  s  being  much  magnified  and  converted  into  side  to  side  movements.     The 


THE    VASCULAR   MECHANISM 


225 


point  of  s  rests  on  the  blackened  surface  represented  in  section  at  a,  and 
scratches  on  this,  when  moving,  a  magnified  record  of  the  expansion  of  the 
artery  under  the  knob  p. 

Either  form  of   sphygmograph  is   generally  applied  to  the   radial   artery 
since  this  is  near  the  surface  and  is  supported  by  bone,  and  the  arm  is  well 

Fig.  121. 


Dudgeon's  sphygmograph,  showing  its  mode  of  application 
to  the  radial  artery. 

adapted  for  the  application  of  the  sphygmograph.  The  pulse-curve  obtained 
by  means  of  a  sphygmograph  varies  according  to  the  artery  employed  and  the 
force  with  which  the  lever  presses  on  the  artery,  but  all  the  curves  present  the 
same  general  features. 

Fig.  122. 


Diagram  of  arrangement  of  recording  lever  in  Dudgeon's 
sphygmograph. 


Fig.  123  represents  a  pulse-curve  taken  from  the  radial 
artery. 

It  will  be  noticed  that  the  elevation  due  to  the  expansion 

15 


22()  PHYSIOLOGY 

of  the  artery  is  sudden  and  uninterrupted.  We  have  ah-eady 
explained  that  this  is  due  to  the  sudden  pumping  of  blood 
into  the  first  part  of  the  aorta,  whence  the  impulse  is  trans- 
mitted as  a  wave  along  the  arteries.  The  curve  descends 
gradually  till  the  next  beat  occurs,  since  the  elastic  reaction 
of  the  arteries,  which  tends  to  keep  up  the  pressure,  acts 
more  constantly  and  steadily  than  the  heart-beat.  On  this 
descending  part  of  the  curve  are  seen  two  or  three  secondary 
elevations,  (b)  is  the  primary  or  '  percussion  '  wave,  (c)  the 
predicrotic  or  '  tidal '  wave,  and  (e)  the  dicrotic  wave.  Eleva- 
tions may  occur  on  the  curve  after  (e),  which  are  called  post- 
dicrotic  waves.  It  is  better  however,  for  reasons  which 
we  shall  see  presently,  to  class  the  elevations  before  the 
dicrotic  notch  (d)  as  systolic  elevations,  and  those  afterwards, 
including  the  dicrotic  elevation  itself,  as  diastolic.     For  the 

Fig.  123. 


Pulse-curve  from  radial  artery. 

exact  understanding  of  these  elevations,  it  is  necessary  to 
take  simultaneous  tracings  of  the  pressure  in  the  left  ven- 
tricle and  the  aorta.  In  this  way  we  may  dissociate  the 
waves  caused  by  the  ventricular  systole  from  those  having 
their  origin  in  the  aorta.  In  Fig.  124  are  represented  typical 
tracings  of  cardiogram,  intraventricular  pressure,  and  aortic 
pressure  taken  simultaneously.  The  dotted  lines  are  drawn 
through  synchronous  parts  of  the  curves.  Considering  first 
the  dotted  part  of  Curve  II  and  Curve  IV,'  we  see  that  the 
contraction  of  the  ventricle  begins  at  a  ;  the  rise  of  intra- 
ventricular pressure  from  a  to  b  is  without  effect  on  the  aortic 
pulse  ;  at  b  the  intraventricular  is  exactly  equal  to  the  aortic 
pressure,  and  then  rapidly  rises  above  it.  Since  the  aortic 
valves  offer  no  resistance  to  the  flow  of  blood  from  ventricles 

'  Curve  IV  in  Fig.  124  must  be  compared  with  the  pulse-tracing  taken 
from  the  radial  artery  in  Fig.  123.  It  will  be  seen  that,  apart  from  the  fact 
that  Fig.  124,  IV,  is  more  lengthened  out  than  Fig.  123,  owing  to  the  great 
rapidity  of  the  recording  apparatus,  the  curves  are  practically  similar. 


THE   VASCULAR  MECHANISM 


227 


to  aorta,  they  must  open  so  soon  as  the  intraventricular 
exceeds  the  aortic  pressure,  and  this  is  shown  to  be  the  case 
by    the   rise   of   pressure   in   the    aorta.     From   b  to   c  the 


See. 


Fig.  124. 


Svstolo 


Diastole 


Diagram  (after  Hiirthle)  showing  simultaneous  cardiogi'aphic, 
endocardiac,  and  aortic  curves.  I.  Cardiogram.  II.  Endo- 
cardiac  pressure.  III.  Aortic  pressure.  IV.  Aortic  pressure, 
corresponding  to  dotted  endocardiac  curve  in  II. 


ventricle  is  still  contracting  and  forcing  the  blood  into  the 
already  distended  aorta,  so  causing  a  rise  of  pressure.  At 
c   the   ventricle   relaxes,  the  intraventricular   pressure   falls 


228 


PHYSIOLOGY 


quickly,  and  at  d  has  fallen  below  the  aortic  pressure.  The 
aortic  valves  must  now  close,  since  the  pressure  is  greater  on 
their  aortic  side.  The  fall  of  pressure  in  the  ventricle  now 
goes  on  uninterruptedly,  but  in  the  aorta  there  is  a  sharp 
elevation  immediately  after  d.  This  elevation  is  the  dicrotic 
wave.  We  thus  see  that  it  comes  immediately  after  closure 
of  the  aortic  valves. 

There  are  several  factors  at  work  tending  to  produce  a 
secondary  wave  at  this  point. 

If  a  column  of  fluid  moving  along  a  tube  (a,  b,  Fig.  125) 
provided  with  a  stopcock  (c)  and  a  manometer  (m)  be  suddenly 
checked  by  turning  the  cock  (c),  the  column  in  front  of  the 
cock,  having  a  certain  momentum,  will  tend  to  go  on  moving, 
and  therefore  produce  a  suction  or  negative  pressure  behind 

Fig.  125. 


"=17=* 


it.  This  will  be  indicated  by  a  depression  of  the  level  of  the 
manometer.  The  fluid  will  flow  back  to  fill  this  partial 
vacuum,  and  so  a  series  of  oscillations  in  the  column,  getting 
smaller  and  smaller,  are  produced,  which  are  recorded  by  the 
manometer  as  oscillations  of  pressure.  The  same  thing 
must  occur  in  the  beginning  of  the  aorta,  when  the  inflow 
of  blood  from  the  ventricle  suddenly  ceases  with  the  end 
of  the  ventricular  contraction.  In  this  case  however  the 
oscillations  are  increased  by  the  elastic  reaction  of  the  arterial 
wall,  just  as  a  weight  which  is  suddenly  applied  to  a  piece  of 
elastic  swings  up  and  down  before  it  comes  to  rest  with  the 
elastic  in  a  permanent  condition  of  tension.  These  two  factors 
combine  in  producing  a  negative  wave  in  the  beginning 
of  the  aorta  at  the  end  of  the  ventricular  systole,  the 
blood  driven  up  against  the  aortic  valves  closing  them  tightly 


THE   VASCULAR   MECHANISM  229 

and  putting  them  on  the  stretch.  The  negative  wave,  even 
in  the  rigid  tube,  is  followed  Ijy  a  positive  wave  in  the 
opposite  direction.  In  the  aorta  this  positive  wave  is  in- 
creased by  the  elastic  reaction  of  the  stretched  aortic  valves, 
so  that  we  may  regard  the  blood  as  being  driven  up  against 
them  by  the  negative  pressure,  and  then  rebounding  like 
a  billiard  ball  from  the  elastic  cushion,  to  give  rise  to  the 
dicrotic  elevation. 

The  post-dicrotic  weaves,  when  present,  are  probably  due  to 
the  waves  of  oscillation. 

We  have  now  to  consider  the  elevations  in  the  first  part 
of  the  curve,  which  we  have  spoken  of  as  systolic  elevations, 
and  which  include  the  pre-dicrotic  elevation.  It  will  be  seen 
that  they  are  also  represented  on  the  ventricular  curve,  and 
occur  while  the  aortic  valves  are  open  and  blood  is  flowing 
from  the  ventricle  into  the  aorta.  They  are  probably  due  to 
the  elastic  vibrations  of  the  aortic  wall,  and  perhaps  of  the 
heart-wall  itself,  started  by  a  sudden  increase  of  tension  in 
the  aorta  and  heart. 

The  general  form  of  the  pulse-curve  varies  with  changes 
in  the  heart,  in  the  arteries,  and  in  the  peripheral  resistance. 
Thus  some  curves  may  present  secondary  elevations  on  the 
ascending  part,  as  in  Fig.  124,  III,  and  are  called  anacrotic, 
while  in  others  all  secondary  elevations  occur  on  the  descend- 
ing part.  This  latter  type  is  called  Icatacrotic,  and  is  the 
tracing  usually  obtained  from  a  normal  radial  artery.  By 
comparing  these  two  types  of  curves  with  the  corresponding 
intraventricular  pressures,  we  find  that,  in  both  cases,  blood 
is  flowing  into  the  aorta  during  the  whole  time  from  the 
beginning  of  the  primary  elevation  to  the  notch  just  before 
the  dicrotic  elevation.  This  is  shown  by  the  fact  that  the 
intraventricular  pressure  is  all  this  time  slightly  higher  than 
the  aortic  pressure.  So  long  as  this  is  the  case  blood  must 
flow  from  ventricle  into  aorta.  (This  fact  shows  that  there 
is  normally  no  part  of  the  cardiac  cycle  during  which  the 
ventricle  remains  contracted  and  empty,  the  ventricle  in  all 
cases  relaxing  before  it  has  completely  emptied  itself  of 
blood.) 

Now  it  is  easy  to  see  the  conditions  which  determine 
whether  the  systolic  plateau  shall  be  ascending  or  descend- 
ing,  and   therefore   when   the   pulse   shall   be   anacrotic   or 


230  PHYSIOLOGY 

katacrotic.  If,  after  the  first  sudden  rise  of  pressure  in  the 
aorta,  the  blood  can  escape  more  rapidly  through  the  peri- 
pheral resistance  than  it  is  thrown  into  the  beginning  of  the 
aorta,  the  systolic  plateau  will  sink,  and  a  katacrotic  pulse 
tracing  is  obtained.  If  on  the  other  hand  the  peripheral 
resistance  is  high,  or  an  extra  large  amount  of  blood  is 
thrown  into  the  aorta  at  each  stroke  of  the  heart  {e.g.  by  pro- 
longation of  the  diastole),  the  aortic  pressure  will  rise  so 
long  as  blood  is  flowing  in,  and  we  get  an  ascending  systolic 
plateau  and  an  anacrotic  pulse.  Thus  we  obtain  an  anacrotic 
pulse  in  old  people  with  Bright's  disease,  in  whom  the  peri- 
pheral resistance  is  very  high,  and  also  in  animals  when  the 
heart  is  slowed  by  vagus  action. 

The  production  of  the  dicrotic  elevation  is  favoured  by 
any  influence  which  increases  the  elastic  resiliency  of  the 
arteries  or  causes  the  primary  elevation  of  the  pulse  to  be 
rapid  and  sharp.  Thus  it  is  much  more  pronounced  in  young 
people  than  in  old  people,  whose  arteries  have  become  rigid. 
"Where  the  peripheral  resistance  is  low  through  relaxation  of 
the  arterioles,  and  the  heart  is  beating  forcibly,  as  in  many 
cases  of  fever  and  also  to  some  extent  after  a  good  meal  with 
alcohol,  the  dicrotic  elevation  becomes  very  marked.  Under 
such  circumstances  it  may  be  easily  felt  with  the  finger  at  the 
wrist,  and  in  many  cases  the  mistake  has  been  committed  of 
taking  the  dicrotic  wave  for  a  normal  beat,  and  so  doubling 
the  rate  of  the  pulse. 

In  tracings  of  the  artificial  pulse  obtained  from  the  arterial  schema, 
secondary  elevations  are  observed  on  the  descending  part  of  the  curve,  which 
are  not  explained  in  any  of  the  above-mentioned  ways.  These  waves  are  the 
reflections  of  the  primary  wave  from  the  peripheral  resistance.  This  is  shown 
by  the  fact  that  the  nearer  to  the  peripheral  resistance  we  record  the  pulsation, 
the  nearer  is  the  secondary  to  the  primary  wave.  Near  the  pump  the  two 
waves  may  be  separated  by  a  considerable  interval  (Fig.  126). 

It  has  been  thought  that  some  of  the  elevations  in  the  normal  pulse-curve 
could  be  explained  as  reflected  waves.  This  theory  is  at  once  excluded  by 
the  fact  that  wherever  we  take  the  pulse  tracing,  whether  from  the  aorta, 
carotid,  radial,  or  dorsalis  pedis,  the  secondary  elevations  are  always  situated 
the  same  distance  from  the  beginning  of  the  primary  elevation,  showing  that 
all  these  waves  are  centrifugal,  and  have  their  origin  in  the  beginning  of  the 
arterial  system. 

Besides,  a  single  reflected  wave  from  the  multitudinous  peripheral  divisions 
would  be  impossible,  as  the  reflected  waves  from  any  one  part  would  be  interfered 
with  and  destroyed  by  the  reflected  waves  coming  from  all  the  other  parts.  A 
reflected  wave  would  be  increased  by  a  high  peripheral  resistance,  and  not 
diminished  as  the  dicrotic  wave  is. 


THE  VASCULAR  MECHANISM 


231 


sovWAAAArVAnA/V/X A  AAA  / 


Pulse-curves  described  by  a  series  of  sphygmographic  levers  placed 
at  intervals  of  20  cm.  from  each  other  along  an  elastic  tube,  mto 
which  fluid  is  forced  by  the  sudden  stroke  of  a  pump.  The  pulse- 
wave  is  travelling  from  left  to  right,  as  indicated  by  the  arrows  over 
the  primary  (a)  and  secondary  (b,  c)  pulse -waves.  The  dotted  vertical 
lines  drawn  from  the  summit  of  the  several  primary  waves  to  the 
tuning-fork  curve  below,  each  complete  vibration  of  which  occupies 
J-  sec  allow  the  time  to  be  measured  which  is  taken  up  by  the  wave 
in  passing  along  20  cm.  of  the  tubing.  The  waves  (a')  are  waves 
reflected  from  the  closed  distal  end  of  the  tubing ;  this  is  indicated  by 
the  direction  of  the  arrows.  It  will  be  observed  that  m  the  more 
distant  lever  (VI)  the  reflected  wave,  having  but  a  slight  distance  to 
travel,  becomes  fused  with  the  primary  wave.  (From  Foster,  after 
Marey.) 


232  PHYSIOLOGY 

Venous  Pulse. — Under  certain  conditions  the  pulse  may  be 
carried  on  from  the  arteries  through  the  capillaries  into  the 
veins,  giving  rise  to  a  venous  pulse.  Thus  stimulation  of 
the  chorda  tympani  nerve  causes  all  the  arterioles  of  the 
submaxillary  gland  to  dilate.  The  peripheral  resistance  is 
in  this  manner  so  lowered  that  it  is  insufficient  to  destroy 
the  arterial  pulse,  and  on  cutting  the  veins  from  the  gland 
blood  spurts  intermittently  from  their  peripheral  end.  Lower- 
ing the  resistance  in  this  case  has  produced  the  same  effect 
as  unclamping  the  tube  c^  (Fig.  100)  in  the  arterial  schema. 

On  attaching  a  manometer  to  the  central  end  of  the 
superior  or  inferior  vena  cava,  elevations  are  observed 
corresponding  to  the  variations  in  the  auricular  pressure. 
These  are  double  for  each  heart-beat.  While  the  ventricle 
is  contracting  there  is  a  slow  rise,  due  to  the  fact  that  the 
blood,  which  is  flowing  in  from  the  veins,  cannot  escape  into 
the  ventricle,  and  so  distends  the  auricle ;  a  second  short 
sharp  elevation  of  pressure  is  produced  by  the  auricular 
systole. 

Alterations  of  venous  pressure,  determined  by  the  respira- 
tory movements,  are  also  to  be  observed  in  the  great  veins, 
the  pressure  sinking  during  inspiration  and  rising  during 
expiration.  These  may  be  spoken  of  as  the  respiratory  venous 
pulse. 


THE   VASCULAK  MECHANISM  233 


Section  4 
CAEDIAC  EHYTHM 

If  the  heart  be  rapidly  cut  out  of  the  body,  it  will  con- 
tinue beating  in  a  normal  fashion  for  some  time— in  the  case 
of  mammals  from  five  to  ten  minutes  ;  in  the  case  of  cold- 
blooded animals,  such  as  the  frog  or  tortoise,  for  some  hours  or 
even  days.  We  say  therefore  that  the  rhythm  of  the  heart  is 
automatic  ;  and  we  have  now  to  discuss  wherein  this  automatic 
rhythmicity  lies.  The  circumstance  that  the  cold-blooded 
heart  goes  on  beating  so  long,  when  severed  from  all  connec- 
tion with  the  body,  has  caused  it  to  be  much  used  in  investi- 
gations on  the  subject,  and  from  it  most  of  our  knowledge  has 
been  acquired. 

Methods  of  Investigation 

The  contractions  of  the  frog's  heart  may  be  recorded  by 
magnifying  its  movements  by  means  of  a  light  lever,  one  end 
of  which  rests  upon  the  ventricle,  while  the  other  end  is  made 

Fig.  127. 


Schiifei's  heart  plethysmograph. 

to  write  upon  a  blackened  surface — or,  as  in  Gaskell's  method, 
by  clamping  the  heart  in  the  auriculo-ventricular  groove, 
and  attaching  threads  from  auricle  and  ventricle  to  two 
levers  which  are  arranged  to  write  one  over  the  other.  Or 
we  may  register  the  changes  in  the  intraventricular  pressure 
by  allowing  dilute  blood  or  some  other  nutrient  fluid  to 
flow  through  a  perfusion  cannula  tied  into  the  ventricle,  and 
attaching  the  exit-tube  of  the  cannula  to  a  small  mercurial 
manometer. 

Another  way  is  to  register  the  changes  in  volume  of  the 


234  PHYSIOLOGY 

heart  (Fig.  127).  The  ventricle  is  tied  round  a  perfusion 
cannula,  and  is  inserted  into  an  air-tight  vessel  containing 
oil.  On  one  side  of  the  vessel  is  a  tube,  in  which  a  lightly 
moving  piston  is  fitted,  to  which  a  writing  lever  is  attached. 
Fluid  is  passed  through  the  heart  by  the  perfusion  cannula  at 
a  constant  pressure.  The  changes  in  volume  are  indicated  by 
the  movements  of  the  piston. 

Anatomy  of  the  Frog's  Heart 

The  frog's  heart  differs  anatomically  in  several  respects 
from  the  mammalian  heart.  It  consists  of  sinus  venosus,  two 
auricles,  one  ventricle,  and  bulbus  arteriosus.  The  venous 
blood  from  the  body  flows  into  the  sinus  venosus  by  the  three 


Diagram  of  frog's  heart  (after  Cyon).  V.  Ventricle.  E.A., 
L.A.  Rijrht  and  left  auricles  (atrium).  S.V.  Sinus  venosus. 
P.V.  Pulmonary  veins.  L.V.C.S.  and  R.V.C.S.  Left  and  right 
superior   vena   cava.     V.C.I.  Vena   cava   inferior.     Tr.A.  Truncus 

arteriosus. 

venae  cavee,  and  thence  into  the  right  auricle.  The  left  auricle 
receives  the  blood  from  the  lungs.  The  ventricle  thus  receives 
mixed  arterial  and  venous  blood. 

The  muscular  fibres  of  the  heart  are  less  highly  developed 
than  those  of  the  mammalian  heart.  They  are  spindle- 
shaped,  and  only  dimly  cross-striated.  The  cross-striation 
becomes  more  distinctly  marked  as  we  proceed  from  sinus 
to  ventricle,  the  sinus  muscle-fibre  representing  the  most 
primitive  condition.  There  is  complete  muscular  continuity 
between  all  the  cavities  of  the  heart.  The  circular  ring  of 
muscle  at  the  junctions  of  sinus  with  auricles,  and  of  auricles 
with  ventricles,  presents  only  slight  traces  of  cross-striation. 

The  heart  is  well  supplied  with  nerve-fibres  and  ganglion- 
cells.  The  two  vagi  enter  the  sinus  venosus,  and  branch  just 
under  the  pericardium.  Here  they  become  connected  with  a 
collection   of   nerve-cells,    spoken   of    as   Remak's   ganglion. 


THE   VASCULAE  MECHANISM  235 

From  the  sinus,  the  two  vagi,  now  called  septal  nerves,  pass 
down  in  the  interauricular  septum,  one  in  front  and  the  other 
behind.  Near  the  auriculo-ventricular  groove  they  enter  two 
collections  of  ganglion  cells,  called  Bidder's  ganglia.  From 
these  ganglia  non-medullated  fibres  are  distributed  to  sur- 
rounding parts  of  the  auricle  and  to  the  whole  of  the  ventricle. 
In  the  upper  third  of  the  ventricle  occur  scattered  ganglion- 
cells  attached  to  the  nerve-fibres.  These  are  quite  absent  in 
the  lower  half  or  two  thirds 

The  Automatic  Contraction  of  the  Frog's  Heart 

The  frog's  heart  in  the  body,  or  when  removed  from  the 
body  intact,  beats  regularly,  the  contraction  starting  in  the 
sinus,  then  travelling  to  auricles,  ventricle,  and  bulbus.  If 
however  the  heart  be  removed  by  cutting  it  across  the  sino- 
auricular  junction,  or  if  the  auricles  be  functionally  separated 
from  the  sinus  by  a  ligature  round  this  junction  (Stannius' 
ligature),  the  auricles  and  ventricle  stop  dead  in  an  uncon- 
tracted  condition  (diastole)  while  the  sinus  goes  on  beating 
regularly.  After  the  lapse  of  a  period  varying  from  five 
minutes  to  half  an  hour  the  detached  part  of  the  heart  begins 
to  beat,  at  first  slowly  and  then  more  rapidly,  but  never 
attaining  the  rate  of  the  sinus.  The  auricles  beat  first,  and 
then  the  ventricle. 

If  now  the  ventricle  be  cut  away  by  an  incision  in  the 
auriculo-ventricular  groove  from  the  auricles,  the  latter  go  on 
beating ;  while  the  former,  after  a  few  beats  due  to  the  excita- 
tion of  the  incision,  stops  still,  and  only  after  a  considerable 
time  may  begin  again  to  contract  very  slowly. 

On  the  other  hand,  a  ventricle-apex  preparation  (that  is  to 
say,  the  lower  two  thirds  of  the  ventricle  separated  func- 
tionally from  the  rest  of  the  heart)  never  beats  again  under 
normal  circumstances.  To  single  stimuli  it  responds  with 
a  single  beat,  not  with  a  series  of  beats  as  the  whole  heart 
does. 

If  the  lower  third  of  the  ventricle  be  separated  functionally 
in  the  living  frog  by  crushing  the  ring  of  tissue  between  it 
and  the  upper  third,  it  never  gives  a  spontaneous  beat  again, 
although  it  is  under  the  most  normal  conditions  possible 
in  the  circumstances.     There  is  thus   a  descending  scale  of 


236  PHYSIOLOGY 

automatic  power  in  the  different  parts  of  the  frog's  heart — 
from  the  sinus,  where  it  is  highest,  to  the  lower  part  of 
the  ventricle,  where  it  is  apparently  absent.  From  this  fact 
it  has  been  thought  that  the  automaticity  of  the  frog's  heart 
is  dependent  on  the  ganglia  present  in  it.  The  contraction 
was  supposed  to  be  started  by  impulses  proceeding  from 
the  sinus  ganglion.  If  this  were  cut  off,  Bidder's  ganglia, 
or  the  scattered  cells  in  the  upper  third  of  the  ventricle, 
could,  it  was  thought,  take  up  its  task  of  originating  impulses. 
The  muscle-cells  under  this  hypothesis  act  as  the  servants  of 
the  ganglion  cells,  just  as  the  voluntary  muscles  wait  on  the 
commands  of  the  cells  in  the  spinal  cord  and  brain. 

But  we  have  evidence  that  the  muscle-cells  do  not  play 
so  subordinate  a  part  as  this  in  the  heart,  and  that  the  larger 
ganglia,  at  any  rate,  are  of  very  little  importance  in  initiating 
the  rhythm.  Eemak's  and  Bidder's  ganglia  may,  with  care, 
be  excised  without  markedly  interfering  with  the  normal 
rhythm  or  sequence  of  contraction.  The  ventricle  apex  may 
be  made  to  contract  rhythmically  if  the  pressure  on  its 
interior  be  slightly  increased,  either  by  clamping  the  aorta  in 
the  living  frog,  or  by  supplying  a  ventricle-apex  preparation 
through  a  perfusion  cannula  with  diluted  blood  at  a  pressure 
rather  above  the  frog's  normal  blood-pressure.  A  strip  cut 
from  the  apex  of  the  tortoise's  ventricle  may  be  made  to 
beat  rhythmically,  if  it  be  hung  in  a  moist  chamber  and 
stimulated  at  intervals  with  weak  induction-shocks.  After 
a  time  the  strip  begins  to  beat  of  its  own  accord,  and  beats 
rhythmically  for  many  hours. 

It  seems  probable  then  that  the  automaticity  of  the  heart 
is  inherent  in  its  muscular  tissue,  the  difference  in  the  auto- 
matic power  of  the  various  parts  being  dependent  on  their 
different  histological  characters.  The  lowly  differentiated 
sinus-cell  has  high  rhythmic  power  and  a  quick  rhythm  of 
beat,  but  contracts  feebly.  The  more  highly  differentiated 
ventricle-cell  has  only  slight  rhythmic  power,  but  beats  for- 
cibly, and  is  a  good  servant  of  the  sinus.  If  all  parts  of  the 
heart  had  equal  rhythmic  power  or  the  same  rate  of  rhythm, 
there  would  be  no  reason  why  any  one  part  of  thei  heart 
more  than  another  should  initiate  the  beat.  As  it  is,  the  beat 
always  starts  in  the  part  beating  with  the  greater  frequency, 
viz.  the  sinus.     In  some  animals  this  difference  of  rhythmic 


THE    VASCULAE   MECHANISM  237 

power  is  less  marked,  and  the  contraction  may  start  from 
either  end  of  the  heart.  Even  in  the  frog,  the  heart-cavities 
may  be  made  to  contract  with  a  reversed  sequence,  i.e.  in  the 
order  of  ventricle,  auricles,  sinus,  by  artificially  stimulating 
the  ventricle  at  a  rate  exceeding  the  normal  sinus  rhythm. 

Propagatio7i  of  the  Wave  of  Contraction  in  the  Frog's 

Heart 

The  next  question  is :  How  is  the  excitatory  process  eon- 
ducted  from  sinus  to  auricles  and  thence  to  ventricle?  It 
was    formerly   thought   that   this    conduction    was    efiected 

Fig.  129. 


Ventricle 


Tortoise's  heart  (after  Gaskell)  as  it  appears  when  suspended 
for  registering  the  auricular  and  ventricular  contractions. 
N,  nerve-trunk  with  fibres  connecting  Kemak's  and  Bidder's 
ganglia  ;  Cor.  v,  coronary  vein. 

through  nerves.  It  has  however  been  shown  by  Gaskell  in 
the  tortoise's  heart,  where  the  nerve-trunks  run  apart  from 
the  auricles  from  sinus  to  ventricle,  that  division  of  these 
trunks  causes  no  alteration  in  the  rhythm  of  the  ventricle.  If 
however  the  auricles  be  cut  through,  leaving  the  ventricle 
attached  to  the  sinus  by  the  nerve-trunks,  the  sequence  of 
beats  is  utterly  lost. 

The  auricles  may  be  slit  up  by  interdigitating  cuts,  by 
which  all  nerve-trunks  must  infallibly  be  divided,  and  yet 
the  wave  of  contraction  passes  from  the  sinus  over  the  auricles 
round  the  interdigitating  cuts  to  the  ventricle. 

There  is  a  distinct  pause  between  the  contractions  of 
auricles  and  ventricle.  This  was  supposed  to  point  to  the 
intermediation  of  nerves  in  the  transmission  of  the  excitatory 
process  across  the  groove.  The  pause  however  is  probably 
due  to  the  fact  that  the  muscle-cells  forming  the  auriculo- 
ventricular  ring  are  very  slightly  differentiated,  and  so  con- 


238  PHYSIOLOGY 

tract  and  conduct  slowly,  i.e.  this  muscular  ring  presents 
a  partial  *  block.' 

The  auricles  may  be  made  to  beat  in  two  halves  by  merely 
dividing  the  sinus  half  from  the  ventricular  half,  leaving  them 
connected  by  a  very  narrow  strip  of  auricular  wall.  In  this 
way  a  block  is  produced  at  the  cut.  The  sinus  contracts, 
then  the  upper  part  of  the  auricles.  This  is  followed  by  a 
distinct  pause,  during  which  the  excitatory  process  is  passing 
the  block.  The  ventricular  half  of  the  auricles  then  con- 
tracts, followed  by  the  ventricle  and  bulbus. 

We  may  conclude  that  a  normal  contraction  of  the  frog's 
heart  originates  in  the  muscular  wall  of  the  sinus,  and 
travels  as  a  wave  from  muscle-cell  to  muscle-cell,  over  the 
auricles  and  ventricle  ;  the  apparent  pauses  between  sinus 
and  auricles  and  auricles  and  ventricle  being  due  to  the  low 
conducting  power  of  the  muscular  tissue  connecting  these 
cavities. 

It  will  be  seen  from  what  we  have  already  said  that  the 
contraction  of  the  heart  is  to  be  looked  upon  as  a  single  con- 
traction wave,  propagated  from  one  end  of  the  heart  to  the 
other,  just  as  a  wave  of  contraction  passes  along  the  sartorius 
(though  at  much  quicker  rate  and  of  shorter  duration)  when 
this  muscle  is  stimulated  at  one  end. 

So7ne  Properties  of  the  Cardiac  Muscle 

Contraction  ahvays  maximal. — In  several  points  however 
the  properties  of  cardiac  muscle  differ  from  those  of  ordinary 
striated  muscle.  Its  automaticity  and  power  of  responding 
rhythmically  to  continuous  stimuli  have  already  been  men- 
tioned. The  height  of  contraction  of  a  voluntary  muscle  is, 
within  certain  limits,  proportional  to  the  strength  of  stimulus. 
If  the  ventricle,  rendered  motionless  by  a  Stannius'  ligature, 
be  stimulated  with  a  single  induction-shock,  it  always  re- 
sponds with  a  maximal  contraction,  whether  the  stimulus 
applied  be  minimal  or  maximal.  There  is  thus  no  propor- 
tionality in  the  heart  between  strength  of  stimulus  and 
height  of  contraction.  The  heart,  if  it  contracts  at  all, 
always  contracts  to  its  utmost.  The  height  of  the  contrac- 
tion is  dependent  on  the  condition  of  the  muscle  at  the  time, 
but  not  on  the  strength  of  stimulus. 

'  Staircase  '  phenomenon. — If  the  frog's  ventricle  has  been 
at  rest  for  some  time,  a  single  contraction  makes  the  heart 


THE   VASCULAR   MECHANISM  239 

more  excitable  and  in  a  better  condition.  So  if,  in  a  Stannius' 
preparation,  we  excite  the  ventricle  with  single  induction- 
shocks  once  in  every  10  seconds,  the  first  four  or  five  con- 

FiG.  130. 


Group  of  pulsations  showing  '  staircase  '  character. 

tractions  form  an  ascending  series,  each  contraction  being 
rather  higher  than  the  preceding  one. 

Summation  of  stimuli. — In  a  similar  preparation  it  is  easy 
to  demonstrate  the  summation  of  stimuli,  which  was  described 
in  dealing  with  unstriped  muscle.  If  the  ventricle  of  a 
Stannius'  preparation  be  stimulated  with  inadequate  shocks, 
it  is  found,  on  repeating  these  shocks  at  short  intervals  of 
time,  that  they  become  adequate,  and  cause  a  contraction  of 
the  ventricle.  The  subsequent  contractions  then  show  the 
progressive  augmentation  just  described  as  the  '  staircase.' 

Influence  of  tension. — Much  the  most  important  factor 
affecting  the  strength  of  the  cardiac  contractions  is  the 
tension  of  the  muscle-fibres.  Within  certain  limits  the 
energy  of  contraction  of  the  cardiac  muscle  increases  with 
the  tension  {i.e.  resistance  to  shortening)  to  which  the 
muscle-fibre  is  exposed.  This  effect  can  be  seen  when  the 
resistance  to  the  outflow  of  blood  from  the  heart  is  increased, 
and  the  resistance  is  only  experienced  during  the  contraction 
of  the  heart-muscle.  It  is  much  better  marked  however 
if  the  alterations  of  tension  take  place  before  the  contraction, 
i.e.  during  the  diastole.  In  this  case  we  observe  that 
within  wide  limits  the  heart  contracts  more  forcibly  the 
greater  its  distension  during  the  diastolic  period.  This  effect 
of  the  initial  tension  on  the  force  of  the  ventricular  beats  is 
well  shown  in  Fig.  131,  representing  tracings  taken  from  the 
frog's  ventricle  under  increasing  initial  tensions.  In  this 
case  the  heart  was  contracting  isometrically,  i.e.  against 
a  strong  elastic  resistance,  so  that  the  curves  are  a  true 
representation  of  the  actual  energy  displayed  by  the  heart. 
In  all  cases  the  heart  was  unable  to  expel  any  of  its  contents, 
so  that  the  difference  in  height  of  the  curves  is  due  solely  to 


240 


PHYSIOLOGY 


the  varying  tension  of  the  fibres  at  the  beginning  of  the  con- 
tractile process. 

Of  course  if  the  initial  tension  be  continually  increased,  we 
finally  arrive  at  a  point  at  which  this  peripheral  reactive 
mechanism  gives  way  ;  the  heart  is  unable  to  contract  against 
the  great  resistance  and  becomes  permanently  stretched  and 
damaged. 

The  refractory  period. — At  each  contraction  of  the  heart- 
muscle  there  is  a  sudden  decomposition  of  contractile  mate- 
rial which,  so  far  at  least  as  concerns  the  incidence  of  an 
external  stimulus,  is  maximal,  i.e.  complete.  Directly  this 
has  occurred  a  process  of  assimilation  or  re-formation 
of   contractile   material   starts.      This   lasts   throughout   the 

Fm.  131. 


Isometric  contractions  of  frog's  ventricle.  The  initial  tension  was 
continually  increased  from  curve  1  to  curve  6,  each  increase  of 
tension  causing  a  greater  energy  of  contraction,     (v.  Frank.) 

diastolic  period,  and  the  store  of  contractile  material  is  at 
its  maximum  just  before  the  next  contraction.  We  may  in  fact 
compare  the  process  to  a  bucket,  into  which  a  stream  of 
water  is  constantly  flowing,  and  which  tips  up  automatically 
and  empties  out  its  contents  as  soon  as  the  water  reaches  a 
certain  height.  It  is  evident  that  the  power  of  the  heart- 
muscle  to  contract  in  response  to  a  stimulus  (its  irritability) 
must  be  at  a  minimum  immediately  after  the  automatic  dis- 
charge, or  decomposition,  has  taken  place,  and  will  con- 
tinually increase  from  this  point  as  the  store  of  contractile 
material  grows,  until  it  arrives  at  such  a  height  that  the 
explosive  discharge  occurs  spontaneously.  Hence  in  each 
cardiac   cycle   there   is   a   period,   known   as   the   refractory 


THE   VASCULAR  MECHANISM 


241 


period,  in  which  stimuli  applied  to  the  heart  have  no  eiTect. 
This  will  be  followed  by  a  period  in  which  a  stimulus  is 
followed  by  an  extra  contraction,  but  with  a  prolonged  latent 


Fi.!.  132. 


■MBWIi 


■■■■ 


Tracings  of  spontaneous  contractions  of  frog's  ventricle,  to  show 
refractory  period.  In  each  series  the  surface  of  the  ventricle 
was  stimulated  by  an  induction  shock  at  e,  as  indicated  by  the 
tracing  of  the  signal.  In  1,  2,  and  3,  this  stimulus  had  absolutely 
no  effect,  since  it  fell  during  the  refractory  period.  In  4,  5,  6,  7, 
the  effect  of  the  shock  was  to  interpolate  an  extra  contraction  in 
the  series,  the  latent  period  (shaded  part)  gradually  diminishing 
from  4  to  7  (diastolic  rise  of  irritability).  In  8  the  irritability 
of  the  preparation  was  already  considerable,  and  the  latent  period 
inappreciable.  The  '  compensatory  pause '  after  the  extra  beat  is 
also  well  shown  in  4,  5,  6,  7,  8.     (Marey.) 

period.     Just  before  the   next   spontaneous   contraction    the 

irritability  is    at   its  height,  and  the  heart-muscle   responds 

with  a  contraction  to  a  minimal  stimulus.     These  facts  are 

well  shown  in  Fig.  132. 

16 


242 


PHYSIOLOGY 


Bhythm  of  Mammalian  Heart 

So  far  as  we  know,  the  process  of  contraction  in  the 
mammalian  heart  is  essentially  the  same  as  in  the  frog's 
heart.  The  contraction  starts  in  the  terminations  of  the 
great  veins,  and  travels  thence  over  the  am-icles.  A  pause 
of  about  one-tenth  of  a  second  occurs,  and  then  the  ventricles 
contract,  the  contraction  starting  at  the  base  and  travelling 
thence  as  a  wave  to  the  apex. 

The  wave-like  progression  of  the  excitatory  condition  in 
the  ventricle  can  be  well  shown,  as  in  the  frog,  by  leading 
off  the  base  and  apex  of  the  exposed  ventricle  to  the  two 
terminals  of  a  capillary  electrometer.  We  then  get  in  many 
cases  a  diphasic  variation  differing  only  in  the  shorter  dura- 
tion of  its  phases  from  that  described  for  the  frog's  heart. 
We  can  in  fact  (as  was  shown  by  Waller)  by  leading  off  from 
the  apex  beat  and  right  hand  obtain  a  photographic  record 
of  the  variation  of   the  heart  in  man.     Fig.  133  represents 


Fig.  133. 


Hsi  (to  apex) 


Acid  (to  base) 

Pulse  tracing 
from  caroMd 


Electrical  variation  of  human  heart.     (BayUss  and  Starling.) 


the  variation  of  the  ventricles  as  obtained  in  this  way.  It 
will  be  seen  that  the  variation  is  triphasic,  implying  that  the 
excitatory  change  starting  at  the  base  of  the  heart  extends 
thence  to  the  apex,  but  the  contraction  lasts  longer  at  the 
base  than  at  the  apex.  The  production  of  the  curve  is  shown 
by  the  following  diagram  (Fig.  134). 

With  a  more  delicate  electrometer  it  is  possible  to  record 
also  the  electrical  change  due  to  the  auricular  contraction. 
This  has  the  appearance  of  a  sharp  spike  immediately  pre- 
ceding the  triphasic  ventricular  variation. 

The  only  essential  difference  between  the  mammalian  and 
amphibian  heart  seems  to  lie  in  the  comparative  automaticity 


THE    VASCULAR  MECHANIS3I 


243 


of  the  ventricles  in  the  two  cases,  the  mammalian  ventricles 
possessing  much  more  automatic  power  than  that  of  the  frog. 
If  the  ventricles  be  functionally  separated  from  the  auricles  by 
crushing  the  auriculo-ventricular  groove,  both  parts  continue 
beating,  but  at  different  rhythms,  the  ventricular  rhythm  being 
as  a  rule  much  slower  than  that  of  the  auricles.  Conduction 
is  probably  effected,  as  in  the  frog's  heart,  by  the  inter- 
mediation of  the  muscular  tissue. 

Fig.  134. 


negative 
base 


excitatory 
change 


Diagram  to  show  mode  of  production  of  electrometer  curve  in 
Fig.  133.  The  primary  '  negativity '  of  the  base  (shaded 
portion  of  lower  diagram)  causes  a  movement  of  the  Hg 
meniscus  downwards  (the  acid  being  connected  to  the  base). 
Immediately  afterwards  the  negativity  spreads  to  the  apex. 
The  whole  heart  is  at  same  potential,  and  the  meniscus 
returns  sharply.  The  excited  condition  then  dies  away,  but 
lasts  longer  at  the  base ;  hence  a  second  excursion  of  the 
meniscus  downwards,  with  a  slow  recovery. 


Although  ganglia  occur  in  large  numbers  in  the  different 
parts  of  the  mammalian  heart,  it  is  possible  to  find  consider- 
able sections,  especially  near  the  apex  of  the  ventricles,  which 
are  quite  free  from  ganglion-cells.  Porter  has  shown  that 
such  a  strip  of  muscle  may  be  kept  alive  for  some  hours  by 
perfusing  it  through  a  branch  of  the  coronary  artery  with 
defibrinated  blood.  Under  these  conditions  the  strip  beats 
with  a  regular  rhythm,  demonstrating  the  absolute  sufficiency 
of  the  muscular  fibres  for  the  initiation  and  propagation  of  the 
contractile  process. 


244 


PHYSIOLOGY 


Section  5 
INNEEVATION   OF   THE   HEAET 

The  heart  is  supplied  with  nerves  from  two  sources — the 
vagi  and  the  sympathetic.  The  fibres  supplying  the  heart  run 
a  sHghtly  different  course  in  the  frog  and  in  the  mammal,  such 
as  the  dog. 

Fig.  135. 

.  y^"^^^-  ^3nql.  Vagus 

-Verl".  I. 


Vago-Symparhe-Hc 


Subclav.  at-r 


Aorra 


-Splanchn.  n, 
lnl•e&^.  arf 


N.VIII 

N.IX. 

Sympathetic  chain  of  frog  (right  side)  to  show  connection  with 
vagus  nerve.  The  sympathetic  ganglia  with  their  branches  are 
black.  Of  the  peripheral  branches  only  the  splanchnic  nerve  is 
represented.     (Modified  from  Ecker.) 

The  sympathetic  fibres  to  the  frog's  heart  leave  the  cord 
by  the  anterior  root  of  the  third  spinal  nerve,  pass  through 
the  ramus  communicans  to  the  corresponding  splanchnic 
ganglion,  and  thence  by  the  second  ganglion,  the  annulus  of 


THE   VASCULAR  MECHANISM 


245 


Vieussens,  and  the  first  ganglion  to  the  cervical  sympathetic. 
This  runs  up  to  join  the  ganglion  trunci  vagi,  and  thence  the 
fibres  run  down  with  the  vagus  nerve  (Fig.  135). 


r.Sp.Ac. 


Diagram  of  cardiac  inhibitory  and  accelerator  fibres  in  the  dog 
.  (from  Foster).  r.Vg.  Roots  of  the  vagus.  r.Sp.Ac.  Roots  of  the 
spinal  accessory.  G.J.  Ganglion  jugulare.  G.h.V.  Ganglion 
trunci  vagi.  Vg.  Trunk  of  vagus  nerve.  C.Sy.  Cervical  sympa- 
thetic. G.C.  Inferior  cervical  ganglion.  A.V.  Annulus  of 
Vieussens.  A.sb.  Subclavian  artery,  n.c.  Cardiac  nerves.  G.St. 
Ganglion  stellatum.  D.2,  D.3,  D.4,  D.5.  Second,  third,  fourth,  and 
fifth  dorsal  spinal  roots.     G.Th.  Ganglia  of  the  thoracic  chain. 


In  the  dog,  the  sympathetic  fibres  to  the  heart  leave  the 
spinal  cord  by  the  anterior  roots  of  the  second  and  third  dorsal 
nerves,  run  in  the  rami  communicantes  to  the  stellate  ganglion, 
and  thence  by  the  annulus  of  Vieussens  to  the  inferior  cervical 


246  PHYSIOLOGY 

ganglion.  The  cardiac  branches  containing  these  fibres  run 
from  the  stellate  ganglion,  the  annulus,  the  inferior  cervical 
ganglion,  and  the  trunk  of  the  vagus  to  the  heart. 

The  accelerator  nerves  are  small  meduUated  fibres  as  they 
leave  the  cord  and  pass  along  the  anterior  roots  and  white 
rami  communicantes  to  the  stellate  ganglion.  In  this  ganglion 
they  end  in  connection  with  its  cells,  and  a  fresh  relay  of 
fibres,  which  are  non-medullated,  carries  the  impulses  on  to  the 
heart.  In  the  heart  they  are  apparently  distributed  directly 
to  the  muscular  fibres,  without  the  intervention  of  any  more 
ganglion-cell  stations. 

The  effect  of  stimulating  these  two  sets  of  nerve-fibres  is 
the  same  in  the  frog  and  mammal.     In  the  frog,  since  the 

Fig    137. 


Vent. 


Allll    1 


ill ! 


piMwm'wninny 


Tracing  to  show  effect  of  stimulation  of  the  vago-sjmpathetic 
nerve  on  the  frog's  heart.  The  rhytlim  is  unaltered,  but  the 
beats  of  auricle  and  ventricle  are  much  decreased  in  size. 
On  ceasing  the  stimulation  the  beats  become  augmented. 
(Gaskell.) 

sympathetic  fibres  run  down  in  the  trunk  of  the  vagus,  it  is 
necessary,  in  order  to  obtain  pure  effects,  to  stimulate  the 
intracranial  part  of  the  vagus  or  the  cervical  sympathetic. 
If  the  intracranial  vagus  be  stimulated  while  the  heart  is 
beating  regularly,  the  heart  stops  at  once  in  diastole,  or  it  may 
give  one  beat  before  stopping.  If  the  stimulation  be  now  dis- 
continued, the  heart  after  a  little  while  begins  to  beat  again, 
at  first  slowly,  and  gradually  comes  back  to  the  normal  rhythm. 
In  many  cases,  if  the  vagus  be  stimulated  repeatedly,  a  distinct 
improving  action  on  the  beat  is  observed,  i.e.  the  heart  beats 
more  forcibly  and  rapidly  when  the  stimulation  is  discontinued 


THE   VASCULAE   MECHANISM 


247 


than  it  did  at  the  commencement  of  the  experiment.  The 
vagus  is  named  the  inhibitory  nerve  of  the  heart.  This  inhi- 
bition may  influence  either  the  rhythm  or  the  force  of  the 
ventricular  contraction,  the  different  results  being  probably 
dependent  on  whether  the  sinus  is  most  affected,  when  the 

Fig.  138. 


A  tracing  similar  to  Fig.  137.  In  this  case  however  the  stimulation 
caused  complete  stoppage  (inhibition)  of  both  auricular  and  ven- 
tricular beats.     (Gaskell.) 

beats  will  be  slowed,  or  the  ventricle,  in  which  case  each  beat 
will  be  weaker.  If  only  a  weak  stimulus  be  applied  to  the 
vagus,  the  effect  may  be  merely  to  weaken  or  slow  the  beats, 
without  causing  a  complete  stoppage. 


Fig.  139. 


Blood-pressure  tracing  from  carotid  of  dog  (taken  with  Hiirthle's 
manometer),  showing  effect  of  excitation  of  vagus  (between  the 
arrows),     o.  Abscissa  line  of  no  pressure. 

Stimulation  of  the  sympathetic  cardiac  nerves  has  exactly 
the  reverse  effect,  causing  increase  in  force  or  rate  of  the 
heart-beats  or  both  results  at  once.  They  are  therefore  said 
to  be  both  augmentor  and  accelerator  nerves. 

The  augmentor  fibres  are  much  less  easily  tired  than  the 


248  iPHYSlOLOGY 

vagus  fibres.  Hence,  if  the  vago-sympathetic  of  the  frog  be 
stimulated,  the  first  effect  is  inhibition  due  to  vagus  action  ; 
the  vagus  nerve-endings  then  tiring,  the  influence  of  the  stimu- 
lation of  the  accelerator  fibres  makes  itself  apparent,  and  the 
heart,  while  stimulation  is  still  going  on,  commences  to  beat 
more  rapidly  and  forcibly  than  it  did  before. 

Stimulation  of  the  vagus  also  lowers  the  conductivity  of 
the  cardiac  tissue,  and,  with  a  carefully  graduated  stimulus,  it 
is  often  possible  to  make  a  block  between  auricles  and  ventricle, 
so  that  the  latter  responds  only  to  every  second  auricular 
beat.  The  accelerator  fibres  have  the  reverse  effect.  We 
may  make  an  artificial  block  between  auricles  and  ventricle 
by  clamping  rather  tightly  the  auriculo-ventricular  groove, 
so  that  the  ventricle  beats  only  once  to  every  two  auricular 
contractions.  If  now  the  accelerator  nerves  be  stimulated  the 
block  is  removed,  and  the  ventricle  beats  in  normal  sequence 
to  the  auricles. 

These  two  sets  of  fibres  have  exactly  the  same  function 
in  the  mammal  (dog).  Vagus  causes  slowing  of  rhythm, 
depression  of  force  and  of  conductivity  ;  accelerators  cause 
acceleration,  augmentation,  and  improvement  of  conducting 
power.  Since  however  the  beat  of  the  heart  is  normally  ruled 
by  the  auricular  beat,'  we  find  that  the  action  of  these  nerves 
is  much  more  pronounced  on  the  auricles  than  on  the 
ventricles.  This  is  illustrated  by  the  fact  that  during  pro- 
longed vagus  excitation  the  ventricles  may  begin  to  beat  with 
a  rhythm  of  their  own,  while  the  auricles  remain  perfectly 
motionless. 

In  many  animals  the  vagus  centre  in  the  medulla  exercises 
a  tonic  or  continuous  inhibitory  action  on  the  heart.  Thus 
in  the  dog,  section  of  one  vagus  causes  a  slight  quickening 
of  the  heart-beat  {e.g.  from  60  to  80  per  minute).  If  now  the 
second  vagus  be  cut,  the  heart -beat  is  markedly  quickened, 
and  may  occur  120  times  in  the  minute.  The  effect  of  vagus 
section  is  still  more  marked  if  the  vagus  centre  in  the  medulla 
be  in  a  condition  of  increased  activity,  as  after  administration 
of  morphia,  or  during  asphyxia. 

'  The  rate  of  the  ventricle  is  determined  by  the  frequency  of  the  excitation 
arriving  at  it  from  the  auricles.  The  strength  of  the  beat  depends  on  the  initial 
tension  of  the  ventricular  muscle,  and  therefore  largely  on  the  amount  of  blood 
sent  into  the  ventricle  by  the  auricular  contraction. 


THE   VASCULAE   MECHANISM 


249 


The  accelerators  are  further  distinguished  from  the  vagus 
in  the  length  of  their  latent  period,  which  in  the  case  of  the 
former  is  excessively  long.  The  latent  period  of  vagus 
excitation  in  the  mammal  is  considerably  less  than  a  second, 
whereas  that  of   the   accelerators    amounts   to   ten   or   even 

Fi(i.  140. 


liitaifcyliiMiiiM] 

(itmfflttfflmtiff^ 


BBBSiBBfiB 


Tracings  of  ventricular  (upper  curve)  and  auricular  contractions  (lower 
curve).  From  xtoy  the  accelerator  nerves  stimulated.  Lowest  line 
=  seconds. 


twenty  seconds  (Fig.  140).  The  effect  of  accelerator  stimu- 
lation lasts  for  an  equal  length  of  time  after  the  stimulation 
is  discontinued. 


Afferent  Nerves  of  the  Heart 

Besides  these  efferent  fibres  going  to  the  heart,  there  are 
other  fibres  running  chiefly  in  the  vagus  which  serve  to 
carry  afferent  impulses  from  the  heart  to  the  nervous  centres. 
Some  of  their  terminal  branches  ramify  over  the  ventricle 
(of  the  dog)  immediately  under  the  pericardium.  We  may 
investigate  their  functions  by  stimulation  of  the  central  ends 
of  the  divided  nerves.  They  may  produce  one  or  more  of 
four  effects  : 

1.  Pain,  as  evinced  by  the  movements  of  an  animal  not 
fully  under  the  influence  of  an  anaesthetic  (we  should  be  more 
correct  if  we  said  that  stimulation  of  these  nerves  produced 
reflex  movement). 

2.  Eeflex  inhibition  of  the  heart.  If  one  vagus  be  cut  and 
its  central  end  stimulated,  there  is  very  often  slowing  of  the 
heart  by  reflex  impulses  which  descend  the  other  vagus. 

3  and  4.  Pressor  and  depressor  effects.  Stimulation  of 
these  nerves  may  cause  a  reflex  raising  (pressor)  or  lowering 
(depressor)  of  the  blood -pressure. 


250  PHYSIOLOGY 

Cardio-inhibitory  Centre 

There  is  one  little  spot  in  the  medulla,  in  the  neighbour- 
hood of  the  origin  of  the  vagus  nerves,  stimulation  of  which 
causes  inhibition  of  the  heart.  If  this  spot  be  destroyed, 
reflex  inhibition  of  the  heart  can  no  longer  be  produced  ; 
hence  it  is  spoken  of  as  the  cardio-inliibitory  centre.  The 
afferent  nerves  from  the  abdomen  and  intestine  seem  to  have 
very  close  connections  with  this  centre,  so  that  reflex  inhibi- 
tion of  the  heart  can  be  easily  produced  in  the  frog  by  tapping 
a  loop  of  intestine  with  the  handle  of  a  scalpel.  This  con- 
nection explains  to  some  extent  the  depressed  condition  of  the 
circulation  in  man  in  severe  abdominal  affections,  such  as 
peritonitis. 

When  the  blood-pressure  is  raised,  as  by  a  general  con- 
striction of  the  arterioles  of  the  abdominal  viscera,  the 
resistance  to  the  outflow  of  blood  from  the  heart  is  increased. 
This  factor  by  itself  would  tend  to  produce  a  stronger  heart- 
beat, without  any  accompanying  slowing.  Under  normal 
conditions  however  we  find  that  the  '  rate  of  the  heart-beat 
is  in  inverse  ratio  to  the  arterial  pressure '  ('  Marey's 
Law'),  a  rise  of  blood-pressure  causing  a  slowing  of  the 
heart,  and  vice  versa.  This  law  holds  good  only  if  the  vagi 
are  intact.  The  slowing  consequent  on  rise  of  pressure  is 
probably  conditioned  by  two  factors  :  (1)  the  increased  pres- 
sure in  the  cranial  cavity  directly  affecting  the  cardio- 
inhibitory  centre ;  and  (2)  the  action  of  the  rise  of  pressure 
on  the  endings  of  the  vagus  in  the  walls  of  the  heart,  stimu- 
lation of  these  fibres  by  the  increased  tension  causing  a  reflex 
inhibition  of  the  heart  through  the  same  nerve-trunk.  Both 
these  effects  would  of  course  be  abolished  by  section  of  the 
two  vagi. 

Influence  of  Drugs  on  the  Heart 

The  cardio-inhibitory  centre  is  stimulated  by  digitaline  and 
morphine,  so  that  the  heart  is  slowed  under  the  influence  of 
these  drugs. 

Muscarine  stimulates  the  nerve-endings  of  the  vagus.  If 
applied  to  the  frog's  heart,  it  causes  gradual  weakening  and 
slowing  of  the  beat,  and  the  heart  finally  stops  still  in  dia- 
stole.    If   a  solution  of  atropine  be  now  applied,  the  heart 


THE   VASCULAR   MECHANISM  251 

commences  beating  again.  It  is  fomid  that  stimulation  of 
the  vagus  is  now  absolutely  ineffectual  in  producing  inhibi- 
tion. Hence  we  argue  that  atropine  paralyses  the  termina- 
tions of  the  vagus  in  the  heart.  The  same  paralysis  of  the 
vagus  is  produced  in  the  mammal  if  atropine  be  injected  into 
the  circulation.  After  administration  of  atropine,  stimulation 
of  the  sino-auricular  junction  (the  so-called  local  inhibitory 
centre)  has  no  effect  on  the  heart. 

If  nicotine  be  injected  into  the  circulation  or  applied 
directly  to  the  heart,  it  first  stimulates  and  then  paralyses  the 
nerve  cells  to  which  the  vagus  fibres  run,  and  which  give  oft' 
the  inhibitory  fibres  to  the  heart  muscle.  After  this  drug 
therefore,  although  stimulation  of  the  vagus  trunk  has  no 
inhibitory  action  (the  impulses  being  blocked  in  the  heart 
ganglia),  stimulation  of  the  sino-auricular  junction  still  causes 
inhibition  in  consequence  of  direct  excitation  of  the  post- 
ganglionic fibres. 

Curare  has  a  paralysing  influence  similar  to  that  of 
atropine,  but  only  when  applied  in  large  doses. 

Physostigmin  has  the  same  action  as  muscarine,  and,  as  in 
this  case,  its  effect  is  removed  by  the  application  of  atropine. 

Adrenaline,  the  active  principle  of  the  medulla  of  the 
suprarenal  bodies,  when  applied  directly  to  the  isolated  heart, 
causes  a  considerable  augmentation  and  strengthening  of  the 
beat.  When  injected  into  the  circulation,  it  produces  a  general 
constriction  of  the  l)lood-vessels  and  a  consequent  rise  of 
blood  pressure,  and  this  rise  is  accompanied  by  extreme  slow- 
ing of  the  heart  brought  about  by  stimulation  of  the  vagus 
centre. 

Dilute  alkalies  (KHO,  1  in  20,000)  cause  the  frog's  heart 
to  stand  still  in  a  tonic  contraction,  the  tone  of  the  heart 
gradually  increasing  till  the  beats  are  no  longer  visible  on  the 
tracing. 

Dilute  acids  have  the  opposite  eft'ect,  removing  the  tonic 
contraction  produced  by  alkalies,  and  finally  causing  a  stand- 
still of  the  heart  in  complete  relaxation. 


252  PHYSIOLOGY 


Section  6 
THE   WORK   OF  THE   HEART 

The  energy  of  the  ventricular  contraction  is  expended  in 
two  ways  :  firstly,  in  forcing  a  certain  amount  of  blood  into 
the  already  distended  aorta  against  the  resistance  presented 
by  the  arterial  blood-pressure,  which  itself  is  directly  con- 
ditioned by  the  resistance  in  arterioles  and  capillaries  ;  and 
secondly,  in  imparting  to  the  mass  of  blood  so  thrown  out  a 
certain  velocity.  Thus  the  energy  of  the  muscular  contraction 
is  converted  partly  into  potential  energy  in  the  form  of  in- 
creased distension  of  the  arterial  wall,  and  partly  into  the 
kinetic  energy  represented  by  the  momentum  of  the  moving 
column  of  blood.  The  work  done  at  each  beat  may  be  cal- 
culated from  the  formula  : 

W  =  QR  ^  "^-^ 

2g 

where  W  stands  for  work,  w  for  the  weight,  and  Q  for  the  quan- 
tity (volume  in  ccms.)  of  Ijlood  expelled  at  each  contraction.  R 
is  the  average  arterial  resistance  or  pressure  during  the  outflow 
of  blood  from  the  heart,  and  v  is  the  velocity  of  the  blood  at  the 
root  of  the  aorta.  In  this  equation  QR  is  the  work  done  in  over- 
coming the  resistance,'  and      -    is  the   energy  expended  in 

2  g 
imparting  a  certain  velocity  to  the  blood. 

We  have  already  discussed  the  means  whereby  the  average 
pressure  and  velocity  of  the  blood  in  the  aorta  may  be 
measured,  and  it  remains  only  to  determine  the  output  of 
the  ventricles  at  each  beat,  in  order  to  have  at  our  disposal  all 
the  factors  necessary  for  the  calculation  of  the  work  of  the 
heart.  The  measurement  of  the  output  presents  considerable 
difficulties.     Stolnikow   and    Pawlow   practically  cut  out  the 

'  This  expression  QB  is  only  approximately  correct.  Supposing  the  pressure 
in  the  aorta  at  the  beginning  of  systole  is  50  mm.  Hg.  and  at  the  end  of  systole 
150  mm.,  the  work  could  not  be  deduced  accurately  from  the  average  pressure, 
but  would  need  a  simple  application  of  the  integral  calculus  for  its  determina- 
tion. The  simple  expression  employed  above  deviates  from  the  real  value  only 
by  about  10  per  cent.,  and  is  therefore  sufficiently  accurate  for  our  purpose. 


THE   VASCULAR   MECHANISM 


253 


systemic  circulation  altogether,  and  caused  the  blood  from  the 
left  ventricle  to  traverse  an  instrument  (current-measurer, 
Strumaiclie)  which  recorded  automatically  the  amount  of  blood 
that  went  through  it  in  a  given  time. 

In  Fig.  141,  I  and  II  are  two  cylinders  containing  accurately  fitting  floats, 
bearing  writing  levers  on  their  upper  ends.  Each  of  these  communicates  below 
with  two  tubes,  a  and  v,  one  of  which  is  connected  to  the  right  carotid  artery, 
while  the  other  is  inserted  into  the  superior  vena  cava.  All  the  other  branches 
of  the  aorta,  as  well  as  the  inferior  vena  cava,  are  ligatured.  At  the  beginning 
of  the  experiment,  cylinder  II  is  filled  with  defibrinated  blood.     This  blood 

Fig.  141. 


Diagram  of  Stolnikow's  instrument. 


passes  down  the  tube  2v  into  the  right  auricle,  and  so  through  the  right 
ventricle  and  lungs,  where  it  is  aerated,  into  the  left  auricle  and  ventricle.  As 
the  heart  continues  beating,  the  left  ventricle  expels  its  contents  into  cylinder  I,  so 
that  the  piston  in  I  is  rising  while  that  in  II  is  falling.  As  soon  as  cylinder  II 
becomes  empty,  the  tubes  1v  and  2a  are  released  and  the  tubes  la  and  iiv  are 
clamped.  The  left  ventricle  now  expels  its  blood  through  2a  into  cylinder  II 
while  cylinder  I  is  emptying  itself  through  Iv  into  the  right  auricle.  We  thus 
get  two  series  of  zigzag  lines  traced  by  the  piston  rods,  and  the  frequency  of  the 
zigzags  is  an  expression  of  the  output  of  the  left  ventricle  in  a  given  time. 

This  method  suffers  from  the  defect  that  the  arterial  pres- 
sure, in  consequence  of  the  absence  of  external  resistance,  is 
extremely  low,  so  that  the  heart  is  throughout  under  highly 


254 


PHYSIOLOGY 


abnormal  conditiuns.  It  enjoys  however  the  corresponding 
advantage  that  K  (the  resistance),  though  low,  is  constant 
throughout  the  experiment,  and  the  work  done  by  the  heart  is 
therefore  directly  proportional  to  the  output. 

Better  methods  are  those  based  upon  the  application  of 
the  plethysmographic  method  to  the  heart  in  situ.  We  may 
either,    as   in    Tigerstedt's   method,  employ  the  pericardium 

Fig.  142. 


Diagram  of  Roy's  cardiometer.  On  the  right  of  the  figure  are  the 
two  quarter  spheres  which  are  clamped  on  to  the  pericardium  at 
the  root  of  the  heart. 


itself  filled  with  oil  or  air  as  the  oncometer,  and  register  the 
changes  in  the  volume  of  the  heart  by  connecting  the  cavity 
of  the  pericardium  with  some  form  of  piston-recorder,  or  we 
may  make  use  of  Roy's  cardiometer.  This  is  a  brass  sphere 
in  three  segments.  The  two  quarter  spheres  (Fig.  142)  are 
applied  round  the  base  of  the  heart  and  clamped  together, 
the  cut  pericardium  being  attached  to  their  constricted  neck, 
the   third    segment,  a  hemisphere,  is  then  applied  over  the 


THE    VASCULAR  MECHANISM  255 

apex  of  the  ventricles  and  clamped  to  the  parts  already  in  situ. 
Attached  to  the  centre  of  this  hemisphere  is  a  modified  piston- 
recorder,  containing  a  piston  working  in  oil,  with  which  the 
whole  of  the  apparatus  is  filled.  At  a  is  a  spring  which  can 
be  adjusted,  so  as  to  exercise  a  constant  pull  upon  the  piston, 
and  reproduce  to  some  extent  the  negative  pressure  under 
which  the  heart  normally  works.  The  piston-rod  carries  a 
lever  which  writes  on  a  blackened  surface.  The  excursions  of 
this  lever  are  proportional  to  the  diminution  in  volume  of  the 
heart  at  each  systole,  and  tlierefore  serve  as  a  measure  of  the 
output  of  the  ventricles. 

An  attempt  has  been  made  to  determine  the  ov;tput  of  the  ventricles  from 
the  time  taken  up  in  the  total  circulation.  It  was  mentioned  earlier  that  a 
solution  of  methylene  blue  injected  into  the  central  end  of  the  jugular  vein 
could  be  detected  in  the  blood  flowing  from  the  peripheral  end  of  the  same 
jugular,  after  twenty-seven  heart-beats.  This  has  been  interpreted  errone- 
ously as  equivalent  to  saying  that  the  whole  blood  made  the  whole  circuit 
of  the  vascular  system  and  therefore  passed  through  the  heart  twice  in 
twenty-seven  heart-beats.  Thus  we  should  have  only  to  divide  the  quantity 
of  blood  in  man  (5,000  grms.  in  a  man  of  65  kilos.)  by  27,  in  order  to  arrive 
at  the  output  of  each  ventricle  at  each  cardiac  systole,  i.e.  about  185  grms. 
It  is  evident  however  that  the  figure  obtained  by  the  methylene-blue  method 
merely  represents  the  shortest  possible  time  in  which  any  given  particle  of 
blood,  taking  all  the  short  cuts  which  may  be  open,  can  travel  round  the  whole 
circulation,  so  that  the  true  output  of  the  left  ventricle  in  man  must  be  con- 
siderably less  than  185  grms.,  and  is  probably  not  more  than  one-third  of  this 
amount. 

From  a  direct  determination  by  the  cardiometer  method  of  the  output  in 
animals,  Tigerstedt  concludes  that  the  output  in  man  at  each  beat  is  probably 
between  50  and  100  ccms.  By  other  methods,  Zuntz  has  come  to  the  conclusion 
that  60  ccms.  represents  the  average  output  in  man. 

Accepting  this  last  figure  as  correct,  we  have  now  all  the  data  for  the  cal- 
culation of  the  work  of  the  heart. 

Qll  =  60  X  0-150  m.  x  13-6  '  =  12'2-4  grammetres. 

Taking  the  velocity  imparted  to  the  column  of  blood  in  the  aorta  as 
I  metre — 

w    v2  _  60  X  0-5- 
2"^?  "    2T9-8'  "  ^'^  gi-ammetres. 

It  is  evident  that  this  latter  factor  is  negligible,  and  that  for  all  practical 
purposes  we  may  regard  the  work  of  the  heart  as  proportional  to  the  product  of 
the  output  and  the  average  arterial  blood-pressure. 

Taking  the  average  pressure  in  the  pulmonaiy  artery  at  25  mm.  Hg,  the 


'  The  specific  gravity  of  blood  is  taken  here  as  approximately  equal  to  that 
of  water.  If  it  be  desired  to  allow  for  this,  it  would  be  necessary  to  divide  the 
product  by  1-07. 


256  PHYSIOLOGY 

work  of  the  j-'igld  ventricle  at  each  beat  would  amount  to  ahout  20  f,'ranmietres, 
a  total  for  the  two  ventricles  of  140  grammetres  per  beat,  which  is  equivalent  to 
about  14,000  kilogrammctres  in  the  twenty-four  hours. 

We  are  now  in  a  position  to  discuss  the  effect  of  various 
conditions  on  the  total  work  of  the  heart.  We  have  seen 
that  this  varies  as  w  x  E,  so  it  is  evident  that  any  condition 
which  increases  either  or  both  of  these  factors  will  increase 
the  work  done  and  vice  versa.  Thus  on  exciting  the  peri- 
pheral end  of  the  vagus,  the  heart  is  slowed,  the  diastole 
prolonged,  and  the  amount  of  blood  expelled  at  each  systole 
increased.  There  is  however  a  fall  of  mean  arterial  pressure, 
and  the  increased  output  is  not  proportional  to  the  diminished 
frequency  of  the  beat,  so  that  the  total  output  in  any  given 
time  is  less  than  that  occurring  before  or  after  the  stimula- 
tion. E  and  w  being  both  diminished,  the  total  work  done 
by  the  heart  is  lessened. 

In  dealing  however  with  the  effect  of  experimental  or 
pathological  conditions  on  the  work  of  the  heart,  it  is  im- 
portant to  remember  the  wonderful  power  of  '  compensation  ' 
possessed  by  this  organ.  In  consequence  of  the  augmentor 
effect  of  increased  tension  on  the  cardiac  muscle,  we  can 
increase  the  resistance  to  be  overcome  by  the  heart  to  three 
or  four  times  the  normal  amount  without  altering  in  any  way 
the  quantity  of  blood  expelled  at  each  beat.  Thus  we  may 
put  a  ligature  round  the  pulmonary  artery  and  gradually 
tighten  it,  until  the  lumen  of  this  vessel  is  reduced  to  a  third 
of  its  normal  extent,  without  causing  any  material  change 
in  the  blood-pressure ;  although,  if  we  connect  a  manometer 
with  the  cavity  of  the  right  ventricle,  we  find  that  the  endo- 
cardiac  pressure  rises  to  three  or  four  times  the  normal 
amount,  in  order  to  expel  the  proper  quantity  of  blood  into 
the  pulmonary  vessels  and  so  into  the  left  heart.  In  this 
case  the  increased  tension  acts  upon  the  ventricular  muscle 
during  its  contraction.  The  same  result  is  observed  if  we 
augment  the  work  thrown  on  the  ventricles  by  increasing  the 
inflow  of  blood  into  them  during  diastole.  This  increased 
diastolic  volume  of  the  heart  may  be  brought  about  either  by 
pressure  on  the  veins  of  the  abdomen,  so  increasing  the  venous 
flow  into  the  heart,  or  by  the  injection  of  large  quantities  of 
fluid  into  the  circulation  (Fig.  143),  or  by  destroying  the 
aortic  valves  and  allowing  regurgitation  to  take  place  from 


THE   VASCULAK  MECHANISM 


257 


the  aorta  during  diastole.  In  either  case,  the  work  done  by 
the  ventricles  is  increased,  causing  a  rise  of  mean  arterial 
pressure  in  the  first  two  instances,  and  preventing  any  fall 
of  arterial  pressure  in  the  last  case.  A  large  number  of 
similar  experiments  may  be  devised,  but  they  all  teach  the 
same  lesson,  viz.  that,  within  very  wide  limits,  the  output  of 
the  heart  is  independent  of  the  resistance  to  the  output. 


Fig.  143. 


Systole 


Caitliometer  tracing  from  clog's  heart  to  show  effect  of  hicreasing  the 
volume  of  circulating  blood  (hydremic  plethora)  on  the  total  output 
and  the  volume  of  the  heart.  Between  the  parts  A  and  B,  30  c.c.  of 
warm  normal  salt  solution  were  injected  intravenously,  and  between 
B  and  C,  20  c.c.  more.  It  will  be  noticed  that  both  the  systolic  and 
the  diastolic  volume  are  increased,  i.e.  the  heart  is  more  distended 
during  diastole,  and  does  not  contract  to  its  normal  size  in  systole. 
The  contraction  volume,  and  therefore  the  output,  are  very  largely 
increased.     (Roy.) 


In  a  fairly  healthy  individual,  increased  work  thrown 
upon  the  heart  by  injury  or  narrowing  of  the  valvular 
orifices  does  not  necessarily  result  in  fatigue  and  failure  of 
the  heart-muscle,  but  this  tissue  may  react  just  as  skeletal 

17 


258  PHYSIOLOGY 

muscle  does  to  increased  work  by  hypertrophy.  Where  this 
hypertrophy  is  sufficiently  developed,  we  say  that  the  heart- 
lesion  is  fully  compensated,  and  such  cases  may  continue  for 
years,  without  the  subject  being  aware  that  there  is  anything 
the  matter  with  his  heart.  It  is  not  easy  to  explain  the 
causation  of  this  hypertrophy,  though  it  is  possible  that  one 
factor  in  the  increased  growth  of  the  tissue-cells  may  be 
the  increased  lymph-flow,  which  is  the  result  of  activity  in 
muscle.  The  katabolic  changes,  which  accompany  activity  of 
muscle,  cause  a  marked  rise  of  osmotic  pressure  within  the 
muscle-fibres,  which  therefore  swell  up  in  consequence  of  im- 
bibition of  fluid  from  the  surrounding  tissue- spaces.  Activity 
thus  induces  increased  supply  of  nutritive  material  to  the 
constituent  elements  of  the  active  tissues. 


THE   VASCULAR   MECHANISM  259 

Section  7 
INNEEVATION    OF   THE   BLOOD-VESSELS 

We  have  already  mentioned  that  the  chief  resistance  to 
the  flow  of  blood  occurs  in  the  arterioles  and  capillaries. 
The  greater  part  of  this  resistance  is  in  the  arterioles,  and 
is  dependent  on  the  continued  contraction  or  tone  of  their 
muscular  walls.  If  the  spinal  cord  of  a  dog  be  divided  just 
below  the  medulla,  artificial  respiration  being  kept  up,  the 
blood-pressure  in  the  carotid  artery  falls  from  120  to  40  mm. 
of  mercury.  This  fall  is  not  due  to  any  action  on  the  heart, 
which  goes  on  beating  well.  It  is  due  to  a  relaxation  of  all 
the  arterioles,  and  also  of  the  portal  and  perhaps  other  veins. 
This  relaxation  causes  a  lowering  of  arterial  pressure  in  two 
ways.  In  the  first  place,  the  peripheral  resistance  is  largely 
diminished,  and  in  the  second  place  the  total  capacity  of 
the  vascular  system  is  increased.  In  consequence  of  this 
increase  in  capacity,  we  find  that  section  of  the  cord  lowers 
the  pressure,  not  only  in  the  arteries  but  also  in  all  the  veins 
of  the  body. 

This  experiment  shows  that  the  tone  of  the  vessels  is 
dependent  on  the  integrity  of  their  connections  with  some 
part  of  the  nervous  system.  If  a  section  be  made  just  above 
the  medulla,  the  blood-pressure  remains  high.  If  however 
a  certain  part  of  the  medulla  be  destroyed,  the  blood-pres- 
sure sinks  as  low  as  if  the  cervical  cord  were  divided. 
Stimulation  of  the  same  part  causes  a  great  rise  of  blood- 
pressure,  due  to  increase  in  peripheral  resistance.  We  learn 
then  that  the  continued  contraction  or  tone  of  the  small 
arteries  is  provided  for  by  a  restricted  region  of  the  medulla, 
which  we  call  the  vaso-motor  centre.  (The  lower  border  of 
this  centre  is  about  4  mm.  above  the  apex  of  the  calamus 
scriptorius,  and  its  upper  border  about  4  mm.  higher.  It 
apparently  coincides  in  position  with  the  antero-lateral  nucleus 
of  Clarke.) 

In  this  section  we  have  to  consider  the  means  by  which 
the  vaso-motor  centre  is  able  to  control  the  calibre  of  the 
blood-vessels,  and  therefore  the  blood-supply  to  various  parts 
of  the  body. 


260  PHYSIOLOGY 

In  order  to  study  the  influence  of  the  nervous  system  on 
the  distribution  of  blood  in  various  parts  of  the  body,  it  is 
necessary  to  determine  not  only  the  local  condition  of  the 
circulation,  but  also  the  general  blood-pressure ;  since  a 
diminished  flow  of  blood  through  any  part  might  be  pro- 
duced either  by  a  local  constriction  of  the  blood-vessels,  or 
by  a  fall  of  general  blood -pressure.  It  is  essential  to  be 
certain  that  any  local  change  which  is  observed  in  the  circu- 
lation is  of  local  production  and  not  a  secondary  result  of 
vascular  constriction  or  dilatation  in  some  other  large  area 
of  the  body.  For  the  registration  of  the  mean  arterial  blood- 
pressure,  the  mercurial  manometer  is  most  advantageously 
employed. 

In  order  to  observe  local  changes  in  the  circulation,  several 
methods  may  be  adopted. 

1.  The  simplest  method  is  that  of  ocular  inspection.  In 
many  cases  it  is  easy  to  tell  by  the  colour  of  the  organ  or 
part  whether  its  blood-vessels  are  constricted  or  dilated.  If 
for  instance  the  abdomen  be  opened  and  the  splanchnic  nerves 
stimulated,  the  intestines  will  be  at  once  observed  to  become 
pale  and  anfemic,  owing  to  this  constriction  of  their  vessels. 

2.  Since  the  temperature  of  the  peripheral  parts  of  the 
body  is  considerably  lower  than  that  of  the  blood  flowing 
into  them,  it  follows  that  their  temperature  must  be  in  some 
degree  proportional  to  the  amount  of  warm  blood  which 
reaches  them,  and  we  can  use  the  surface  temperature  of  a 
limb  as  an  index  to  the  changes  in  the  circulation  through 
the  limb.  Thus  if  thermometers  be  tied  between  the  toes  of 
the  two  hind  paws  of  an  animal  and  the  right  sciatic  nerve 
be  divided,  the  thermometer  on  the  right  side  will  show  a 
higher  temperature  than  on  the  left,  owing  to  the  vaso-motor 
paralysis  produced  in  the  right  leg  by  the  section  of  the 
nerve. 

3.  Changes  in  the  calibre  of  the  blood-vessels  of  any  part 
will  alter  the  rapidity  of  the  blood-flow  through  that  part, 
provided  that  there  is  no  concomitant  opposing  change  in  the 
general  blood-pressure.  These  changes  in  velocity  of  the 
blood  might  of  course  be  investigated  on  the  arterial  side  by 
one  of  the  methods  already  described.  It  is  more  convenient 
however  in  most  cases  to  carry  out  the  experiment  on  the 
venous  outflow,  the  blood  being  prevented  from  coagulating 


THE    VASCULAR   MECHANISM 


261 


in  the  course  of  the  experiment  by  some  artificial  means,  such 
as  the  injection  of  leech  extract,  or  the  defibrination^  of  the 
circulating  blood. 

A  convenient  way  of  measuring  the  venous  outflow  is  to 
let  the  blood  drop  on  a  mica  disc  attached  to  a  Marey's 
tambour,  from  which  a  tube  is  carried  to  a  registering 
tambour.  Every  drop  is  recorded  on  a  moving  surface  by  a 
little  elevation  of  the  lever  of  the  registering  tambour. 

4.  The  volume  of  an  organ  is  largely  dependent  on  the 
amount  of  blood  contained  within  its  vessels.  "VVe  can  there- 
fore judge  of  the  nature  of  changes  in  the  circulation  through 
an  organ  by  taking  a  graphic  record  of  its  volume.  For  this 
purpose  we  use  an  instrument  known  as  a  plethysmograph  or 
oncometer,  of  which  many  forms  have  been  devised. 

Fig.  144. 


Diagram  of  oncometer. 


Fig.  144  represents  diagrammaticaily  the  structure  of 
Roy's  kidney  oncometer.  This  is  a  metal  capsule,  the  two 
halves  of  which  are  jointed  together,  and  are  accurately  fitted 
to  one  another  except  at  (h),  where  a  small  hole  is  left  for 
the  exit  of  the  kidney  vessels  and  ureters.  A  delicate  animal 
membrane  (m)  is  attached  to  the  rim  of  each  half  of  the 
oncometer,  the  space  between  this  and  the  brass  capsule  being 
filled  with  warm  oil.     The  kidney  (k)  rests  inside,  supported 

'  In  dogs  and  cats  the  blood  may  be  rendered  uncoagulable  by  drawing  off 
half  the  blood,  defibrinating  it,  and  reinjecting  it  into  the  circulation.  This 
process  is  repeated  five  or  six  times,  when  it  is  found  that  the  blood  is  no  longer 
coagulable — a  condition  which  lasts  for  many  houi's. 


262 


PHYSIOLOGY 


on  the  bed  of  warm  oil,  from  which  it  is  separated  by  the 
membrane.  The  tube  (o)  leads  from  the  cavity  between  the 
brass  capsule  and  membrane  to  the  registering  apparatus 
or   oncograph,    represented   in   Fig.    145.      Any   swelling   of 


g: 


i^ 


Fig.  145. 


^W^ 


Diagram  of  oncograph. 


to  oncometer 


the  kidney  will  drive  oil  out  of  (o)  into  the  oncograph,  and 
will  thus  raise  the  piston  of  the  latter.  The  excursions  of 
the  piston  are  recorded  by  the  lever  (1),   wliich  is  arranged 


Fifi.  146. 


-dZTUaa 


Diagram  to  show  structure  of  a  piston  recorder  (Hiirthle's  pattern). 
This  instrument  consists  of  an  accurately  turned  vulcanite  piston, 
moving  in  a  glass  cylinder.  To  the  piston  is  attached  a  light 
counter-weighted  lever.  The  piston  moves  very  easily,  has  very 
little  tendency  to  swing  on  its  own  account,  and  gives  excursions, 
which  are  directly  proportional  to  the  changes  of  volume  of  air  or 
fluid  in  the  attached  oncometer. 


to  write  on  a  })lackened  surface.  Plethysmographs  for  the 
limbs  and  other  organs  have  been  constructed  on  a  similar 
principle. 

A  very  simple  form  of  air-plethysmograph  has  been  devised  by  Schafer.  A 
box  is  made  of  vulcanite  adapted  to  the  size  of  the  organ  whose  volume  is  the 
object  of  investigation.  In  one  side  of  the  box  there  is  a  depression  sufficient  to 
accommodate  easily  the  vessels  and  nerves  going  to  the  organ.  The  oncometer 
is  covered  by  a  glass  lid,  the  connections  being  made  air-tight  by  means  of 
vaseline.  A  glass  tube  is  fixed  in  one  corner  of  the  box.  This  is  connected  by 
a  rubber  tube  with  a  piston  recorder  (Fig.  146)  or  a  tambour.  Every  variation 
in   the  volume  of   the   organ  causes   a   movement  of   air  into   or  out  of  the 


THE   VASCULAR  MECHANISM 


263 


oncometer,  and  thus  gives  rise  to  a  corresiJonding  movement  of  the  lever  of  the 
piston-recorder. 

A  plethysmograph  must  always  be  used  in  connection  with 
a  blood-pressure  manometer  if  we  wish  to  investigate  the 
active  condition  of  the  vessels  of  the  organ  under  considera- 
tion.    Fig.  147  will  serve  as  an  example  to  show  the  mode 

Fig.  147. 


Blood- 

""^'^'"'^^^'V.-V.-v^ 

pressure 

'       ■                           . 

Kidney- 

volume 

A''              \  X — -f - 

.  .     ,....,.,        .  ■    \..  .  .  ..■  ..  .  .  A  .  .  .  1 1,.  ,,-L 

y 

Simultaneous  tracings  of  carotid  blood-pressure  and  volume  of  kidney. 
Between  x  and  x  the  peripheral  end  of  the  divided  10th  nerve  was 
stimulated.     Time-marking  =  seconds.     (Bi-adford.) 

in  which  these  two  forms  of  instruments  are  employed  for 
the  investigation  of  vascular  conditions.  The  upper  curve 
is  the  carotid  blood-pressure,  recorded  by  means  of  a  mer- 
curial manometer.  The  lower  is  the  tracing  recorded  by  the 
lever  of  an  oncograph  in  connection  with  an  oncometer,  in 
which  the  kidney  of  the  animal  is  placed.  At  the  point 
marked  with  a  cross  on  the  tracing,  the  peripheral  end  of 
the  anterior  root  of  the  tenth  dorsal  nerve  was  stimulated. 
It  will  be  seen  that  this  stimulation  is  followed  by  a  slight 
rise  of  general  blood-pressure,  but  a  marked  shrinking  of 
the  kidney-volume.  The  rise  of  blood-pressure  shows  us 
that  there  must  be  somewhere  a  constriction  of  arterioles 
giving  rise  to  increased  peripheral  resistance,  since  the  heart- 
beat is  obviously  unaffected.  An  increased  blood -pressure 
would  by  itself  tend  to  force  more  blood  into  the  kidney,  and 
so  would  cause  an  expansion  of  the  kidney. 


264  PHYSIOLOGY 

It  is  evident  that  there  must  be  an  active  contraction  of 
the  arterioles  of  the  kidney,  emptying  this  organ  of  blood, 
and  so  causing  it  to  diminish  in  size.  If  we  had  used  the 
oncometer  alone,  we  should  have  been  in  doubt  whether  the 
shrinking  might  not  be  due  to  failure  of  the  heart's  activity. 
Again,  without  the  oncometer  we  should  only  have  known 
that  there  was  increased  peripheral  resistance  in  the  blood- 
vessels in  some  part  of  the  body,  but  we  should  not  have  been 
able  to  localise  it. 

This  experiment  has  taught  us  already  that  stimulation  of 
certain  nerves  causes  constriction  of  the  arterioles  in  definite 
parts  of  the  body.  This  influence  of  nerves  on  the  calibre 
of  the  arterioles  is  still  better  shown  in  the  ear  of  the  rabbit. 
If  this  be  held  up  to  the  light,  the  arteries  and  veins  can  be 
plainly  seen.  If  now  the  sympathetic  in  the  neck  be  divided, 
the  ear  on  the  same  side  will  instantly  become  redder  and 
warmer  than  the  other,  and  on  holding  it  up  to  the  light, 
all  the  vessels  will  be  observed  to  be  much  dilated,  and  many 
small  vessels  will  be  evident  that  could  not  previously  be 
seen.  On  stimulating  the  upper  end  of  the  cut  sympathetic 
the  reverse  effect  is  produced  ;  the  vessels  contract,  and  the 
ear  becomes  once  more  cool  and  pale.  In  the  same  way 
constriction  of  the  vessels  in  the  web  of  the  frog's  foot  may 
be  observed  under  the  microscope  to  follow  stimulation  of 
the  sciatic  nerve.  Similar  experiments  to  these  have  shown 
that  the  muscular  walls  of  all  the  arteries  in  the  body  are 
under  the  control  of  the  central  nervous  system,  and  that 
they  are  held  in  a  condition  of  continued  contraction  or 
tone  under  the  influence  of  the  vaso-motor  centre.  Division 
of  the  spinal  cord  in  the  neck  cuts  off  the  arteries  in  the 
trunk  and  limbs  from  the  vaso-motor  centre,  and  these  in 
consequence  become  dilated,  and  the  blood-pressure  falls. 
Division  of  the  cord  in  the  dorsal  region  similarly  causes 
dilatation  of  the  vessels  in  the  lower  limbs.  If  the  animal 
be  kept  alive  •  after  this  operation  for  some  time,  the  vessels 
recover  their  tone,  but  lose  it  again  if  the  spinal  cord  be 
destroyed.  We  see  then  that,  although  the  chief  vaso- 
motor centre  lies  in  the  medulla,  there  are  also  subsidiary 
centres  in  the  cord,  which  are  able,  after  a  time,  to  take  up 
by  themselves  the  work  of  regulating  the  condition  of  the 
blood-vessels   in   parts    of   the   body  suijplied  by  the  spinal 


THE   VASCULAR  MECHANISM  265 

nerves.  The  nerves  that  convey  impulses  causing  constric- 
tion of  the  arteries  are  called  vaso-constrictors  or  vaso- 
motors. 

Course  of  the  Vaso-constrictor  Nerves 

In  investigating  the  course  of  the  vaso-constrictor  fibres 
we  have  to  determine 

1.  The  origin  of  the  fibres  from  the  central  nervous 
system ; 

2.  The  course  of  the  fibres  on  their  way  to  their  peripheral 
distribution  on  the  blood-vessels  ; 

3.  Their  connections  with  nerve-cells. 

The  first  two  details  can  be  found  by  stimulating  various 
nerves  and  nerve-roots  in  different  parts  of  their  course  and 
observing  the  effects  produced  on  the  local  and  general 
circulation.  The  importance  of  the  third  heading  is  due  to 
the  fact  that  the  vascular  nerves,  like  the  visceral  nerves 
generally,  do  not  have  their  last  cell-station  in  the  spinal 
cord.  The  fibres  carrying  vaso-constrictor  impulses,  which 
leave  the  cord,  do  not  pass  direct  to  the  blood-vessels,  but 
come  to  an  end  in  a  peripheral  collection  of  ganglion  cells, 
which  may  belong  to  the  main  chain  of  the  sympathetic  or  be 
situated  more  distally  and  belong  to  the  group  of  collateral  or 
peripheral  ganglia. 

These  fibres,  as  they  leave  the  central  nervous  system,  are 
small  medullated  nerves.  These  come  to  an  end  in  a  gan- 
glion, where  a  fresh  relay  of  fibres  starts  and  carries  the 
impulse  on  to  the  muscle-cells  of  the  vessels.  These  post- 
ganglionic fibres  difter  from  the  pre-ganglionic  fibres  in  being 
non-medullated. 

The  discovery  of  the  exact  ganglia,  with  which  any  given 
set  of  nerve-fibres  is  connected,  is  rendered  easy  by  the  fact 
that  in  many  animals  the  sympathetic  ganglion-cells  are  para- 
lysed by  nicotin  (Langley).  The  nicotin  may  be  painted  on 
the  ganglion  or  may  be  injected  into  the  blood-stream.  The 
first  efi'ect  of  the  drug  is  a  powerful  stimulation  of  the  gan- 
glion-cell, so  that,  if  the  drug  be  injected,  there  is  an  enormous 
rise  of  blood-pressure  owing  to  the  universal  vaso -constriction 
that  is  produced.  This  stimulation  gives  place  to  a  condition 
of  paralysis ;  the  blood-pressure  falls  below  normal  owing  to 
the  cutting  off  of  the  peripheral  vascular  nerves  from  the 


266  PHYSIOLOGY 

vaso-motor  centre.  Stimulation  of  the  pre-ganglionic  fibre  is 
now  without  effect,  although  the  normal  results  follow  stimu- 
lation of  the  post-ganglionic  non-medullated  fibre. 

By  these  methods  it  has  been  determined  that  all  the 
vaso-constrictor  nerves  of  the  body  leave  the  spinal  cord  by 
the  anterior  roots  of  the  spinal  nerves  from  the  first  dorsal 
to  the  third  or  fourth  lumbar  inclusive.  From  the  roots 
they  pass  by  the  white  rami  communicantes  to  the  ganglia  of 
the  sympathetic  chain  lying  along  the  front  of  the  vertebral 
column.  Here  they  take  different  courses  according  to  their 
destination. 

The  fibres  to  the  head  and  neck  leave  by  the  first  four 
thoracic  nerves,  pass  into  the  sympathetic  chain  through  the 
ganglion  stellatum  and  ansa  Vieussenii  to  the  inferior  cervical 
ganglion,  and  up  the  cervical  sympathetic  trunk  to  the 
superior  cervical  ganglion.  Here  they  end,  and  the  impulses 
are  carried  by  a  fresh  relay  of  fibres,  which  start  from  cells 
in  this  ganglion  and  travel  as  non-medullated  fibres  on  the 
walls  of  the  carotid  artery  and  its  branches. 

The  constrictors  to  the  fore  limb  in  the  dog  leave  the 
cord  by  the  white  rami  of  the  fourth  to  the  tenth  thoracic 
nerves.  The  fibres  run  up  the  sympathetic  chain  to  the 
stellate  ganglion,  where  they  all  end  in  synapses  round  the 
cells  of  this  ganglion.  The  impulses  are  carried  on  by  non- 
medullated  fibres  along  the  grey  rami  of  the  sympathetic 
to  the  cervical  nerves  which  make  up  the  brachial  plexus, 
and  run  down  in  the  branches  of  this  plexus  to  be  distributed 
to  the  vessels  of  the  fore  limb. 

The  constrictor  impulses  to  the  hind-limb  in  the  dog  arise 
from  the  nerve-roots  between  the  eleventh  dorsal  and  third 
lumbar  roots.  All  the  fibres  end  in  connection  with  cells 
in  the  sixth  and  seventh  lumbar  and  first  and  second  sacral 
ganglia  of  the  sympathetic  chain,  whence  the  impulses  are 
carried  by  grey  rami  to  the  nerves  making  up  the  sacral 
plexus. 

The  most  important  vaso-motor  nerve  of  the  body  is  the 
splanchnic  nerve.  This  nerve  receives  most  of  the  fibres 
forming  the  white  rami  from  the  lower  seven  dorsal  and 
upper  two  or  three  lumbar  roots,  the  latter  fibres  often 
taking  a  separate  course  as  the  lesser  splanchnics.  The 
fibres  can  be  seen  to  pass  through  the  sympathetic  chain  of 


THE   VASCULAR  MECHANISM  267 

the  thorax  without  interruption  and  for  the  most  part  have 
their  cell-station  in  the  large  ganglia,  especially  the  semi- 
lunar ganglia,  of  the  solar  plexus,  whence  a  thick  meshwork 
of  non-medullated  fibres  is  distributed  along  all  the  vessels 
of  the  abdominal  viscera.  The  area  of  the  vessels  innervated 
by  this  nerve  is  so  large,  that  section  of  this  nerve  on  each 
side  causes  a  large  fall  in  the  general  blood-pressure.  This 
fall  is  more  marked  in  animals  such  as  the  rabbit  and  other 
herbivora,  in  which  the  alimentary  canal  is  proportionately 
very  much  developed,  and  has  consequently  a  very  large 
blood-supph\ 

Vaso-dilator  Nerves 

Since  the  arteries  are  in  a  constant  condition  of  moderate 
contraction,  a  dilatation  might  be  brought  about  by  a  relaxa- 
tion of  this  tone  by  an  inhibition  of  the  normal  constrictor 
impulses  proceeding  to  the  vessels  from  the  vaso-motor  centre. 
We  find  however  in  many  parts  of  the  body  evidence  of  the 
existence  of  a  nerve-supply  to  blood-vessels  antagonistic  in  its 
function  to  the  vaso-constrictors.  Thus,  if  the  chorda  tymjjani 
nerve  going  to  the  submaxillary  gland  be  cut,  no  change  is 
evident  in  the  blood-vessels  of  the  gland.  But  if  its  peripheral 
end  be  stimulated,  there  is  instantl}'  free  secretion  of  saliva 
from  the  gland,  and  all  the  blood-vessels  are  largely  dilated. 
In  consequence  of  this  dilatation  the  blood  rushes  through 
the  capillaries  so  quickly  that  it  has  no  time  to  lose  much  of 
its  oxygen ;  the  blood  flowing  from  the  vein  is  therefore 
bright  arterial  in  colour,  and  is  increased  to  six  or  eight 
times  the  previous  amount.  If  atropin  be  injected  into  the 
animal,  the  action  of  the  chorda  tympani  on  the  blood- 
vessels is  unaffected,  although  the  secretion  on  stimulation 
is  abolished.  The  chorda  tympani  is  therefore  said  to  con- 
tain vaso-dilator  fibres  for  the  vessels  of  the  submaxillary 
gland.  Other  examples  of  vaso-dilator  (or  dilatator)  nerves 
are  the  small 2^etrosal  nerve  to  the  parotid  gland,  the  lingual 
nerve  to  the  blood-vessels  of  the  tongue,  and  the  nervi 
erigentes  or  pelvic  visceral  nerves  to  those  of  the  penis.  It  has 
been  thought  that  all  the  vessels  in  the  body  have  a  double 
nerve-supply,  vaso-constrictor  and  vaso-dilator.  The  presence 
of  the  latter  variety  in  a  mixed  nerve  is  often  difficult  to 
prove,  since  on  ordinary  faradic  stimulation  the   constrictor 


268 


PHYSIOLOGY 


effect  is  always  more  pronounced.  Moreover  the  dilators 
do  not  seem  to  conduct  any  tonic  influences  to  the  vessels. 
Hence,  after  section  of  a  mixed  nerve,  the  only  effect 
observed  is  that  due  to  the  removal  of  the  tonic  constrictor 
influences,  and  the  vessels  in  the  area  of  distribution  of  the 
nerve  are  therefore  dilated.  Two  methods  however  have  been 
made  use  of  to  demonstrate  the  existence  of  vaso-dilators  in 
a  mixed  nerve-trunk. 

(rt)  If  the  sciatic  nerve  be  cut,  the  vessels  of  the  leg  and 
foot  dilate.  This  paralytic  dilatation  passes  oft'  after  two  or 
three  days,  and  the  vessels  resume  their  normal  calibre.  If 
now  the  peripheral  end  of  the  sciatic  nerve  be  stimulated, 
dilatation  of  the  vessels  is  produced  (Fig.  148  b).     It  seems 

Fig.  148. 


/ 

Nerve  freshly  divided. 
Constriction. 


Nerve  4  days  degenerated. 
Dilatation. 


Pletliysmograpliic  tracing  of  hind  limbs,  showing  effect  of  stimu- 
lating the  sciatic  nerve  on  the  volume  of  the  limb,  a,  immediately 
after  section  of  the  nerve ;  b,  4  days  after  section.  The  nerve 
was  stimulated  between  the  two  vertical  lines.  Curves  to  be  read 
from  right  to  left.     (Bowditch  and  Warren.) 


that  the  degenerative  processes  affect  the  constrictor  fibres 
earlier  than  tlie  dilator  fibres,  so  that  at  a  certain  period  after 
nerve-section  the  latter  alone  respond  to  stimulation. 

{h)  If  a  mixed  nerve  be  stimulated  with  shocks  slowly 
repeated  at  intervals  of  one  second,  instead  of  with  the 
ordinary  faradic  current,  vaso-dilator  effects  are  often  obtained, 
whereas  stimulation  of  the  same  nerve  with  the  faradic 
current  produces  vaso-constriction  (Fig.  149).  Thus  rapid 
stimulation  of  the  anterior  root  of  the  tenth  dorsal  nerve  in 
the  dog  produces  shrinking  of  the  kidney  from  contraction 
of  its  blood-vessels.  If  the  same  nerve  be  rhythmically 
stimulated  with  single  shocks  repeated  at  slow  intervals,  the 
kidney  swells,  showing  that  its  vessels  have  dilated. 

The  course  of  the  typical  dilator  nerves  differs  very  much 


THE   VASCULAR   MECHANISM 


269 


from  that  of  the  constrictors.  Instead  of  leavmg  the  central 
nervous  system  in  a  particular  area,  and  running  through  the 
sympathetic  chain  before  proceeding  to  their  destinations,  it 
seems  that  the  dilators  may  leave  the  brain  or  cord  by  any 
cerebro-spinal  nerve.  Thus  the  chorda  tympani  springs  from 
the  root  of   the   facial  nerve,  the   nervi  erigentes    from   the 

Fig.  149. 


I   per  sec. 

4  per  sec. 

16  per  sec. 
64  per  sec. 


Effect  on  the  volume  of  the  hind  hnibs  of  the  cat  of  stimulating  the 
sciatic  nerve  with  induction  shocks  at  different  rates.  It  will  be 
noticed  that  with  one  shock  per  second,  there  is  hardly  any  con- 
striction but  considerable  dilatation,  whereas  with  64  shocks  per 
second  the  only  effect  produced  is  vaso-constriction.  Curves  to 
be  read  from  right  to  left.     (Bowditch  and  Warren.) 


second  and  third  sacral  nerves.  All  these,  however,  are  probably 
interrupted  in  a  ganglion  or  collection  of  nerve-cells  before 
reaching  their  destination.  Although  in  some  cases,  as  in  the 
case  of  the  vaso-dilators  to  the  lips  and  gums  of  the  dog,  these 
cell-stations  may  be  situated  in  the  sympathetic  chain  (stellate 
ganglion),  in  the  best  marked  vaso-dilators,  such  as  the  chorda 
and  nervi  erigentes,  there  is  no  connection  at  all    with  the 


270  PHYSIOLOGY 

main  chain  of  the  sympathetic.  The  nerve-cells  of  the  chorda 
tympani  lie  buried  in  the  hilum  of  the  submaxillary  gland, 
while  the  vaso-dilators  of  the  pelvic  nerve  are  connected  with 
cells  in  the  so-called  pelvic  ganglion  and  distributed  over  the 
base  of  the  rectum  and  bladder. 

The  so-called  dilator  nerves  to  the  limb  vessels  seem  to  be  fundamentally 
different  from  the  typical  vaso-dilators  just  mentioned.  It  has  been  shown  by 
Bayliss  that  these  nerves  have  their  trophic  centre  in  the  spinal  root  ganglia,  and 
are  apparently  identical  with  the  afferent  nerves  of  the  limbs.  Mechanical 
or  electrical  stimulation  of  the  peripheral  end  of  a  cut  posterior  root,  taking  part 
in  the  formation  of  a  limb  plexus,  gives  rise  to  well-marked  vascular  dilatation 
of  the  limb  vessels,  especially  those  of  the  skin. 

There  is  a  striking  analogy  between  the  nerves  distributed 
to  the  blood-vessels  and  those  going  to  the  heart — which  is 
indeed  only  a  specialised  part  of  the  general  blood -tubes  of  the 
body.  These  nerves,  according  to  their  action  on  the  meta- 
bolic activity  of  the  tissues  supplied,  are  divided  by  Gaskell 
into  anaholic  and  katahoUc  nerves. 

The  anabolic  nerves,  as  indicated  by  their  name,  cause 
a  building  up  or  regeneration  of  the  contractile  tissue.  They 
therefore  act  as  inhibitory  nerves,  and  bring  about  a  condition 
of  rest  in  the  tissue.  This  class  of  nerves  would  include  the 
vagus  and  the  vaso-dilator  fibres. 

The  katabolic  nerves  cause  an  increased  activity  of  the  con- 
tractile tissue  and,  as  was  shown  in  treating  of  voluntary 
muscle  (p.  132),  active  contraction  is  associated  with  and 
derives  its  energy  from  a  disintegration  or  katabolism  of  the 
complex  and  unstable  muscle  molecule  (inogen).  An  ordinary 
motor  nerve  to  a  muscle  is  therefore  a  katabolic  nerve.  This 
class  would  include  the  accelerator  nerves  to  the  heart  and 
the  vaso-constrictors.  The  course  of  these  two  sets  of  nerves 
bears  out  this  comparison,  the  path  taken  by  the  accelerator 
nerves  being  identical  at  first  with  that  of  the  vaso-constrictor 
fibres  to  the  head  and  neck. 

Beflex  Alterations  of  the  Blood-vessels 

Life  is  reaction ;  every  vital  act  is  a  reaction  of  the 
organism  to  changes  in  its  environment.  Hence  we  have 
not  completed  our  view  of  the  changes  affecting  the  vessels 
until  we  have  not  only  considered  the  means  by  which  the 
nervous  system  acts  on  the  vessels,  but  also  the  means  by 


THE   VASCULAR   MECHANISM 


271 


which  the  centres  are  excited  to  action.  In  fine,  we  must 
complete  the  reflex  arc  affecting  the  vessels  by  considering  the 
afferent  impulses  to  the  vaso-motor  centre. 

The  afferent  impulses  to  this  centre  may  be  divided  into 
pressor  and  depressor  ;  and  these  names  are  also  applied  to 
the  nerves  that  carry  such  impulses. 

There  is  in  the  rabbit,  cat,  and  horse  a  small  nerve  in  the 
neck  that  runs  up  from  the  heart  to  join  the  vagus  or  its 
superior  laryngeal  branch.     If  after  section  of  both  vagi  (to 


Sup-,  lar.  n.  J 


Depressor 


« 


sec. 


Sup-,  lar.  n.-^ 


---S 


ymp. 


, -Vagus 


RABBIT 


--- Sup-.  Cerv.  Gang. 
-Depressor 
Cerv.  symp.  n. 


Vago.  symp. 


DOG 


Diagrams  of  the  connections  of  the  depressor  nerve  in  the  rabbit 
and  dog,  according  to  Cyon.  It  will  be  noticed  that  in  the  latter 
animal  the  depressor  nerve  runs  in  the  vagus  trunk  for  the  greater 
part  of  its  course. 


prevent  reflex  inhibition  of  the  heart)  this  nerve  be  cut  and 
its  central  end  stimulated,  while  the  blood -pressure  is  being 
registered  by  means  of  a  mercurial  manometer  connected  with 
the  carotid  artery,  a  marked  fall  of  blood-pressure  is  at  once 
observed  (Fig.  151).  This  fall  of  pressure  is  hardly  notice- 
able after  section  of  the  splanchnic  nerves,  showing  that  the 
stimulation  of  the  depressor  has  affected  the  vaso-motor  centre, 
inhibiting  the  constrictor  impulses  that  normally  pass  down 
the  splanchnic  nerves. 

The  splanchnic  dilatation  that  is  brought  about  by  excita- 
tion of  the  depressor  nerve  may  be  demonstrated  by  enclosing 


272  PHYSIOLOGY 

any  organ  of  the  abdomen  in  a  plethysmograph.  Fig.  152 
represents  a  curve  of  the  splenic  volume,  and  shows  the  marked 
expansion,  together  with  fall  of  general  blood-pressure  resulting 
from  stimulation  of  the  depressor  nerve. 

All  sensory  nerves  are  pressoj-  nerves,  i.e.  stimulation 
of  their  central  end  causes  a  marked  rise  of  blood -pressure 
in  animals  under  curare  and  morphia.  Thus  a  rise  of  the 
general  blood-pressure  follows  stimulation  of  the  central  end 

Fig.  151. 


Blood-pressure   curve    from    rabbit   showing   effect    of    excitation    of 
central  end  of  depressor  nerve  (mercurial  manometer).     (Bayliss.) 


of  the  cut  sciatic  or  superior  laryngeal  nerves  (Fig.  153). 
This  rise  of  pressure  is  due  to  constriction  of  the  arterioles, 
especially  in  the  splanchnic  area.  The  effect  however  of 
excitation  of  a  purely  sensory  nerve  is  not  quite  so  simple  as  at 
first  appears.  In  many  cases  stimulation  of  the  central  end 
of  a  sensory  nerve  causes  general  arterial  constriction  with  a 
rise  of  blood-pressure,  and  at  the  same  time  a  vaso-dilatation 
in  the  area  of  distribution  of  the  nerve.  This  can  be  demon- 
strated by  exciting  the  central  ends  of  the  posterior  roots  of 
the  nerves  to  a  limb,  which  causes  a  swelling  of  the  limb  due 


THE   VASCULAR   MECHANISM 


273 


to  dilatation  of  its  vessels,  accompanied  by  rise  of  general 
blood -pressure  owing  to  constriction  of  vessels  in  the  splanch- 
nic area  and  elsewhere.     The  physiological  purpose  of   this 


Fig.  152. 


B.P, 


Spleen 


Simultaneous  tracing  of  arterial  blood-pressure  and  splenic  volume 
fi'om  a  rabbit,  showing  the  marked  swelling  of  the  spleen  asso- 
ciated with  fall  of  general  blood-pressure  on  stimulation  of  the 
central  end  of  the  depressor  nerve.  The  nerve  was  excited 
between  a  and  b.     (Bayliss.) 


arrangement  is  obvious.  Thus  when  a  limb  is  injured  and 
inflamed,  and  a  good  supply  of  blood  is  required  for  reparative 
processes,  the  stimulation  of  the  sensory  nerves  in  the  injured 

Fig.  153. 


Blood-pressure  curve  from  carotid  of  dog.  Between  the  arrows  the 
central  end  of  a  sensory  nerve  was  stimulated  (Hiirthle's  mano- 
meter). 


area  calls  forth  reflexly  a  dilatation  of  the  blood-vessels  in  this 
area.  This  dilatation  alone  allows  an  increased  flow  of  blood 
through  the  part ;  but  this  flow  is  still  further  increased  by 

]8 


274  PHYSIOLOGY 

the  rise  of  blood-pressure  which  is  caused  by  the  general 
arterial  constriction  also  induced  reflexly  by  the  stimulation 
of  the  same  nerve. 


Factors  influencing  the  Vaso-motor  Centre  directly. 
Asphyxia 

In  addition  to  its  power  of  response  to  the  effects  of  peri- 
pheral stimuli,  the  vaso-motor  centre  in  the  medulla  may  also 
react  to  changes  occurring  in  the  blood  with  which  it  is  supplied. 
Thus  administration  of  digitalis  or  strophanti!  us  to  an  animal 
causes  a  marked  rise  of  general  blood-pressure  due  to  the 
constriction  of  the  peripheral  vessels  and  brought  about  by 
impulses  from  the  centre. 

The  changes  occurring  in  the  blood-pressure  in  asphyxia 
are  important,  and  depend  partly  on  the  abnormal  stimulation 
of  the  vaso-motor  and  vagus  centres  by  the  venous  blood,  and 
partly  on  the  affection  of  the  heart  itself.     These  phenomena 
are  best  observed  in  a  curarised    animal,  and  we  will    first 
consider  them  with  both  vagi  cut,  in  order  to  shut  out  the 
action  of  the  vagus  centre.     The  blood-pressure  is  registered 
by  means  of  a  mercurial  manometer  in  connection  with  the 
carotid  artery.     On  leaving  off  the  artificial  respiration,  the 
blood -pressure  remains    at   the    same   height    for  twenty  or 
thirty  seconds,  the  only  change  noticed  being  the  absence  of 
the  respiratory  oscillations.     At  this  point  the  blood-pressure 
suddenly  rises  rapidly,  and  in  another  ten  seconds  may  reach 
a  height  twice  as  great  as  it  was  previously.     The  heart  beats 
a  little  more  forcibly  in  consequence  of  the  increased  cardiac 
tension,  but  its  frequency  is  almost  unaltered.     The  blood- 
pressure  remains  at  this  height  for  about  a  minute,  and  then 
gradually  falls,  the  heart-beats  becoming  smaller  and  smaller, 
until  the  pressure  has  sunk  to  a  point  very  little  above  the 
abscissa  line  (level  of  no  pressure).     This  fall  in  pressure  is 
due  to  the  failure  of  the  heart.     The  heart,  badly  supplied 
with  oxygen,  cannot  overcome  the  enormous  resistance  pre- 
sented Iby  the  contracted  arterioles ;  it  gets  over-filled,  and 
gradually  loses  the  power  of  expelling  any  of  its  contents.     If, 
when  the  blood-pressure  has  sunk   to   its  lowest  point,  the 
heart  be  rapidly  cut  out  of  the  body,  it  will  at  once  begin  to 
beat  fairly  forcibly,  being  relieved  of  the  excessive  internal 


THE   VASCULAR  MECHANISM  275 

tension.  The  vessels  however  remain  constricted  until  the 
death  of  the  animal.  This  is  shown  by  two  facts.  If,  while  the 
pressure  is  sinking,  artificial  respiration  be  recommenced,  the 
heart  supplied  with  oxygen  at  once  begins  to  beat  more  forcibly, 
and  the  blood-pressure  may  rise  to  an  even  greater  height 
than  immediately  after  the  commencement  of  the  asphyxia. 
Again,  if  the  volume  of  the  kidney  be  recorded  by  means  of 
the  oncometer,  the  rise  of  general  blood-pressure  produced  by 
asphyxia  is  seen  to  be  accompanied  by  a  marked  shrinking  of 
the  kidney,  and  this  shrinking  endures  until  the  animal  dies. 

Fig.  154. 


Vwv^V 


Curve  of  blood-pressure  tracing  during  asphyxia.  The  tracing  was 
taken  by  a  manometer  connected  with  the  femoral  artery  of  a 
dog  under  curare.  Artificial  respiration  discontinued  at  X.  Both 
vagi  had  been  previously  divided. 

showing  that  the  fall  of  blood-pressure  following  the  rise  is 
due  not  to  a  giving  way  of  the  arterial  resistance,  but  solely 
to  a  failure  of  the  heart. ^ 

If  in  the  dog,  and  to  a  less  extent  in  other  animals,  the 
vagi  be  left  intact,  the  blood-pressure  tracing  during  asphyxia 
has  quite  another  appearance.  At  the  point  of  the  tracing 
corresponding  to  the  rapid  rise  in  the  previous  experiment, 
there  is  in  this  case  only  a  slight  rise  of  pressure,  but  the 
heart  begins  to  beat  very  slowly.     At  each  beat  it  necessarily 

^  There  are  no  grounds  for  the  statement  sometimes  made  that  constric- 
tion of  the  lung  arterioles  plays  any  part  in  this  fall  of  blood-pressure.  The 
increased  distension  of  the  right  side  of  the  heart  after  asphyxia  by  ligature 
of  the  trachea  is  due  to  the  thinner  walls  and  greater  distensibility  of  this 
side  of  the  heart,  and  to  the  fact  that  the  forced  respiratory  movements  tend 
to  fill  the  lung-vessels  and  the  great  veins,  and  so  affect  chiefly  the  right 
heart. 


276 


PHYSIOLOGY 


sends  out  a  greater  volume  of  blood  than  when  it  is  beating 
more  frequently,  and  hence  the  oscillations  on  the  blood-pres- 
sure curve  caused  by  the  heart-beats  become  very  large.  This 
slow  beat  is  due  to  the  action  of  the  vagus  centre  which  is  ex- 
cited by  the  venous  blood,  and  it  is  at  once  abolished  by  section 
of  the  two  vagi.  The  sparing  of  the  heart  by  means  of  this 
vagus  action  enables  it  to  last  longer,  and  the  final  fatal  fall 
of  blood-pressure  due  to  heart  failure  comes  on  rather  later 
than  when  the  vagi  are  divided. 

During  the  period  of  increased  pressure,  waves  are  often 
observed  on  the  blood -pressure  curve,  which  must  arise  in  a 

Fig.  155. 


Blood-pressure  tracings  showing  Traube-Hering  curves.     (C.  J.  Martin.) 


slow  rhythmic  variation  of  the  constrictor  impulses  sent  out 
from  the  vaso-motor  centre.  These  waves  are  known  as  the 
Traube-Hering  curves,  and  are  not  to  be  confused  with  the 
waves  on  an  ordinary  pressure-curve  due  to  respiration,  being 
much  slower  in  their  rhythm  than  the  latter.  They  are 
observed  not  only  during  asphyxia,  but  may  occur  in  blood- 
pressure  tracings  from  normal  dogs,  and  are  frequent  in  dogs 
poisoned  with  morphia.  Fig.  155  represents  tracings  obtained 
from  a  dog  under  the  influence  of  morphia  and  curare.  The 
upper  curve,  taken  while  artificial  respiration  was  being  carried 


THE   VASCULAR  MECHANISM  277 

on,  shows  the  three  forms  of  curves — the  oscillations  due 
to  the  heart-beat,  next  in  size  those  due  to  the  respiratory 
movements,  which  in  their  turn  are  superposed  on  the  slow 
prolonged  curves,  i.e.  the  Traube-Hering  curves.  The  lower 
curve  is  taken  immediately  after  cessation  of  the  artificial 
respiration,  and  shows  only  the  heart-beats  and  the  Traube- 
Hering  curves. 

The  presence  of  Traube-Hering  curves  may  generally  be  ascribed  to  a  state 
of  abnormal  excitation  of  the  vaso-motor  centre.  This  excitation  may  arise  in 
various  ways.  A  very  frequent  cause  is  the  one  just  described,  viz.  increased 
venosity  of  the  blood  supplied  to  the  centre.  Well-marked  Traube  curves  are 
often  observed  in  cases  of  hfemorrhage.  In  spite  of  the  loss  of  blood,  the  vaso- 
motor centre  at  first  maintains  a  normal  arterial  blood-pressure  by  means  of 
vascular  constriction.  As  the  bleeding  continues,  this  means  becomes  inade- 
quate, and  at  this  point  the  efforts  of  the  centre  take  on  a  rhytlimic  character, 
giving  well-marked  Traube  curves,  just  as  the  arm  of  a  man  holding  up  a  weight 
begins  to  shake  before  he  is  obliged  to  give  way  through  fatigue.  If  the  bleeding 
still  continues,  the  pressure  sinks  steadily  and  the  curves  disappear.  The 
curves  may  also  be  often  observed  during  operations  involving  exposure  of  the 
cord,  and  may  possibly  be  ascribed  in  this  case  to  abnormal  irritations  ascending 
the  posterior  columns. 

Spinal  Vaso-motor  Centres 

As  already  mentioned,  the  spinal  cord  contains  a  series  of 
subsidiary  vaso-motor  centres  presiding  over  the  local  vascular 
reactions  of  the  different  segments  of  the  body.  If  an  animal 
be  kept  alive  by  means  of  artificial  respiration  for  some  hours 
after  division  of  the  cord  just  below  the  medulla,  these  centres 
gradually  resume  their  tonic  influence,  and  the  blood -pressure 
slowly  rises.  If,  after  two  or  three  hours,  artificial  respiration  be 
discontinued,  the  asphyxia  excites  the  centres  of  the  cord,  just  as 
it  does  those  of  the  bulb  in  the  normal  animal.  The  motor  dis- 
charge reveals  itself  however  in  a  single  prolonged  spasm,  not 
in  a  series  of  convulsions  as  is  the  case  when  the  connections 
of  the  bulb  are  intact.  At  the  same  time  a  universal  constric- 
tion of  the  blood-vessels  occurs,  which  outlasts  the  spasm  of 
the  skeletal  muscles  and  causes  a  very  considerable  rise  of 
blood-pressure  {v.  Fig.  155a). 

Peripheral  Vascular  Beactions 

Even  after  destruction  of  all  connection  with  the  central 
nervous  system,  the  blood-vessels  still  possess  considerable 
powers  of  maintaining   a   tone,  and  adapting  themselves  to 


278 


PHYSIOLOGY 


changed  conditions.  Like  all  muscular  tissues,  the  arterial 
wall  is  largely  influenced  by  tension — increased  tension  acting 
as  an  excitant  to  increased  contraction.  Hence  increased 
blood-pressure  will  cause  contraction  of  the  arterial  wall,  while 


50-= 


lOSECS. 


Blood  pressure  tracing  taken  by  a  mercurial  manometer  from  carotid 
artery  of  a  dog,  three  hours  after  section  of  the  cord,  just 
below  the  medulla  oblongata.  At  o  the  artificial  respiration 
was  discontinued.  A  general  spasm  of  the  skeletal  muscles 
occurred  between  x  and  x  .  The  muscles  then  relaxed,  and 
were  flaccid  during  the  rest  of  the  rise  of  blood-pressure. 

diminished  blood-pressure  will  cause  relaxation — a  state  of 
things  eminently  adapted  to  the  maintenance  of  a  continuous 
flow  of  blood  through  a  part,  whatever  may  be  the  alterations 
of  general  blood-pressure  conditioned  by  changes  occurring  in 
other  parts  of  the  body. 

Influence  of  Gravity  iipon  the  Circulation. 

In  dealing  with  the  mechanics  of  the  circulation  we  spoke 
of  a  mean  systemic  or  general  blood-pressure.  By  this  term 
we  imply  that  the  vascular  system  under  normal  circum- 
stances is  always  slightly  distended,  even  if  the  blood  is  at 
rest.  The  effect  of  the  heart-beat  is,  by  pumping  blood  from 
the  venous  to  the  arterial  side,  to  depress  the  pressure  in  the 
veins  below  the  mean  pressure,  and  to  raise  that  in  the  arteries 
above  the  mean  pressure.  On  account  of  the  much  greater 
distensibility  of  the  veins,  the  fall  of  pressure  on  the  venous 
side  is  not  nearly  as  marked  as  the  rise  of  pressure  on  the 
arterial  side.  This  mean  systemic  pressure  probably  amounts 
in  the  dog  to  about  10  mm.  Hg.     It  is  difficult  to  measure 


THE   VASCULAR   MECHANISM  279 

it  with  accuracy,  owing  to  the  fact  that  any  cessation  of 
the  blood-flow,  as  by  stoppage  of  the  heart-beat,  will  tend 
indirectly  to  alter  the  state  of  contraction  of  the  blood-vessels, 
and  therefore  the  tension  on  the  vascular  walls. 

Owing  to  the  distensibiHty  of  the  vascular  system,  the 
pressure  at  any  part,  and  the  circulation  as  a  whole,  must 
be  largely  dependent  on  the  influence  of  gravity.  If  we  take 
a  continuous  tube  of  soft  rubber  and  fill  it  with  fluid  to  a 
pressure  of  10  mm.  Hg.,  so  long  as  the  tube  is  horizontal  the 
pressure  on  its  walls  will  be  the  same  throughout  its  whole 
circuit.  If  however  the  tube  be  hung  up,  the  weight  of  the 
fluid  will  tend  to  bulge  out  the  lower  dependent  parts  of 
the  tu])e,  so  that  the  upper  part  may  be  entirely  collapsed 
and  empty.  The  vascular  system,  being  also  distensible, 
would  behave  in  a  similar  manner,  if  the  state  of  contraction 
of  its  walls  remained  unaltered  during  changes  of  position. 
In  man  in  the  upright  position  the  pressure  in  the  femoral 
artery  is  45  mm.  Hg.  higher  than  the  pressure  in  the  carotid 
artery.  If  there  is  paralysis  of  the  blood-vessels,  as  occurs 
during  administration  of  chloroform,  or  to  a  lesser  degree  as 
the  result  of  illness,  this  increased  pressure  in  the  lower  part 
of  the  body,  including  the  abdomen,  which  ensues  on  change 
from  a  horizontal  to  a  vertical  position,  may  cause  such  a 
bulging  of  these  vessels  that  they  accommodate  the  greater 
part  of  the  blood  in  the  body.  An  insufficient  amount  is 
therefore  returned  to  the  heart,  and  fainting  ensues  from 
anaemia  of  the  brain.  In  the  normal  individual  the  effects  of 
change  of  position  are  at  once  compensated  for  by  a  change 
in  the  distensibiHty  of  the  vessels,  chiefly  of  the  abdomen. 
These  become  more  rigid  in  consequence  of  vaso-constrictor 
impulses  arriving  at  them  from  the  bulbar  centres,  and 
accommodate  therefore  no  more  blood  than  they  did  in  the 
horizontal  position  at  a  lower  pressure.  If,  from  any  reason, 
this  compensatory  action  of  the  vaso-motor  centre  is  im- 
perfectly carried  out,  the  brain  is  insufficiently  supplied  with 
blood  and  the  respiratory  centre  is  also  set  into  increased 
activity.  The  increased  respiratory  movements  thus  set  up 
act  as  a  respiratory  pump,  the  contractions  of  the  diaphragm, 
alternating  with  strong  expiratory  contractions  of  the  abdo- 
minal muscles,  serving  to  support  the  yielding  abdominal 
vessels  and  to  drive  their  contents  on  into  the  heart. 


280  PHYSIOLOGY 


Section  8 
THE   CAPILLARY   CIRCULATION 

The  capillaries  may  be  regarded  as  the  chief  part  of  the 
circulation,  since  the  whole  object  of  the  varied  arrangements 
of  the  heart  and  arterioles  is  to  secure  an  adequate  flow  of 
blood  through  these  smallest  vessels — that  is,  a  supply  of 
blood  adequate  to  the  needs  of  the  tissues  in  which  the 
capillaries  are  embedded.  The  transudation  of  lymph  and  the 
chemical  interchange  between  the  tissues  and  the  blood  take 
place  only  in  the  region  of  the  capillaries  and  small  veins. 
At  present  we  have  no  evidence  of  an  influence  of  the  nervous 
system  on  the  calibre  of  the  capillaries,  or  on  the  interchange 
taking  place  between  them  and  the  surrounding  tissues, 
although  the  circulation  here  is  indirectly  affected  by  changes 
induced  in  the  calibre  of  the  arterioles. 

The  condition  of  the  endothelial  wall  of  the  capillaries  and 
its  influence  on  the  blood-stream  seem  to  be  chiefly  dependent 
on  the  nutrition  of  the  surrounding  tissues.  This  is  well 
exemplified  in  the  series  of  phenomena  classed  under  the  head 
of  inflammation.  By  inflammation  we  understand  those 
processes  wherein  the  organism  reacts  to  a  destructive  lesion 
of  its  tissues  ;  and  these  processes  in  the  higher  animals  are 
connected  with  marked  vascular  changes,  which  can  be  well 
studied  on  the  tongue  of  the  frog.  If  this  be  spread  out  and 
arranged  for  microscopical  observation,  a  beautiful  picture  of 
the  normal  circulation  of  the  blood  through  the  arterioles, 
capillaries,  and  veins  is  afforded.  As  a  destructive  lesion  to 
call  forth  inflammatory  changes,  a  small  piece  of  the  tongue 
may  be  cut  off,  or  the  tongue  may  be  painted  with  a  very 
weak  solution  of  croton  oil.  The  following  series  of  pheno- 
mena are  then  observed.  At  first  the  injury  is  followed  by  a 
dilatation  of  all  the  vessels,  consequent  upon  dilatation  of  the 
arterioles  ;  the  blood  rushes  through  the  capillaries,  and  many 
vessels  make  their  appearance  which  were  before  invisible. 
After  a  time,  the  vessels  still  remaining  dilated,  the  stream  of 
blood  becomes  slower,  and  it  is  then  seen  that  in  the  small 
veins  there  are  two  layers  :  a  layer  next  the  vessel  wall,  in 


THE   VASCULAR  MECHANISM  281 

which  large  numbers  of  leucocytes  are  present,  and  which 
remains  almost  stationary  ;  and  an  inner  layer  of  slowly 
moving  red  corpuscles.  Since  this  slowing  of  the  circulation 
is  unattended  by  any  narrowing  of  the  calibre  of  the  vessels, 
it  must  be  due  to  an  increased  friction  between  the  blood- 
plasma  and  its  contents  and  the  capillary  wall.  It  has  been 
explained  by  saying  that  the  layer  of  endothelium  is  more 
adhesive.  In  the  capillaries  the  endothelium  is  also  thickly 
covered  with  leucocytes,  but  here  the  red  corpuscles  are 
mixed  with  the  leucocytes,  and  there  is  not  such  a  division 
into  two  layers  as  in  the  veins.  Very  soon,  at  one  or  two 
spots,  it  will  be  observed  that  a  leucocyte  is  squeezing  itself 
or  being  squeezed  through  the  capillary  wall,  so  that  half  of 
it  lies  inside,  half  of  it  outside  the  vessels,  and  the  emigra- 
tion  speedily   becomes   complete.     This  emigration  of  white 

Fig.  156. 


Emigration  of  leucocytes  through  capillary  wall.     (Arnold.) 

blood -corpuscles  increases  in  extent,  and  at  the  end  of  seven 
or  eight  hours  the  tissues  in  the  immediate  neighl;ourhood 
of  the  small  veins  and  capillaries  are  infiltrated  with  masses 
of  leucocytes.  At  the  same  time  the  amount  of  lymph  that 
transudes  through  the  vessel-walls  is  largely  increased,  so 
that  it  cannot  be  carried  off  quickly  enough  by  the  lymphatics, 
and  remains  in  the  interstices  of  the  tissues,  causing  a  swelling 
or  oedema. 

The  true  significance  of  this  process  of  inflammation 
has  been  pointed  out  in  recent  years  by  Metchnikoft'.  This 
observer  has  shown  that  all  the  vascular  phenomena  of  in- 
flammation are  directed  towards  furthering  the  emigration  of 
leucocytes,  and  that  these  leucocytes,  or  ■pliagocytes,  have  the 
power  of  devouring  the  irritant  body  if  it  be  a  micro-organism, 
or  of  removing  the  tissues  killed  by  the  lesion,  and  so  clearing 
the  ground  for  a  regeneration  of  the  tissue.     It  is  probable 


282  PHYSIOLOGY 

that  the  ingestion  of  micro-organisms  is  often  preceded  by 
the  secretion  of  some  material  from,  e.g.  the  eosinophile 
cells,  which  kill  the  bacteria  or  prepare  them  in  some  way 
for  their  ingestion.  Such  a  condition  of  phagocytosis — 
that  is  a  collection  of  wandering  cells  to  devour  and  remove 
disintegrated  tissues,  foreign  bodies,  or  micro-organisms — 
has  been  shown  to  occur  in  all  animals,  even  in  those 
destitute  of  a  vascular  system.  The  animals  with  such 
a  system  have  the  advantage  over  the  lower  animals, 
in  that  the  circulating  blood  is  always  bringing  up  fresh 
relays  of  leucocytes  to  vanquish  and  destroy  the  offending 
body.  In  many  cases  the  chemical  or  microbic  influence 
destroying  the  tissues  is  too  powerful  for  the  leucocytes  to 
overcome  ;  they  also  are  destroyed,  and  the  dead  leucocytes 
collect  in  the  tissues  and  form  pus.  If  the  leucocytes  are 
successful  in  removing  the  irritant  body,  they  disappear, 
perhaps  wandering  back  into  the  blood-stream,  and  the  lost 
tissue  is  replaced  by  regeneration  of  the  surrounding  tissues. 


THE   VASCULAR  MECHANISM  283 

Section  9 
VARIATIONS   IN   THE   QUANTITY   OF  BLOOD 

From  a  few  observations  on  executed  criminals  it  has  been 
determined  that  the  amount  of  blood  in  the  human  body 
forms  about  one-thirteenth  of  the  body-weight.  Thus  a  man 
weighing  65  kilos  would  have  about  5  kilos  of  blood  in  his 
vessels. 

The  determination  of  the  total  hloocl  is  carried  out  in  tlie  following  way.  A 
small  sample  of  blood  is  taken  and  diluted  one  hundred  times  with  distilled 
water.  The  animal  is  then  bled  to  death,  the  blood  defibrinated,  and  the 
vessels  washed  out  with  water,  until  the  outflowing  fluid  is  no  longer  tinged 
with  hfgmoglobin.  The  organs  very  rich  in  blood,  such  as  the  liver  and  spleen, 
are  minced  and  washed  free  fi'om  h.Tmoglobin.  All  the  blood  and  the  washings 
are  then  mixed  together,  and  diluted  with  water  until  the  tint  of  the  mixture  is 
exactly  identical  with  that  of  the  first  sample.  We  then  have  only  to  divide 
the  total  volume  by  100  to  arrive  at  the  amount  of  blood  contained  in  the  animal's 
vessels. 

A  method  has  been  devised  by  Haldane  by  means  of  which  the  total  volume  of 
blood  can  be  determined  in  the  living  subject.  The  animal  or  man  experimented 
on  is  made  to  breathe  in  and  out  of  a  bag  containing  a  measured  quantity  of 
CO  gas,  mixed  with  pure  oxygen.  The  expired  air  is  allowed  to  pass  through 
soda  lime  on  its  way  back  to  the  bag  in  order  to  absorb  the  carbon  dioxide,  and 
fresh  oxygen  is  supplied  from  a  reservoir  as  that  in  the  bag  is  used  up.  When 
the  oxygen  has  displaced  all  the  CO  gas  into  the  lungs,  the  bag  is  disconnected, 
and  a  sample  of  blood  taken,  the  relative  amounts  of  CO-htemoglobin  and  of 
oxyhemoglobin  being  determined  by  a  colorimetric  method.  The  amount  of 
CO-ht'Emoglobin  can  be  estimated  from  the  amount  of  CO  absorbed  in  the 
course  of  the  experiment ;  and  since  the  proportion  of  CO-hasmoglobin  to  total 
haemoglobin  has  been  also  determined,  it  is  easy  to  calculate  the  total  amount 
of  hfemoglobin  in  the  body. 

A  determination  of  the  relative  content  of  the  blood  in  haemoglobin  now 
enables  us  to  calculate  the  total  mass  of  blood.  Estimations  carried  out  in  this 
way  by  Haldane  and  L.  Smith  led  to  the  conclusion  that  the  amount  of  blood 
in  the  body  is  not  J^  as  generally  assumed,  but  less,  varying  in  different  indi- 
viduals from  jig  to  3L,  the  average  being  J^... 

PletJiora  and  HydrcBmic  Plethora 

The  effects  of  increasing  the  total  volume  of  circulating 
fluid  may  be  studied  by  injecting  several  hundred  cubic 
centimetres  of  defibrinated  blood  or  normal  saline  fluid  into 
a  vein.  In  the  latter  case,  since  the  blood  is  rendered  more 
dilute,  the  condition  is  called  hydraemic  plethora. 

On  the  arterial  pressure  the  result  of  guch  an  injection  is 


284  PHYSIOLOGY 

not  very  marked.  There  is  a  slight  initial  increase  in  the 
pressure,  but  the  increase  is  by  no  means  proportional  to  the 
amount  of  fluid  injected,  showing  that  the  fluid  is  not  to  any 
large  extent  contained  in  the  arterial  system.  On  examining 
the  pressure  in  the  veins  however  we  find  a  very  great  relative 
rise  of  pressure,  and  on  opening  the  abdomen  it  is  seen  that  all 
the  veins  are  distended  and  that  the  liver  is  swollen.  The 
effect  of  increasing  the  volume  of  circulating  fluid  would  be  to 
increase  the  mean  systemic  pressure,  and  therefore  one  would 
expect  to  find  a  large  increase  both  in  arterial  and  venous 
systems.  But  the  organism  prevents  the  rise  on  the  arterial  side 
by  relaxing  the  whole  system  of  arterioles,  so  that  the  distri- 
bution of  pressures  is  altered,  and  the  venous  approximates 
more  closely  to  the  arterial  pressure.  This  arterial  dilatation 
however  augments  the  velocity  of  the  blood :  it  has  been 
found  that  the  velocity  may  be  accelerated  to  six  or  eight 
times  the  normal  rate  by  injecting  an  amount  of  salt  solution 
equivalent  to  50  per  cent,  of  the  total  blood. 

The  high  venous  pressure  causes  increased  diastolic  filling 
of  the  heart,  and  therefore  augments  both  strength  and  fre- 
quency of  the  beat.  Thus  the  work  of  the  heart  is  increased 
in  three  ways,  viz.  by  : — 

(1)  Piise  of  arterial  pressure. 

(2)  Greater  frequency  of  beat. 

(3)  Increased  output  at  each  beat  (Fig.  143). 

These  series  of  changes  result  however  in  the  relief  of  the 
vascular  system.  The  heightened  pressure  in  the  abdominal 
veins  and  capillaries  causes  a  great  leakage  of  fluid  in  the 
form  of  lymph  from  the  capillaries  of  the  intestines  and  liver, 
while  the  increased  pressure  and  velocity  of  the  blood  in  the 
glomeruli  of  the  kidney  induce  a  copious  secretion  of  urine, 
so  that  within  a  couple  of  hours  after  the  injection  of  salt 
solution  the  volume  of  the  circulating  fluid  may  have  returned 
to  normal. 

This  recovery  is  effected  with  greater  difficulty  if  the 
plethora  has  been  brought  about  by  the  injection  of  defibri- 
nated  blood,  since  this  fluid  cannot  escape  rapidly  from 
the  capillaries,  nor  can  it  be  excreted  unchanged  by  the 
kidneys.  Hence  it  is  easy  to  kill  an  animal  by  wearing  out 
its  heart,  if  too  large  quantities  of  defibrinated  blood  be  in- 
jected.    The  ultimate  fate  of  the  injected  blood  is  to  be]_used 


THE   VASCULAK   MECHANISM  285 

as  food  by  the  tissues,  and  to  be  eliminated  by  the  ordinary 
channels. 

It  must  be  remembered  that  the  blood  serum  of  one  animal  is  often  poisonous 
for  the  corpuscles  of  another.  Thus  a  few  e.c.  of  dog's  serum  injected  into 
the  peritoneal  cavity  of  a  rabbit  will  cause  death.  This  poisonous  action 
is  also  shown  by  mixing  dog's  serum  with  defibrinated  rabbit's  blood,  in 
which  case  the  red  corpuscles  of  the  latter  are  broken  up,  setting  free  hemo- 
globin (licemohjsis). 

The  Effects  of  Hcemorrhage.    Ancemia 

Any  diminution  of  the  total  volume  of  the  blood,  as  by 
bleeding,  would  tend  to  lower  the  pressure  on  both  sides  of 
the  system.  The  vaso-motor  centre  however  strives  to  main- 
tain a  normal  arterial  pressure,  and  so  the  circulation  through 
the  brain  unaltered.  This  object  is  carried  out  by  a  general 
vascular  constriction,  which  diminishes  the  total  capacity  of 
the  system  and  alters  the  distribution  of  pressure  throughout 
the  system,  tending  to  keep  the  blood  as  much  as  possible 
on  the  arterial  side.  Thus  a  slight  loss  of  blood  has  no 
influence  on  the  arterial  blood-pressure,  but  causes  a  fall  of 
pressure  in  the  veins,  blanching  of  the  abdominal  organs,  and 
diminished  flow  of  urine.  The  heart  beats  very  frequently, 
and  so  aids  in  emptying  the  venous  into  the  arterial  system. 

The  deficiency  of  circulating  fluid  caused  by  bleeding  is 
soon  remedied  by  a  transfer  of  fluid  from  the  tissues  to  the 
blood.  This  transfer  is  independent  of  the  flow  of  lymph 
from  the  thoracic  duct  into  the  blood,  and  is  the  direct 
consequence  of  the  universal  fall  of  capillary  pressure  which 
results  from  the  bleeding.  The  abstraction  of  fluid  from  the 
tissues  is  responsible  for  the  extreme  thirst  which  is  the  result 
of  haemorrhage,  and  which  directs  the  animal  to  take  up  by 
the  alimentary  canal  the  fluid  which  is  wanting  to  the  body. 

This  transfer  of  fluid  from  tissues  to  blood  is  extremely 
rapid  ;  even  during  the  course  of  a  bleeding  it  is  found  that 
the  later  samples  of  blood  are  more  dilute  than  those  obtained 
at  the  beginning. 

This  mechanism  sufiices  only  to  make  up  the  supply  of 
circulating  fluid.  After  a  bleeding  however  an  animal  has 
lost  proteids  and  blood-corpuscles,  and  these  constituents  of 
the  blood  are  but  slowly  restored,  the  former  directly  from 
the  food,  the  latter  by  an  increased  activity  of  the  blood- 
forming  cells  in  the  red  marrow. 


286  PHYSIOLOGY 


CHAPTER   VII 
LYMPH    AND    TISSUE-FLUIDS 

In  no  part  of  the  body  does  the  blood  come  m  actual 
contact  with  the  living  cells  of  the  tissue.  In  all  parts 
the  blood  flows  in  capillaries  with  definite  walls  consist- 
ing of  a  single  layer  of  cells,  and  is  thus  separated  from  the 
tissue-elements  by  these  walls  and  by  a  varying  thiclmess 
of  tissue.  In  some  organs,  such  as  the  liver  and  lung,  every 
cell  is  in  contact  with  the  outer  surface  of  some  capillary ; 
while  in  others,  such  as  cartilage  (which  is  quite  avascular), 
a  considerable  thickness  of  tissue  may  separate  any  given 
cell  from  the  nearest  capillary.  A  middleman  is  thus  needed 
between  the  blood  and  the  tissues,  and  this  middleman  is 
the  lymph  which  fills  spaces  between  all  the  tissue-elements, 
so  that  any  tissue  can  be  regarded  as  a  sponge  soaked  with 
lymph. 

Throughout  these  spaces  we  find  a  close  network  of 
vessels  lined,  and  separated  from  the  tissue  spaces,  by  a 
layer  of  extremely  thin  endothelial  cells,  and  this  plexus  com- 
municates with  definite  channels — lymphatics,  by  which  any 
excess  of  fluid  in  the  part  is  drained  off.  The  lymphatics 
all  run  towards  the  chest,  where  those  of  the  limbs  join 
a  large  vessel  (the  receptaculum  chyli),  which  carries  the 
lymph  from  the  alimentary  canal,  to  form  the  thoracic  duct. 
This  runs  up  on  the  left  side  of  the  oesophagus,  to  open  into 
the  great  veins  at  the  junction  of  the  left  internal  jugular 
with  the  subclavian  vein.  A  small  vessel  on  the  right  side 
drains  the  lymph  from  the  right  upper  extremity  and  right 
side  of  the  chest  and  neck. 

The  lymph  may  be  looked  upon  as  a  part  of  the  plasma 
which  exudes  through  the  capillary  wall,  bathes  all  the 
tissue-elements,  passes  between  the  endothelial  cells  into  the 
peripheral  lymphatic  network,  whence  it  is  carried  by  lym- 


LYMPH   AND   TISSUE-FLUIDS  287 

phatic  trunks  into  the  thoracic  duct,  by  which  it  is  returned 
again  to  the  blood. 

It  is  easy  to  obtain  lymph  for  examination  by  putting 
a  cannula  (a  small  tube  of  glass  or  metal)  into  the  thoracic 
duct,  and  collecting  the  fluid  that  drops  from  it  in  a  glass 
vessel. 

We  may  also  tap  in  a  similar  way  one  of  the  large  lym- 
phatic trunks  of  the  limbs  ;  but  in  the  latter  case  we  have  to 
use  artificial  means  to  induce  a  flow  of  lymph,  since  little  or 
none  can  be  obtained  in  a  normal  animal  from  a  limb  at  rest  ; 
the  only  part  of  the  body,  where  there  is  normally  a  constant 
flow  of  lymph,  being  the  alimentary  canal.  And  thus  we 
cannot  regard  the  flow  of  lymph  from  a  part  as  any  index  of 
the  chemical  changes  going  on  at  that  part.  In  a  limb  at 
rest  foodstuffs  are  being  taken  up  from  the  blood  and  being 
burnt  up  by  the  muscles  with  the  production  of  CO,,  although 
we  may  not  be  able  to  obtain  a  drop  of  lymph  from  a  cannula 
in  one  of  the  lymphatics. 

The  lymph  is  thus  truly  a  middleman ;  as  any  substance, 
oxygen  or  foodstuff,  is  taken  up  by  a  tissue-cell  from  the  lymph 
surrounding  it,  this  latter  recoups  itself  at  once  at  the  expense 
of  the  blood. 

Thus  there  would  seem  to  be  no  need  for  lymphatics  to 
drain  the  limb,  were  it  not  that  under  many  conditions  which 
we  shall  study  directly,  the  exudation  of  lymph  from  the 
blood-vessels  is  so  excessive  that,  if  it  were  not  carried  oft"  at 
once  and  restored  to  the  blood,  it  would  accumulate  in  the 
tissue-spaces,  give  rise  to  dropsy,  and  by  pressure  on  the  cells 
and  blood-vessels  affect  them  injuriously. 

Properties  of  Lymph 

Lymph  obtained  from  the  thoracic  duct  of  an  animal 
varies  in  composition  and  appearance  according  to  the  con- 
dition of  the  animal,  whether  recently  fed  or  fasting.  From 
a  fasting  animal  the  lymph  is  a  transparent  liquid,  generally 
slightly  yellowish,  and  sometimes  reddish  from  admixture  of 
blood-corpuscles.  When  obtained  from  an  animal  shortly 
after  a  meal,  it  is  milky  from  the  presence  of  minute  particles 
of  fat  that  have  been  absorbed  from  the  alimentary  canal. 
In  the  latter   case,  if   the   intestines  be  exposed,  the   small 


288  PHYSIOLOGY 

lymphatics  are  to  be  seen  as  white  lines  running  from  the 
intestine  to  the  attached  part  of  the  mesentery.  It  is  owing 
to  this  fact  that  these  lymphatics  have  received  the  special 
name  lacteals,  the  lymph  in  them  being  called  the  chyle. 
The  fatty  particles  form  the  molecular  basis  of  the  chyle. 

On  microscopic  examination  the  transparent  lymph  of 
fasting  animals  presents  colourless  corpuscles  similar  to  those 
of  blood,  or  perhaps  we  ought  to  say  identical,  since  the 
leucocytes  of  the  blood  are  probably  derived  from  the  cor- 
puscles that  have  entered  with  the  lymph  through  the 
thoracic  duct. 

All  the  lymphatics  pass  at  some  point  of  their  course 
through  lymphatic  glands,  which  we  may  look  upon  as  fac- 
tories of  leucocytes,  since  these  are  much  more  numerous 
in  the  lymph  after  it  has  traversed  the  gland  than  before. 
Leucocytes  are  also  formed  in  all  the  numerous  localities 
where  we  find  adenoid  tissue,  such  as  the  tonsils,  air  passages, 
alimentary  canal  (Peyer's  patches  and  solitary  follicles), 
Malpighian  bodies  of  the  spleen,  and  thymus. 

The  lymph  is  alkaline,  has  a  sp.  gr.  of  about  1015,  and 
clots  at  a  variable  time  after  it  has  left  the  vessels,  forming 
a  colourless  clot  of  fibrin,  just  like  blood-plasma.  It  contains 
about  6  per  cent,  of  solid  matters  ;  the  proteins  consisting  of 
fibrinogen,  paraglobulin,  and  serum  albumen.  The  salts  are 
similar  to  those  of  the  liquor  sanguinis,  and  are  present  in 
the  same  proportions. 

The  Production  of  Lymph 

Many  physiologists  have  thought  that,  in  the  transudation 
of  the  fluid  which  forms  the  lymph,  there  is  an  active  inter- 
vention on  the  part  of  the  endothelial  cells  composing  the 
capillary  wall,  and  that  lymph  is  therefore  to  be  regarded  as 
a  true  secretion.  A  careful  investigation  of  the  known  experi- 
mental facts  has  failed  to  show  that  the  endothelial  cells  act 
otherwise  than  passively,  as  filtering  membranes  of  variable 
permeability.  The  factors  which  are  responsible  for  the 
transudation  of  lymph  may  be  divided  into  two  classes — 
mechanical  and  chemical,  the  former  depending  largely  on 
the  pressure  of  the  blood  in  the  vessels,  and  the  latter  chiefly 
on  the  metabolism  of  the  cells  outside  the  vessels. 


LYMPH   AND   TISSUE-FLUIDS  '289 

According  to  the  views  here  laid  down,  the  formation  of 
lymph  may  be  compared  to  a  process  of  filtration.  If  this 
view  is  correct,  the  amomit  of  lymph  formed  in  any  given 
capillary  area  must  be  dependent  on  the  difi'erence  of  pres- 
sure between  the  blood  in  the  vessels  and  the  fluid  in  the 
extravascular  tissue-spaces.  This  latter  pressure  is  normally 
extremely  low,  so  that  in  attempting  to  test  the  truth  of  this 
view,  we  must  try  the  effects  of  altering  the  pressure  inside 
the  vessels,  in  the  expectation  of  finding  that  the  Ijanph  pro- 
duction will  rise  and  fall  as  the  capillary  pressure  is  increased 
or  diminished.  On  attempting  to  carry  out  such  experiments 
in  different  parts  of  the  body,  we  have  to  recognise  another 
factor  besides  the  capillary  pressure,  viz.  the  permeability  of 
the  vessel-wall.  Whereas  the  capillary  walls  in  the  limbs 
and  connective  tissues  generally  present  a  very  considerable 
resistance  to  the  filtration  of  lymph  through  them,  and  keep 
back  the  larger  portion  of  the  proteins  of  the  blood-plasma, 
the  intestinal  capillaries  are  much  more  permeable,  givmg  at 
moderate  capillary  pressures  a  continual  flow  of  lymph  and 
separating  off  only  a  small  proportion  of  the  proteins.  It  is 
in  the  liver  however  that  we  find  the  greatest  permeability. 
Here  a  very  small  pressure  suffices  to  produce  a  great  transu- 
dation of  lymph,  containing  practically  the  same  amount  of 
protein  as  the  blood-plasma  from  which  it  is  formed. 

The  ease  with  which  fluid  passes  out  from  the  capillaries  of  the  liver  is 
probably  due  to  the  fact  that  these  vessels,  unlike  most  other  capillaries  of  the 
body,  have  not  a  complete  endothelial  lining.  Thus  it  is  impossible  to  display 
a  continuous  endothelial  lining  by  means  of  silver  nitrate.  The  cells  surround- 
ing the  capillaries  are  large  and  branched,  and  possess  marked  phagocytic 
powers,  so  that  after  an  injection  of  carmine  granules  or  bacteria  into  the 
blood  stream  these  bodies  are  found  in  quantity  within  the  cells.  Owing  to  the 
incompleteness  of  this  investment,  the  liver  cells  in  many  places  abut  on 
the  lumen  of  the  capillary.  On  injecting  the  blood  system  of  the  liver,  the 
injection  is  found  to  run  with  ease  into  channels  situated  within  the  cells 
themselves,  and  it  is  reasonable  to  conclude  that  the  blood  plasma  takes  the 
same  course  through  these  intracellular  channels,  by  which  it  passes  into 
the  lymphatics  which  lie  at  the  periphery  of  the  lobules. 

In  experiments  on  the  lymph  production  in  the  limbs 
alterations  of  capillary  pressure  have  but  slight  effect.  The 
lymph-flow  from  a  limb  lymphatic  is  practically  unaltered  by 
changes  in  its  arterial  supply,  although  a  definite  increase 
may  be  obtained  by  ligaturing  all  the  veins  of  the  limb  so  as 
to  cause  a  very  great  rise  of  capillary  pressure.     The  lymph- 

19 


290  PHYSIOLOGY 

flow  from  the  intestines  can  be  measured  by  collecting  the 
lymph  from  the  thoracic  duct.  If  the  lymphatics  which 
leave  the  liver  in  the  portal  fissure  be  previously  ligatured, 
the  whole  of  the  thoracic  duct  lymph  in  an  animal  at  rest  is 
derived  from  the  intestines.  It  will  be  found  that  lowering 
of  the  capillary  pressure  in  these  organs  by  obstructing  the 
thoracic  aorta  stops  the  flow  of  lymph  absolutely,  whereas  a 
rise  of  capillary  pressure,  such  as  that  produced  by  ligature 
of  the  portal  vein,  causes  a  four-  or  five-fold  increase  of  the 
lymph. 

The  effect  of  rise  of  capillary  pressure  on  the  lymph-flow 
is  still  more  striking  in  the  case  of  the  liver.  If  the  inferior 
vena  cava  be  obstructed  just  above  the  opening  of  the  hepatic 
veins,  there  is  a  great  fall  of  arterial  pressure,  but,  owing  to 
the  damming  back  of  the  blood,  a  rise  of  pressure  in  the  liver 
capillaries  to  three  or  four  times  the  normal  amount.  This 
rise  causes  an  enormous  increase  in  the  lymph -flow  from  the 
thoracic  duct.  The  lymph  may  be  increased  eight  to  ten 
times  in  amount,  and  it  contains  more  protein  than  before. 
If  the  portal  lymphatics  be  previously  ligatured,  obstruction 
of  the  inferior  cava  has  no  effect  on  the  lymph-flow,  showing 
that  the  whole  of  this  increase  is  derived  from  the  one  region 
of  the  body  where  the  capillary  pressure  is  increased,  viz.  the 
liver. 

We  must  conclude  that  in  those  regions  of  the  body  where 
the  capillaries  are  fairly  permeable,  the  most  important  factor 
in  the  lymph  production  is  the  intracapillary  pressure. 

In  the  case  of  the  limbs  and  connective  tissues  generally, 
the  pressure  factor  is  probably,  under  normal  conditions,  of 
less  importance,  so  that  the  second  condition,  the  chemical, 
comes  here  more  into  prominence.  The  capillary  wall  not 
only  permits  of  filtration  under  certain  pressures,  but  also 
allows  the  passage  of  water  and  dissolved  substances  by  diffu- 
sion and  osmosis.  These  osmotic  interchanges  between  blood 
and  cell  through  the  intermediation  of  the  lymph  are  con- 
stantly going  on  in  the  normal  life  of  the  tissue,  and  are  quite 
independent  of  the  amount  of  lymph  produced.  Thus  a 
gland-cell  may  use  up  oxygen,  calcium,  or  sugar,  and  create 
a  vacuum  of  these  substances  in  the  layer  of  lymph  imme- 
diately surrounding  the  cell.  There  is  at  once  a  disturbance 
of  the  equilibrium,  and  a  flow  of  these  substances  from  blood 


LYMPH   AND   TISSUE-FLUIDS  291 

to  lymph  is  set  up.  In  consequence  of  the  wonderful  arrange- 
ments m  the  tissues  for  ensuring  the  intimate  contact  of 
blood  and  lymph  without  intermingling,  these  changes  can 
occur  with  great  rapidity.  We  find,  for  instance,  that  if  a 
very  large  amount  (40  grms.)  of  dextrose  be  injected  into  the 
circulation,  osmotic  equilibrium  between  blood  and  lymph  is 
established  within  half  a  minute  of  the  termination  of  the 
injection.  In  this  case  the  rise  of  osmotic  pressure  ^  caused 
l)y  the  injection  of  the  sugar  attracts  water  from  the  lymph, 
and  this  in  its  turn  from  the  tissue-cells,  until  the  osmotic 
pressure  inside  and  outside  the  vessels  is  the  same.  By 
this  means  the  volume  of  the  circulating  blood  is  increased 
at  the  expense  of  the  tissues.     A  process  of  this  character 

Fic.  157. 

^L^mph  spaces 


'Secreting 
cells. 

•''^-- Blood 


I 
I 
Duct. 


\ 

Basement 
membrane 


capillarjf. 


Diagram  to  show  relation  of  the  secreting  cells  of  a  gland  to  the 
blood  and  lymph  supply. 

may  however  work  under  normal  circumstances  in  the 
reverse  direction,  and  lead  to  a  passage  of  fluid  from  blood 
to  tissues  and  tissue-spaces.  Every  active  contraction  of 
a  muscle,  for  instance,  is  attended  by  the  breaking  down 
of  a  few  large  molecules  into  a  number  of  smaller  ones,  and 
this  increase  in  the  number  of  molecules  causes  a  rise  of 
osmotic  pressure  in  the  muscle-fibre  and  surrounding  lymph, 
and  therefore  a  passage  of  fluid  from  blood  to  lymph.  In  the 
same  way  a  cell  of  the  submaxillary  gland,  when  stimulated 
by  means  of  its  nerve,  pours  out  a  quantity  of  fluid  into  the 
gland-duct,  and  so  into  the  mouth.     This  fluid  comes  in  the 

'  A  fuller  description  of  the  phenomena  of  osmotic  pressure  will  be  found  in 
Chapter  X. 


21)2  PHYSIOLOGY 

first  instance  from  the  cell  itself,  but  the  cell  recoups  itself 
from  the  surrounding  lymph,  raising  the  concentration  of  this 
fluid,  and  the  difference  in  concentration  thus  caused  at  once 
induces  a  passage  of  water  from  blood  to  lymph  (Fig.  157). 
Hence  salivary  secretion  is  associated  with  a  large  flow  of 
fluid  through  the  capillary  walls  of  the  gland.  In  this  passage 
the  endothelial  cells  of  the  capillaries  play  no  part,  the  whole 
process  being  conditioned  by  changes  in  the  extravascular 
gland-cell.  We  have  only  to  paralyse  the  gland -cell  by  means 
of  atropin  in  order  to  see  that  the  active  flushing  of  the  gland 
which  accompanies  activity  produces  merely  a  minimal  in- 
crease in  the  lymph -flow  from  the  gland. 

The  influence  of  tissue-activity  in  the  production  of  lymph 
is  still  better  shown  in  the  case  of  a  large  gland,  such  as  the 
liver.  Stimulation  of  this  organ  by  the  injection  of  bile  salts 
into  the  blood-stream  causes  a  large  increase  in  the  lymph- 
flow  from  the  organ,  and  therefore  in  the  lymph-flow  from  the 
thoracic  duct. 

It  is  important  to  remember  that  the  relative  insuscep- 
tibility of  the  limb  capillaries  to  pressure  holds  only  for  the 
absolutely  normal  capillary.  Any  factor  which  leads  to  im- 
paired nutrition  of  the  vascular  wall,  such  as  deficiency  of 
supply  of  blood  or  oxygen,  the  presence  of  poisons  in  the  blood 
or  in  the  surrounding  tissues,  scalding  or  freezing,  increases 
at  the  same  time  its  permeability.  Under  such  conditions 
the  limb  capillary  reacts  to  changes  of  pressure  like  a  liver 
capillary,  the  slightest  increase  of  pressure  causing  an  appreci- 
able increase  in  the  lymph  production.  This  increased  lymph 
production  may  be  too  great  to  be  carried  off  by  the  lymphatic 
channels,  so  that  the  exuded  fluid  stays  in  the  tissue-spaces, 
distending  them  and  causing  the  condition  known  as  oedema 
or  dropsy. 

Lymphagogues. — Among  the  substances  which  have  a 
direct  action  on  the  vessel  wall  are  a  number  of  bodies  which 
were  described  by  Heidenhain  as  lymphagogues  of  the  first 
class.  As  their  name  implies,  these  bodies  on  injection  into 
the  blood-stream  cause  an  increased  flow  of  lymph  from  the 
thoracic  duct.  They  may  be  extracted  from  the  dried  tissues 
of  crayfish,  mussels,  or  leeches  by  simple  boiling  with  water. 
Commercial  peptone  has  a  similar  effect.  Heidenhain  regarded 
these  bodies  as  direct  excitants  of  the  secretory  activities  of 


LYMPH   AND   TISSUE-FLUIDS 


293 


the  endothelial  cells.  They  are  however  general  poisons, 
having  a  special  action  on  the  vascular  system,  and  their 
effect  on  the  lymph  production  is  probably  due  simply  to  their 


Fig.  158. 


1  1 

1 

\ 
\ 
1 

— 

I 
\ 

\ 

\ 

\ 

1 

1 

a      1 

^ 

X 
X 

} 

^ 

=:r-: 

X 

[-» .^  < 

n- 

::r 

F-- 

"^^^^ 

— -- 

---- 

---, 







0123*5678910 


Inj  of  40  grams   dextros 
10 


- 

■— ^ 

r"^ 

1 — -^ 

'-■■;■'      1 

1 

^\: 

^ 

\ — ■ — 

""" 

* 

'f^lT*"*' 

^^ 

-.►- 

— 



^.-- 

1 

0I2J45678910  20  30  40  50  60  minutes 

Bled  to  240  ccm    Inj  13  grams   dextrose 

Curves  to  show  the  influence  of  intravenous  injection  of  dextrose  on 
the  arterial  and  venous  pressures  and  on  the  flow  of  lymph  from 
the  thoracic  duct.  The  upper  diagram  represents  the  effect  of 
the  injection  of  dextrose  in  a  normal  animal,  i.e.  rise  of  arterial 
and  venous  pressures,  and  large  increase  in  lymph.  The  lower 
curve  shows  the  effect  of  injecting  dextrose  after  a  preliminary 
bleeding.  In  this  case  the  fluid  attracted  into  the  vessels  by  the 
sugar  only  just  suffices  to  make  up  for  that  lost  in  the  bleeding. 
Hence  the  venous  and  arterial  pressures  are  little  altered  from 
normal,  and  there  is  very  little  increase  in  the  lymph  flow. 

In  both  diagrams — 

Thick  continuous  line  =  arterial  blood-pressure  in  cm.  Hg. 
Thin         „  ,,     =  portal  ,,  in  cm.  water. 

Thin  dotted  line  =  vena  cava      ,,  ,,         „ 

Thick        „  =  lymph-flow  in  c.e.  per  ten  minutes. 

deleterious  action  on  the  capillary  wall.  Although  these 
bodies  act  chiefly  on  the  liver  capillaries,  so  that  the  main 
increase  in  the  thoracic  duct  lymph  is  derived  from  the  liver, 


294  PHYSIOLOGY 

they  can  be  shown  also  to  have  some  effect  in  the  same 
direction  on  the  intestinal  and  skin  capillaries.  In  fact  the 
injection  or  ingestion  of  these  bodies  often  gives  rise  to  a 
copious  eruption  of  nettle-rash,  i.e.  swellings  of  the  skin  due 
to  an  increased  exudation  of  lymph  into  the  meshes  of  the 
cutis. 

An  increased  lymph -flow  from  the  thoracic  duct  may  be 
produced  also  by  the  injection  of  large  amounts  (10  to  40 
grms.)  of  innocuous  crystalloids  such  as  dextrose,  urea,  or 
sodium  chloride  into  the  circulation.  In  this  case  the  lymph 
becomes  much  more  dilute.  The  explanation  of  the  action  of 
these  bodies  is  very  simple.  We  have  already  seen  that  injec- 
tion of  large  amounts  of  dextrose  into  the  circulating  blood 
raises  the  osmotic  pressure  of  this  fluid.  The  blood  there- 
fore imbibes  water  from  the  tissues  and  swells  up,  i.e.  a  con- 
dition of  hydraemic  plethora  is  brought  about  as  surely  as  if 
several  hundred  cubic  centimetres  of  normal  salt  solution  were 
injected  into  the  circulation.  This  increase  in  the  total  volume 
of  the  blood  causes  a  rise  of  pressure  throughout  the  vascular 
system, — arteries,  capillaries,  and  veins, — and  the  increased 
capillary  pressure  combined  with  the  watery  condition  of  the 
blood  induces  a  great  transudation  of  lymph,  especially  in 
the  abdominal  organs.  The  lymph  is  more  watery  because 
the  blood  also  is  diluted.  That  the  action  of  these  bodies 
is  purely  mechanical  is  shown  by  the  fact  that,  if  the  rise  of 
capillary  pressure  be  prevented  by  bleeding  the  animal  imme- 
diately before  the  injection,  the  increase  in  the  lymph-flow  is 
also  prevented  (Fig.  158,  p.  293),  although  the  concentration 
of  the  sugar  or  salt  in  the  blood  is  still  greater  than  in  the 
experiments  in  which  bleeding  was  not  performed. 

Movement  of  Lymph 

In  the  frog  the  circulation  of  lymph  is  maintained  by 
rhythmically  contracting  muscular  sacs,  which  are  placed  in 
the  course  of  the  main  lymph-channels,  and  pump  the  lymph 
into  the  veins.  In  the  higher  animals  and  in  man,  the  onward 
flow  of  lymph  is  effected  partly  by  the  pressure  at  which  it  is 
secreted  from  the  capillaries  into  the  interstices  of  the  tissues, 
but  also  to  a  large  extent  by  the  contractions  of  the  skeletal 
muscles.      In   the   smaller   lymph -radicles    the   pressure   of 


LYMPH   AND   TISSUE-FLUIDS  295 

lymph  may  attain  8  to  10  mm.  soda  solution.  In  the  thoracic 
duct,  at  the  point  where  it  opens  into  the  great  veins  of  the 
neck,  the  pressure  is  obviously  the  same  as  in  these  veins, 
that  is  to  say,  from  —  4  to  0  mm.  Hg,  the  negative  pressure 
being  occasioned  by  the  aspiration  of  the  thorax.  This  differ- 
ence of  pressure  is  sufficient  to  cause  a  certain  amount  of 
flow.  It  must  be  remembered  however  that  under  normal 
circumstances  no  lymph  at  all  flows  from  a  resting  limb. 
The  only  part  of  the  body  which  gives  a  continuous  stream 
of  lymph  during  rest  is  the  alimentary  canal,  the  lymph 
in  which  is  poured  out  into  the  lacteals,  and  thence  makes  its 
way  through  the  thoracic  duct.     Movement,  active  or  passive, 

Fij.  159. 


A  lymi)liatic  vessel  laid  open  to  show  arrangement  of  the  valves. 
(Testut.) 

of  the  limbs  at  once  causes  a  flow  of  lymph  from  them. 
Since  the  lymphatics  are  all  provided  with  valves  (Fig.  159), 
the  effect  of  external  pressure  on  them  is  to  cause  the 
lymph  to  flow  in  one  direction  only,  i.e.  towards  the  thoracic 
duct  and  great  veins.  Hence  we  may  look  upon  muscular 
exertion  as  the  greatest  factor  in  the  circulation  of  lymph. 
The  flow  of  lymph  from  the  commencement  of  the  thoracic 
duct  in  the  abdominal  cavity  to  the  main  part  of  it  in  the 
thoracic  cavity  is  materially  aided  by  the  respiratory  move- 
ments ;  since,  with  every  inspiration,  the  lacteals  and  abdo- 
minal part  of  the  duct  are  subjected  to  a  positive  pressure, 
and  the  intrathoracic  part  of  the  duct  to  a  negative  pressure, 
so  that  lymph  is  continually  being  sucked  into  the  latter. 


The  Absorption  of  Lymph  and  Tissue-fluids 

On  injecting  a  coloured  solution  or  suspension  into  the 
connective  tissues  of  any  part  of  the  body,  and  gently  kneading 
the  part,  it  is  found  that  the  fluid  fills  all  the  lymphatic 


296  PHYSIOLOGY 

channels  running  from  the  part ;  and  we  can  in  this  way 
inject  the  lymphatics  of  the  limb  and  trace  their  course  on 
to  the  thoracic  duct.  The  same  path  is  taken  by  micro- 
organisms as  they  spread  in  the  tissues,  or  by  particles  of 
carmine  or  Indian  ink  which  have  been  introduced  in  tattoo- 
ing. It  is  on  account  of  these  facts  that  the  lymphatics  are 
often  spoken  of  as  the  absorbent  system. 

This  process  of  lymphatic  absorption  is  however  a  slow 
one,  unless  aided  to  a  large  extent  by  passive  or  active 
movements  of  the  surrounding  parts,  and  cannot  therefore 
account  for  the  rapid  symptoms  of  poisoning  which  super- 
vene within  two  or  three  minutes  after  the  hypodermic 
injection  of  a  solution  of  strychnine  or  other  poison.  That 
this  absorption  is  not  dependent  on  the  lymphatics  is  shown 
by  the  fact  that  the  symptoms  occur  almost  as  quickly,  when 
all  the  tissues  of  the  limb  have  been  severed  with  the  excep- 
tion of  the  main  artery  and  vein.  In  the  same  way,  after 
injecting  methylene  blue  or  indigo  carmine  into  the  pleural 
cavity  or  subcutaneous  tissues,  the  dye-stuff  appears  in  the 
urine  long  before  any  trace  of  colom*  can  be  perceived  in  the 
lymph  flowing  from  the  thoracic  duct.  The  absorption  in 
these  cases  is  by  the  blood-vessels,  and  consists  in  an  inter- 
change between  blood  and  extravascular  fluids,  apparently 
dependent  entirely  upon  processes  of  diffusion  between  these 
two  fluids.  So  long  as  any  difference  in  composition  exists 
between  the  intra-  and  extra-vascular  fluids,  so  long  will 
diffusion -currents  be  set  up,  tending  to  equalise  this  difference. 

More  difficulty  is  presented  by  the  question  of  the 
mechanism  of  absorption  by  the  blood-vessels  of  the  normal 
tissue-fluids — such  an  absorption  as  we  have  seen  to  occur 
after  loss  of  blood  by  haemorrhage.  It  seems  probable 
however  that  this  absorption  depends  on  the  small  proportion 
of  protein  contamed  in  the  tissue-fluid  as  compared  with  the 
blood-plasma.  If  blood-serum  be  placed  in  a  bell-shaped 
vessel  (the  mouth  of  which  is  closed  by  a  gelatinous  mem- 
brane which  does  not  permit  the  passage  of  protein),  and 
suspended  in  normal  salt  solution,  it  is  found  that  the  serum 
absorbs  the  salt  solution  until  the  manometer  attached  to  the 
bell -jar  indicates  a  pressure  of  25-30  mm.  Hg.  Thus  we 
may  conceive  that  there  is  normally  a  balance  in  the  capil- 
laries between  the  processes  of  exudation  and  of  absorption. 


LYMPH  AND  TISSUE-FLUIDS  297 

the  former  being  conditioned  by  the  capillary  blood-pressure 
and  the  latter  hy  the  difference  in  protein  content,  and  there- 
fore of  osmotic  pressure  between  the  blood-plasma  and  tissue- 
lymph.  A  rise  of  capillary  pressure  will  upset  this  balance 
in  favour  of  transudation  and  the  blood  will  become  more 
concentrated,  whereas  a  fall  of  pressure  will  turn  the  scale  in 
favour  of  absorption  and  the  volume  of  blood  will  be  increased 
at  the  expense  of  the  tissue-fluids. 

The  Part  played  by  the  Lymph  in  the  Nutrition  of 
THE  Tissues 

The  fact  that  the  tissue-cells  are  separated  by  the  lymph 
and  the  capillary  wall  from  the  blood  shows  that  in  all  inter- 
changes between  the  blood  and  tissues,  the  lymph  must  act 
as  the  medium  of  communication.  The  l^anph-flow  however 
plays  very  little  part  in  this  process.  The  muscles  of  a 
resting  limb  are  taking  up  nourishment  as  well  as  oxygen 
from  the  blood  and  giving  off  their  waste  products — carbonic 
acid  and  ammonia,  though  not  a  drop  of  lymph  may  flow 
from  a  cannula  placed  in  a  lymphatic  trunk  of  the  limb.  In 
fact  the  interchange  of  material  between  tissue-cell  and  blood 
through  the  mediation  of  the  lymph  is  carried  out  m  the 
same  way  as  are  the  gaseous  interchanges,  viz.  by  a  process 
of  diffusion.  This  explanation  however  holds  good  only  for 
the  diffusible  constituents  of  the  blood  and  will  not  account 
for  the  supply  of  the  indiftusible  protein  molecules  to  the  cell. 
Apparently  the  only  way  in  which  the  tissues  can  obtam  their 
supply  of  protein  is  from  the  small  proportion  of  this  substance 
which  has  filtered  through  the  vessel-wall  into  the  lymph. 
The  increased  exudation  of  concentrated  lymph  to  the  tissues 
which  occurs  in  inflammatory  conditions  or  as  the  result  of 
injuiy  is  therefore  of  advantage,  since  it  furnishes  an  abun- 
dant suppl}'  of  protein  food  to  be  used  up  in  the  regeneration 
of  the  damaged  cells. 


298  PHYSIOLOGY 


CHAPTER   VIII 
THE    MECHANISMS    OF    DIGESTION 

Section  1 

GENEEAL  CHARACTEES  OF  THE  PROCESSES   OF 
DIGESTION 

We  have  already  mentioned  that  the  cells  derived  from 
the  hypoblast  of  the  embryo,  and  lining  the  inner  surface  of 
the  tube  from  which  the  body  is  formed,  are  alimentary  in 
function,  i.e.  they  have  the  office  of  taking  up  the  various 
foodstuffs  and  converting  them  into  a  form  suitable  for 
assimilation  by  the  other  tissues  of  the  body.  In  some  of  the 
lower  animals  the  cells  lining  the  alimentary  canal  devour 
the  food  particles  in  the  same  manner  that  the  amoeba  does, 
secreting  around  them,  after  ingestion,  a  fluid  which  seems 
to  dissolve  them.  In  the  higher  Vertebrates  this  process  is 
simplified,  in  that  the  cells  lining  the  canal  are  differentiated 
into  those  that  secrete  a  fluid  capable  of  dissolving  foodstuffs, 
and  those  which  have  the  duty  of  absorbing  the  foodstuffs 
that  have  been  rendered  soluble  by  the  action  of  the  digestive 
fluids.  The  secreting  cells  are  collected  together  in  depressions 
or  outgrowths  of  the  epithelial  lining  of  the  alimentary  canal 
to  form  glands  ;  and  the  secretions  in  different  parts  of  the 
canal  have  different  properties,  some  being  adapted  to  rendering 
soluble  the  starchy  constituents  of  food,  while  the  action  of 
others  is  limited  to  proteins. 

During  the  time  that  the  food  is  in  the  mouth  it  is  acted 
upon  by  the  mixed  secretions  of  the  parotid,  submaxillary, 
and  sublingual  salivary  glands.  And  here  we  find  the  chief 
digestive  action  consists  in  the  conversion  of  insoluble  starch 
into  soluble  dextrins  and  sugar. 

In  the  stomach  the  food  is  acted  on  by  the  gastric  juice, 
the   secretion   of   a   number  of   simple   tubular   glands  with 


THE   MECHANISMS   OF   DIGESTION  299 

which  the  mucous  membrane  is  thickly  set.  Its  chief  action 
is  on  the  proteins,  hydrating  these  and  converting  them  into 
albumoses  and  peptones.  In  the  duodenum  the  food  is  acted 
on  by  the  pancreatic  juice  and  the  bile,  the  secretion  of  the 
liver.  The  former  has  a  digestive  influence  on  all  three 
classes  of  foodstuffs,  converting  starch  into  sugar,  proteins 
into  peptones,  and  splitting  up  neutral  fats  with  the  forma- 
tion of  glycerin  and  free  fatty  acids.  In  the  small  and 
large  intestine  the  mucous  membrane  is  thickly  set  with  a 
number  of  simple  tubular  glands,  which  are  called  Lieber- 
kiihn's  follicles.  These  secrete  an  alkaline  juice,  which  has 
only  slight  digestive  powers.  It  contains  invert  ferments 
which  convert  cane-sugar  into  laevulose  and  dextrose,  and 
maltose  into  dextrose,  and  also  a  ferment,  crepsin,  which  has 
the  proj)erty  of  causing  a  complete  hydrolysis  of  the  albumoses 
and  peptones  already  formed  by  the  agency  of  gastric  and  pan- 
creatic juices,  and  converting  them  into  amino-acids  and  bases. 

Ferments. — All  the  digestive  juices  owe  their  powers  to 
the  presence  in  them  of  certain  ferments  ;  and  we  ma}^  take 
the  opportunity  of  saying  a  few  words  with  regard  to  ferment 
action  in  general.  Ferments  enter  or  are  said  to  enter  into 
most  of  the  physiological  processes  of  the  body.  To  a  fer- 
ment has  been  ascribed  a  prominent  part  in  the  coagulation 
of  the  blood  ;  and  we  shall  meet  with  them  later  on  in  con- 
sidering the  functions  of  the  liver  and  kidney.  But  it  is  in 
digestion  that  these  bodies  play  the  most  important  part.  In 
all  the  changes  that  are  effected  by  their  agency  there  is  the 
conversion  of  a  body  of  high  potential  energy  into  one  with 
less  potential  energy ;  and  this  conversion  is  in  most  cases 
associated  with  hydrolysis,  i.e.  the  original  body  is  combined 
with  one  or  more  molecules  of  water  to  form  the  new  substance 
of  lower  potential  energy. 

In  inquiring  into  the  nature  of  ferments  we  are  met  at  the 
outset  with  the  difficulty  that  probably  no  one  has  ever  pre- 
pared a  pure  ferment ;  so  that  we  can  only  study  their  pro- 
perties by  studying  those  of  the  fluids  or  precipitates  presumed 
to  contain  a  ferment  from  the  fact  that  they  can  give  rise  to 
certain  changes  in  other  substances.  First,  as  to  the  condi- 
tions of  their  activity.  A  ferment  such  as  diastase  can  convert 
an  indefinite  amount  of  starch  into  sugar,  provided  that 
the  product  of  its  activity  {i.e.  the  sugar)  be  not  allowed  to 


300  PHYSIOLOGY 

accumulate  in  too  large  a  quantity.  Thus  l)y  increasing  the 
strength  of  a  ferment  solution  we  do  not  increase  the  amount 
of  substance  it  is  able  to  transform,  but  merely  the  rapidity 
of  its  action. 

The  exact  effect  of  increasing  the  amount  of  ferment  varies  according  to  the 
nature  of  the  ferment  in  question.  Thus  in  some  cases,  as  with  diastase,  the 
rapidity  of  change  of  the  starch  is  in  direct  proportion  to  the  amount  of  fer- 
ment present.  If  a  given  solution  takes  four  minutes  to  convert  a  starch  solu- 
tion to  the  '  achromic '  point  {i.e.  the  stage  at  which  no  coloration  is  given  with 
iodine),  a  feniient  solution  of  double  the  strength  will  take  only  two  minutes  to 
effect  the  same  change.  In  the  case  of  other  ferments,  such  as  pepsin  and 
trypsin,  it  is  necessary  to  increase  the  concentration  of  the  ferment  four  times 
in  order  to  double  the  rate  of  change  produced. 

A  ferment  is  active  only  within  certain  limits  of  tempera- 
ture ;  and  for  each  ferment  there  is  a  certain  optimum 
temperature  at  which  its  activity  is  greatest.  This,  for  the 
ferments  met  with  in  the  bod}'-,  is  between  40°  and  45°  C. 
For  the  diastase  of  malt  it  is  between  60°  and  65°  C.  At  a 
temperature  of  0°  the  activity  of  all  ferments  that  occur  in 
warm-blooded  animals  is  indefinitely  checked,  although  it  has 
been  shown  that  the  pepsin  or  gastric  ferment  of  fishes  still 
preserves  some  power  at  this  temperature.  At  65°  C.  all  fer- 
ments met  with  in  the  body  are  destroyed,  and  do  not  recover 
on  cooling. 

They  are  soluble  in  distilled  water,  and  precipitated  from 
their  solutions  by  alcohol.  The  digestive  ferments  are  pre- 
cipitated by  saturation  of  their  solutions  with  ammonium 
sulphate.  This  method  has  been  used  for  obtaining  them  in 
a  state  approaching  purity,  and  they  have  been  found  to  have 
the  general  composition  of  proteins.  The  amount  that  can 
be  collected  however  is  so  small  that  it  is  impossible  to  make 
an  accurate  study  of  their  properties,  and  even  then  we  do 
not  know  whether  the  substance  we  have  represents  the  pure 
ferment,  or  is  merely  a  protein  to  which  the  ferment  is  inti- 
mately adherent. 

A  ferment  is  therefore  a  body  which  can  effect  changes  in 
a  surrounding  fluid  of  certain  bodies  of  high  potential  energy 
into  bodies  of  more  stable  composition  with  an  evolution  of 
kinetic  energy  in  the  form  of  heat,  without  itself  being  used 
up  in  the  process. 

It  has  long  been  known  that  the  action  of  a  ferment 
was  impeded  and  finally  checked  by  the  accumulation  of  the 


THE   MECHANISMS   OF   DIGESTION  301 

products  of  its  activity  in  the  solution.  Thus  diastase  added 
to  a  strong  solution  of  starch  will  convert  a  certain  amount  of 
it  into  maltose,  but  the  process  will  cease  before  the  whole  of 
the  starch  is  so  converted.  If  now  the  solution  be  diluted,  or 
the  maltose  removed  by  dialysis,  the  action  will  recommence. 
In  this  respect  the  reaction  exactly  resembles  those  known  in 
chemistry  as  reversible  reactions. 

The  question  arises  whether  a  ferment  action  can  also  be 
reversible.  An  answer  to  this  question  has  been  supplied  by 
Croft  Hill  in  the  affirmative.  The  inverting  ferment,  maltase, 
contained  in  yeast  and  in  the  succus  entericus,  converts 
maltose  into  dextrose.  If  however  the  ferment  be  allowed  to 
act  on  strong  solutions  of  dextrose,  it  converts  a  small  pro- 
portion of  this  back  into  maltose.  The  changes  in  the  two 
cases  are  as  follows  : 

C,2H,20,,  +  H.,0  =  2C,H,,0,- 
2C,3H,,0,3  =:  C,;H,,0„  +  H,0 

This  observation  is  of  importance  as  showing  that  synthetic 
changes  can  occur  in  the  body  apart  from  any  activity  of 
cells,  provided  only  that  the  right  conditions  are  present. 
It  is  evident  that,  if  the  sugar  were  contained  in  a  vessel 
whose  walls  were  permeable  to  or  had  a  distinct  affinity  for 
maltose,  so  that  this  was  removed  as  fast  as  it  was  formed, 
the  ferment  that  normally  converts  maltose  into  dextrose 
(a  downward  change)  might  convert  the  whole  of  the  dextrose 
into  maltose. 

The  changes  brought  about  by  ferments  can  in  most 
cases  be  also  effected  by  ver}^  simple  means,  such  as  heating 
with  water  under  pressure  or  warming  with  dilute  acids. 
The  changes  are  in  nearly  all  cases  hydrolytic,  the  original 
substance  taking  up  water,  and  splitting  into  simple  sub- 
stances. In  this  process  the  ferment  adds  neither  to  the 
products  of  the  reaction  nor  to  the  energy  evolved  in  the 
reaction.  The  only  factor  altered  by  the  ferment  is  the 
velocity  of  the  reaction.  Cane-sugar  in  the  presence  of  water 
is  slowly  inverted  into  Ifevulose  and  dextrose.  If  a  little 
mineral  acid  be  present,  or  some  invertin  be  added,  the 
reaction  takes  place  within  a  few  minutes. 

The  nature  of  ferment  action  may  be  better  conceived 
if  we  compare  it  with  certain  changes  that  have  long  been 


302  PHYSIOLOGY 

known  in  inorganic  chemistry,  and  are  spoken  of  as  katalytic 
changes.  Thus  nitrogen  trichloride  may  be  made  to  explode 
by  contact  with  various  substances,  such  as  phosphorus  or 
oil.  In  its  explosion  it  splits  up  into  free  nitrogen  and 
chlorine— molecules  which  are  more  stable,  and  have  there- 
fore less  potential  energy,  than  those  of  the  original  nitrogen 
trichloride.  In  the  manufacture  of  sulphuric  acid,  nitric 
oxide  is  used  as  a  carrier  of  oxygen  from  the  atmosphere  to 
the  sulphur  dioxide  produced  by  the  burning  of  sulphur. 
Thus  : 

NO       +  0     =     NO,. 

Nitric  oxide  Nitrogen  tetroxide 

SO,  +  NO.3  -!-  H,0  =  H2SO,  +  NO. 

Sulphur  dioxide  Sulphuric  acid     Nitric  oxide 

In  this  case  we  have  at  the  end  of  the  reaction  the  same 
amount  of  nitric  oxide  that  we  started  with  ;  and  it  would 
be  theoretically  possible,  by  using  a  small  quantity  of  nitric 
oxide  as  oxygen-carrier,  to  convert  an  indefinite  amount  of 
sulphur  dioxide  into  sulphuric  acid.  Since  in  this  reaction 
we  know  the  exact  chemical  processes  that  go  on,  the  w^ord 
'  katalysis  '  is  not  used.  On  the  other  hand,  we  employ  this 
word  in  speaking  of  the  splitting  up  of  hydrogen  peroxide  by 
means  of  spongy  platinum  into  water  and  oxygen  : 

2H2O.  =  2H,0  +  0,. 

Hydrogen  peroxide 

But  the  difference  is  probably  merely  one  of  degree.  The 
substance  which  acts  katalytically  exercises  an  attraction  on 
one  of  the  atoms  in  the  unstable  molecule,  which  is  sufficient 
to  give  the  impetus  to  its  decomposition,  although  not  leading 
to  an  actual  combination  of  the  two,  as  in  the  case  of  the 
nitric  oxide  quoted  above.  So  we  may  suppose  that  the 
invert  ferment,  for  instance,  combines  with  a  molecule  of 
water,  and  passes  it  on  to  the  cane-sugar  ;  or  it  may  be  that 
it  merely  exercises  an  attraction  on  some  of  the  constituents 
of  the  cane-sugar  molecule,  so  increasing  its  tendency  to 
break  up  and  unite  with  the  surrounding  molecules  of  water, 
with  the  evolution  of  heat  and  the  production  of  the  more 
stable  bodies,  laevulose  and  dextrose. 


THE   ]\IECHANISMS   OF    DIGESTION  303 

Tho  ferments  which  play  so  important  a  part  in  the 
digestive  functions  belong  to  the  class  of  unorganised  fer- 
ments. The  term  '  ferment '  has  been  applied  to  another  class 
of  bodies,  which  are  distinguished  as  the  organised  ferments. 
These  are  living  organisms  which  have  the  power  of  inducing 
definite  changes  in  the  media  in  which  they  live.  This 
faculty  is  intimately  bound  up  with  the  life  of  these  organisms. 
Destruction  of  this  by  the  action  of  small  amounts  of 
chloroform,  or  by  subjecting  them  for  some  time  to  the  action 
of  absolute  alcohol,  irrevocably  destroys  their  fermentative 
properties.  With  the  organised  as  with  the  unorganised 
ferments  there  is  a  change  of  the  affected  substance  from 
a  condition  of  high  to  one  of  lower  potential  energy.  The 
changes  induced  however  are  often  much  more  than  a  mere 
hydrolysis.  The  yeast  fungus,  for  instance,  converts  sugar 
into  alcohol.  In  this  process  it  has  been  thought  that  the 
change  represents  the  metabolism  of  the  organism  itself. 
But  it  is  becoming  more  and  more  difficult  to  draw  a 
hard  and  fast  line  between  the  two  kinds  of  action.  The 
ammoniacal  fermentation  of  urine  depends  on  the  presence 
of  a  micro-organism,  the  Micrococcus  urecc,  which  converts 
urea  into  ammonium  carbonate. 

/NHj  /ONH, 

COv  +  2H.,0  =  C0( 

^NH.,  '  \ONH, 

Urea  Ammonium  carbonate 

This  action  is  stopped  by  antiseptics.  It  is  possible  how- 
ever to  kill  the  micro-organisms  and  extract  from  the  dead 
cells  an  unorganised  ferment  which  has  exactly  the  same 
effect.  In  the  same  way  it  has  been  shown  that,  if  yeast 
cells  be  thoroughly  broken  up  and  pressed,  an  unorganised 
juice  is  obtained  which  converts  dextrose  into  alcohol.  In 
fact  the  organised  ferments  seem  to  act  by  ferments  enclosed 
within  their  constituent  cells,  whereas  the  unorganised  fer- 
ments are  formed  by  living  cells  and  excreted  so  as  to  act 
outside  the  cell. 

Eecent  research  tends  more  and  more  to  enhance  the 
significance  of  these  ferment-like  bodies,  which  acting  in 
infinitesimal  quantities  are  able  to  exert  so  important  an 
influence  on  the  direction  and  velocity  of  chemical  reactions. 


304  PHYSIOLOGY 

In  the  alimentary  canal  the  following  ferments  are 
found :  — 

Ptyalin  (saliva)  ;  amylopsin  (pancreatic  juice).  Starch  to 
maltose. 

Invertin.  Cane-sugar  to  invert  sugar  (dextrose  and 
laevulose). 

Lactase.     Lactose  to  dextrose  and  galactose. 

Maltase.     Maltose  to  dextrose. 

Pepsin.     Proteins  to  albumoses  and  peptones. 

Trypsin.     Proteins  to  peptones,  amino-acids,  etc. 

Erepsin.  Albumoses  and  peptones  to  amino-acids  and 
hexone  bases. 

Steapsin.     Neutral  fats  to  fatty  acids  and  glycerin. 

Bennin.     Caseinogen  to  casein. 

A  whole  array  of  bodies  of  this  class  are  moreover  found 
in  the  cells  and  body-fluids  under  normal  and  pathological 
conditions,  e.g.  toxins  and  antitoxins,  coagulins  and  haemo- 
lysins  with  their  '  anti '  bodies,  oxydases,  etc. ;  and  it  seems 
not  improbable  that  all  chemical  processes  of  cells  may  in 
time  be  reduced  to  ferment  actions,  the  functions  of  the  living 
protoplasm  being  confined  to  the  determination  of  the  direc- 
tion of  the  process  at  any  moment. 


THE   MECHANISMS  OF  DIGESTION 


805 


Section  2 
SALIVAEY  DIGESTION 

The  saliva  is  a  mixture  of  the  secretions  of  the  sub- 
maxillary, sublingual,  parotid,  and  small  mucous  and  serous 
glands  of  the  buccal  cavity  (Fig.  160).  It  has  a  low  specific 
gravity,  1002  to  1009 ;  it  is  slightly  alkaline,  and  slimy  from 
the  presence  of  mucin.  On  microscopic  examination  it  is  seen 
to  contain  epithelial  scales  and  *  salivary  corpuscles  ' — small 

Fig.  160. 


Dissection  to  display  the  salivary  glands,  a,  sublingual  gland  ; 
b,  submaxillary  gland ;  c,  parotid  gland ;  d,  common  opening 
of  ducts  of  submaxillary  and  sublingual  glands ;  i,  opening  of 
duct  of  parotid  gland. 

round    cells    with    granular   contents,   which    are   probably 
leucocytes  escaped  from  the  tonsils.     It  consists  of — 

Water. 

Salts,  especially  potassium  and  sodium  chlorides. 

Traces  of  albumen. 

Mucin. 

A  diastatic  ferment  (ptyalin). 

Occasional  traces  of  potassium  sulphocyanide. 

Gases,  especially  carbon  dioxide,  with  traces  of  oxygen  and 
nitrogen. 

The  amount  of  saliva  secreted  in  twenty-four  hours  varies 
from  one-half  to  two  litres.  The  greater  part  is  re-absorbed 
in  the  alimentary  canal. 

20 


306  PHYSIOLOGY 


Action  of  the  Saliva  on  the  Food 

Saliva  chiefly  serves  to  moisten  foodstuffs,  and  so  aid  in 
mastication  and  deglutition.  This  is  indeed  in  carnivora  its 
sole  function.  In  herbivora  and  man  it  exercises  a  digestive 
action  on  starch  by  virtue  of  the  ptyalin  it  contains.  If  some 
saliva  be  added  to  some  boiled  starch  in  a  test-tube,  and 
the  mixture  be  kept  at  35°  C.  for  some  time,  the  starch  is 
gradually  converted  into  a  mixture  of  maltose  and  dextrin. 
The  first  stages  of  this  conversion  are  extremely  rapid,  as  is 
shown  by  the  fact  that  if  a  warm  decoction  of  starch  be  taken 
into  the  mouth,  kept  there  for  15  to  30  seconds,  and  then 
ejected,  the  coloration  with  iodine  has  entirely  disappeared, 
all  the  starch  having  in  this  short  time  been  converted 
into  dextrin  and  maltose.  The  stages  of  the  process  are  as 
follows  : 

1.  Starch,  opalescent  solution,  blue  with  iodine. 

2.  Soluble  starch,  clear  solution,  blue  with  iodine. 

3.  A  mixture  of  dextrins,  erythro-  and  achroodextrin. 
The  former  with  iodine  gives  a  mahogany-red  colour. 

4.  Maltose  and  achroodextrin,  the  erythrodextrin  being 
converted  into  maltose,  while  some  of  the  achroodextrin  re- 
mains unaffected.  The  liquid  now  reduces  Fehling's  solution 
by  means  of  the  maltose  it  contains,  and  gives  no  coloration 
with  iodine.  Addition  of  large  excess  of  absolute  alcohol 
gives  a  white  precipitate  of  achroodextrin.  This  ferment 
action  is  dependent  on  temperature,  is  most  active  at  about 
40°  C,  and  is  finally  abolished  at  about  60°  C.  It  can  only 
take  place  in  a  neutral  or  slightly  alkaline  medium,  the 
ferment  being  destroyed  in  the  presence  of  acid. 

It  may  seem  strange  that  the  hydrolysis  of  starch  with  the  formula  CuHuOj 
should  result  in  the  formation  of  dextrin  with  the  same  formula.  It  must  be 
remembered  however  that  these  formulae  are  the  simplest  possible,  and  that 
the  colloidal  starch  molecule  is  probably  at  least  one  hundred  times  as  large  as 
the  molecule  represented  by  CJi^^Oy  The  hydrolysis  of  the  starch  takes  place 
by  stages  resulting  in  the  production  of  a  number  of  dextrins  intermediate 
between  starch  and  maltose.  The  formation  of  maltose  begins  at  the  very 
onset  of  the  hydrolysis,  so  that  we  may  conceive  the  soluble  starch  molecule  as 
made  up  of  a  number  of  groups  3(CsH|o05).  Each  of  these  groups  would  take 
up  one  molecule  of  water  with  the  formation  of  maltose  and  a  dextrin. 

3(C,H,A)  +  H,0  =  C,H,A  +  C,,H,,0„. 

Dextrin  Maltose 


THE   :\IECHANISMS    OF   DIGESTION 


307 


The  dextrins  themselves  however  are  in  most  cases  more  complex  bodies  than 
is  here  represented,  belonging,  like  the  parent  molecule,  to  the  class  of  bodies 
known  as  colloids. 


The  Mechanism  of  Salivary  Secretion 

The  digestive  juices  are  formed  by  the  agency  of  glands. 
These  are  recesses  or  branching  tubules  lined  with  a  con- 
tinuation of  the  general  alimentary  epithelium.  This  secre- 
tory epithelium  is  separated  by  a  basement  membrane  from 
the  surrounding  connective  tissue,  in  which  ramify  blood- 
vessels, lymphatics,  and  nerves.  The  secretory  cells  are 
bathed  by  the  lymph  that  exudes  from  the  capillaries  ;  and 
from  this  lymph  they  select  the  substances  necessary  for 
their  nourishment,  and  form  therefrom  the  special  ingredients 

Fici.  161. 


A,  serous  gland  ;  B,  pure  mucous  gland  from  mouth.     (Kolliker.) 
«,  ducts  ;  /,  fat  cells. 

of  their  secretion,  which  they  turn  out  into  the  lumen  of 
the  gland  tubule.  This  process  is  an  act  of  vital  selection 
by  the  cell,  and  is  not  a  mere  filtration  or  transudation 
of  certain  constituents  of  lymph  through  the  epithelial 
membrane. 

The  salivary  glands  are  divided  into  three  classes,  the 
mucous  glands,  the  serous  glands,  and  glands  which  partake 
of  the  characters  of  both  these  groups.  Chief  among  the 
last  class  are  the  demilune  glands,  which  are  often  spoken  of 
as  mucous. 

Pure  mucous  glands  are  found  scattered  over  the  tongue 
and  mucous  membrane  of  the  mouth  (Fig.  161,  b).  They 
have  large  acini  which  are  lined  with  large  clear  cells,  dis- 
tended with  mucin.  These  acini  open  into  a  short  duct  lined 
with  striated  cubical  epithelium.     Their   secretion    is  thick 


308  PHYSIOLOGY 

and  viscid  and  consists  chiefly  of  a  solution  of  mucin.  The 
serous  glands  occur  as  small  follicles  scattered  over  the 
lining  of  the  oral  cavity.  They  have  small  tubular  acini 
which  are  lined  with  polyhedral  granular  cells  (Fig.  161,  a). 
Their  secretion  is  watery  and  consists  chiefly  of  water,  salts, 
and  ptyalin  with  a  trace  of  albumen  and  globulin. 

Of  the  three  large  salivary  glands  in  man,  the  parotid, 
submaxillary,  and  lingual,  the  parotid  represents  a  pure 
serous  gland.  The  other  two  belong  to  the  class  of  demilune 
glands  :  in  the  submaxillary  of  man  however,  lobules  of  this 
nature  are  mixed  with  pure  serous  acini. 

The  demilune  type  conforms  to  the  purely  mucous  gland 
in  that  its  acini  are  large  and  are  lined  (in  hardened  speci- 
mens) by  a  complete  layer  of  large  clear  swollen  cells,  which 
yield  a  mucous  secretion.  Between  these  cells  and  the  base- 
ment membrane  however  is  another  type  of  cells  which  stain 
deeply  with  haematoxylin,  are  granular,  and  are  denoted  from 
their  shape  crescentic  or  demilune  cells.  These  cells  were 
formerly  supposed  to  take  the  place  of  the  mucous  cells 
destroyed  in  the  process  of  secretion,  but  they  have  their  own 
system  of  ducts,  and  there  seems  little  doubt  that  they  are 
serous  cells  taking  an  active  part  in  the  secretory  process. 

All  these  glands  pour  their  secretions  into  the  mouth,  the 
active  secretion  being  excited  by  the  presence  of  food  in  the 
mouth,  and  by  the  movements  of  mastication. 

Changes  accompanying  Activity 

The  salivary  glands  may  be  made  to  secrete  by  adminis- 
tration of  pilocarpin  or  by  stimulation  of  their  nerves.  On 
histological  examination  it  is  found  that  activity  is  associated 
with  marked  changes  in  appearance  of  the  cells.  If  we 
examine  a  section  of  a  mucous  gland  that  has  been  in  a 
resting  condition  for  some  time  ('  resting  gland  '),  the  acini 
are  seen  to  be  distended  with  large  cells  having  clear  hyaline 
contents,  so  close  together  that  no  lumen  can  be  seen  (Fig. 
162).  The  nuclei  situated  at  the  outer  border  of  the  cells, 
near  the  basement  membrane,  appear  shrivelled,  with  irregu- 
lar margins. 

In  a  section  made  through  a  discharged  gland  {i.e.  one 
that  has  been  actively  secreting  for  some  time),  the  acini  and 


THE   MECHANISMS   OF  DIGESTION 


B09 


the  cells  are  smaller,  the  lumen  quite  distinct,  and  the  nuclei 
round  and  swollen  (Fig.  163).  The  whole  section  appears 
darker  from  the  fact  that  the  cells  have  taken  up  the  stain- 
ing fluid  more  readily.  The  difference  between  the  sections 
depends  chiefly  on  the  fact  that,  in  the  sections  of  the  resting 
gland,  the  cells  are  distended  with  mucin,  which  does  not  take 
up  the  staining  agent  and  gives  the  cells  their  clear  hyaline 

Fig.  162. 


Submaxillary  gland  of  dog,  after  prolonged  rest  (Eanvier).     I,  lumen 
of  alveolus  ;  g,  mucous  cells  ;  c,  demilune  cells. 


Same  gland  attei  piolonyd  activity. 

appearance.  When  secretion  occurs,  the  mucin  is  discharged 
into  the  lumen,  so  that  the  cells  shrink  and  consist  more 
largely  of  protoplasm. 

If,  instead  of  examining  sections  of  hardened  glands,  we 
examme  fresh  glands  teased  in  normal  saline  fluid,  or  fixed 
with  osmic  acid  vapour,  the   appearance  presented   is  quite 


310 


PHYSIOLOGY 


different.  The  cells  of  the  restmg  gland  are  not  clear  and 
hyalme,  but  are  full  of  coarse  granules.  When  secretion 
occurs  these  granules  disappear.  If  to  a  fresh  specimen  of 
resting  gland  any  of  the  ordinary  hardening  agents  (such  as 
spirit  or  Muller's  fluid)  be  added,  the  granules  are  seen  to 
swell  up  and  fuse  together  to  form  a  hyaline  mass  distend- 
ing the  cell.  In  fact  the  ordmary  picture  of  the  hardened 
resting  gland  is  reproduced  (Fig.  164). 

We  see  then  that  the  resting  gland  in  a  normal  condition 
does  not  contain  mucin,  but  contains  a  precursor  of  mucin — 
mucigen,  which  appears  in  the  form  of  granules.  As  these  are 
turned  out  of  the  cell   they  undergo  some  change,  perhaps 

Fig.  164. 


Mucous  cells  from  a  fresh  submaxillary  gland  of  a  dog  (Langley). 
a.  Mucous  cell  examined  fresh  from  a  resting  gland,  a'.  The 
same  cell  treated  with  weak  alcohol,  b  and  b'.  Cells  from  a 
discharged  gland  before  and  after  treatment  with  weak  alcohol. 

associated  with  imbibition  of  water,  and  are  transformed  into 
mucin. 

A  similar  change  occurs  in  the  serous  glands  when  secre- 
tion takes  place.  The  cells  of  the  resting  parotid  gland, 
examined  in  normal  saline  fluid,  are  swollen  and  full  of  fine 
granules.  With  activity  these  granules  are  discharged,  and 
the  cells  shrink  and  become  clearer  (Fig.  165). 

In  the  pancreas  of  the  rabbit,  which  is  very  similar  in 
structure  to  a  serous  salivary  gland,  the  changes  coincident 
with  secretion  can  be  observed  in  the  living  animal,  since 
here  the  gland  is  spread  out  between  the  layers  of  the  mesen- 
tery, so  that  individual  acini  may  be  examined  under  high 


THE   MECHANISMS   OF   DIGESTION 


311 


powers.  When  the  resting  gland  is  observed  in  this  way, 
each  acinus  is  seen  to  be  composed  of  two  zones,  an  outer 
clear  zone  and  an  inner  granular  zone.  The  outlines  of  the 
cells  cannot  be  distinguished  (Fig.  166).  When  secretion  is 
excited  by  the  injection  of    pilocarpin  or    other  means,  the 

Fig.  165. 


Acini  of  a  serous  salivary  gland.     A.  Besting  condition. 
B.  Discharged  condition.     (Langley.) 

inner  zone  clears  up,  the  granules  being  discharged  into  the 
lumen  ;  the  homogeneous  outer  zone  becomes  wider,  while 
the  nuclei  and  borders  of  the  individual  cells  can  be  clearly 
made  out. 

The  amount  of  ferment  to  be  extracted  from  the  pancreas 
seems  to  be  directly  proportional  to  the  number  of  granules 

Fi.i.  106. 


A  terminal  lobule  of  the  pancreas  of  the  rabbit.     A.  In  resting  condition. 
B.  After  active  secretion.     (Kiihne  and  Sheridan  Lea.) 


present  in  the  cells.  But  we  have  evidence,  best  marked 
perhaps  in  the  case  of  the  gastric  glands,  that  these  granules 
do  not  themselves  represent  the  ferment  but  are  merely 
precursors  of  the  ferment,  just  as  the  mucigen  granules 
in  the  submaxillary  gland  are  precursors  of  mucin.  These 
precursors  of  ferments  are  spoken  of  as  zymogens. 


312  PHYSIOLOGY 

Two  changes  occur  in  the  cell  when  secretion  takes  place. 

1.  A  transformation  of  zymogen  granules  into  ferment,  of 
mucigen  into  mucin,  which  substances  are  then  discharged 
from  the  cell  into  the  lumen  of  the  gland. 

2.  A  building  up  or  reintegration  of  protoplasm,  as  evi- 
denced by  the  growth  in  extent  of  the  stainable  protoplasmic 
parts  of  the  cell. 

During  rest  a  twofold  process  is  probably  going  on. 

1.  A  further  building  up  (anabolism)  of  the  protoplasm 
of  the  cell  out  of  the  constituents  of  the  surrounding  lymph. 

2.  A  katabolism  or  breaking-down  of  the  cell-protoplasm, 
with  the  formation  of  zymogen  granules,  which  are  stored  up 
in  the  cell  till  the  economy  requires  that  they  should  be  con- 
verted into  ferment  and  discharged  into  the  lumen. 

Active  secretion  is  associated  in  the  living  body  with — 

1.  Increased  blood-supply.  If  secretion  be  excited  in  the 
submaxillary  gland  by  stimulation  of  the  chorda  tympani 
nerve,  a  cannula  having  been  previously  inserted  into  the 
distal  end  of  a  vein  coming  from  the  gland,  the  amount  of 
blood  flowing  from  the  vein  is  increased  eight  or  ten  times. 
Before  excitation  the  blood  drops  slowly  from  the  cannula  ; 
during  excitation  it  runs  freely,  has  a  bright  arterial  red 
colour,  and  the  stream  may  present  pulsations,  transmitted 
from  the  arteries  through  the  capillaries. 

2.  Increased  production  of  CO.,,.  The  venous  blood  how- 
ever appears  bright  red,  since  this  increased  production  is 
more  than  compensated  by  the  increased  flow  of  blood  through 
the  gland. 

3.  Electrical  changes. 

If  the  outer  surface  and  hilus  of  the  submaxillary  gland  be  led  off  to  a 
galvanometer,  it  is  found  that  there  is  a  slight  resting  current  passing  in  the 
dog's  gland  from  hilus  to  outer  surface.  "When  secretion  is  excited,  this  current 
undergoes  modification  in  a  positive  or  negative  direction,  and  it  seems  that  the 
two  events  of  secretion — cell  change  and  passage  of  fluid  through  the  gland — 
may  give  rise  to  opposite  results.  Thus  stimulation  of  the  chorda  tympani  nerve 
gives  a  positive  followed  by  a  negative  variation  ;  stimulation  of  the  sympathetic 
gives  a  pure  negative  effect.  The  outgoing  current  therefore  seems  to  be  asso- 
ciated with  the  passage  of  fluid  through  the  gland-cells. 

It  might  be  thought  that  the  secretion  was  a  result  of  the 
larger  flow  of  blood  through  the  gland,  and  indeed  of  the 
raised  pressure  in  the  capillaries,  consequent  upon  the  dila- 
tation of   the    arterioles  causing  an  increased   transudation. 


THE   MECHANISMS   OF   DIGESTION  313 

The  following  facts  however  show  that  secretion  is  an  active 
process  of  the  epithelial  cells,  and  is  not  dependent  on  filtra- 
tion. 

1.  If  manometers  be  inserted  in  the  carotid  artery  and 
in  the  duct  of  the  submaxillary  gland,  the  pressure  of  the 
secretion  may  be  double  as  high  as  the  blood-pressure  in  the 
carotid,  so  that  fluid  is  flowing  from  the  blood-vessels  at  a 
low  pressure  into  the  duct  at  a  high  pressure — a  process  not 
explicable  by  any  theory  of  filtration. 

2.  If  atropin  be  administered,  stimulation  of  the  chorda 
tympani  nerve  produces  no  secretion  in  the  submaxillary 
gland,  although  dilatation  of  the  blood-vessels  takes  place  as 
usual. 

3.  Some  secretion  may  be  caused  by  stimulating  the 
chorda  tympani  nerve  in  a  head  recently  severed  from  the 
hodj. 

4.  If  the  submaxillary  gland  be  enclosed  in  a  plethys- 
mograph,  stimulation  of  the  chorda  tympani  nerve  causes 
a  shrinkage  of  the  gland,  in  spite  of  the  concomitant  vascular 
dilatation,  showing  that  the  fluid  secreted  is  supplied 
immediately  by  the  cells,  and  that  these  only  recoup  them- 
selves later  from  the  lymph  and  blood.  A  primary  increased 
transudation  from  the  blood-vessels  would  of  course  be 
accompanied  by  an  increase  in  volume  of  the  gland. 

Nerve-supply 

The  salivary  glands  have  a  double  nerve-supply,  from  the 
sympathetic  and  from  the  cranial  nerves.  The  submaxillary 
gland  receives  its  sympathetic  fibres  from  branches  of  the 
cervical  sympathetic  which  ramify  on  the  facial  artery,  and 
its  cranial  fibres  from  the  chorda  tympani  nerve.  These  fibres 
run  for  a  short  time  with  the  lingual  nerve,  and  then  leave 
it  as  a  slender  nerve  which,  reaching  Wharton's  duct  (duct 
of  submaxillary  gland),  runs  along  this  to  the  gland.  The 
fibres  are  connected  in  the  hilus  of  the  gland  with  nerve-cells. 
A  small  collection  of  nerve-cells — the  '  submaxillary '  ganglion 
— is  found  m  the  triangle  between  the  chorda  tympani  nerve, 
lingual  nerve,  and  duct.  With  the  cells  of  this  ganglion  are 
connected  fibres  of  the  chorda  tympani  going  to  supply  the 
sublingual  gland  (Langley). 


314 


PHYSIOLOGY 


Different  effects  are  obtained  according  as  the  chorda 
tympani  or  the  sympathetic  fibres  are  stimulated.  Stimula- 
tion of  the  chorda  tympani  in  the  dog  gives  rise  to  an  active 
dilatation  of  the  vessels  of  the  gland,  and  a  copious  watery 
secretion  containing  only  a  small  amount  of  mucin  and 
formed  elements. 

Stimulation  of  the  sympathetic  causes  constriction  of  the 
vessels,  and  a  scanty  flow  of  very  thick  viscid  saliva,  rich  in 

Fig.  107. 


Diagram  of  nerve-supply  to  submaxillary  gland.  Sm.G.  Sub- 
maxillary gland.  N.L.  Lingual  nerve.  Ch.T.  Chorda  tympani. 
Sm.Gl.  Submaxillary  ganglion.  Sm.D.  Wharton's  duet.  V.J. 
Jugular  vein.  C.A.  Carotid  artery.  G.C.S.  Superior  cervical 
ganglion.  N.S.  Sympathetic  fibres  ramifying  on  facial  artery. 
(After  Foster.) 


mucin  and  formed  elements.  The  changes  that  occur  in  the 
cells  are  much  more  marked  under  sympathetic  than  under 
chorda  stimulation.  In  consequence  of  the  differences  in 
the  action  of  these  two  sets  of  nerve-fibres,  they  have  been 
supposed  to  have  two  distinct  functions.  The  chorda  fibres 
are  vaso-dilator  and  secreto-motor  of  water ;  the  sympathetic 
fibres  are  vaso-constrictor  and  secreto-motor  of  organic 
matter.  The  latter  have  been  also  denoted  trophic,  because 
of  the  marked  change  in  the  cells  that  is  caused  by  their 
stimulation.  They  might  well  be  called  the  katabolic  fibres 
of  the  gland. 

There  is  no  doubt  that  the  difference  in  the  action  of  the  two  sets  of  nerves 
is  in  some  degree  dependent  on  the  variations  in  the  blood-supply  produced  at 
the  same  time,  and  that  the  chorda  saliva  is  more  watery  because  the  gland- 
cells  have  more  fluid  at  their  disposal. 

According  to  Langley  the  actions  of  the  gland-fibres  of  the  sympathetic  and 


THE   MECHANISMS   OF  DIGESTION  31.5 

chorda  tympani  nerves  are  probably  identical,  the  differences  in  the  saliva 
obtained  by  stimulation  of  the  two  sets  of  nerves  being  conditioned  by  the 
concomitant  vascular  changes.  Against  this  view  may  be  urged  the  fact  that 
while  atropin  paralyses  the  secretory  fibres  of  the  chorda  tympani,  it  has 
practically  no  effect  on  those  derived  from  the  sympathetic. 

The  parotid  gland  has  also  a  double  nerve-supply:  fibres 
from  the  cervical  sympathetic,  and  cerebro-spinal  fibres  run- 
ning in  the  auriculo-temporal  branch  of  the  fifth  nerve,  but 
originating  probably  from  the  glosso-pharyngeal  and  running 
through  the  tympanic  branch  of  this  nerve  (nerve  of  Jacobson). 
Stimulation  of  the  cerebro-spinal  fibres  produces  in  the  rabbit 
and  dog  a  copious  flow  of  limpid  saliva  of  low  specific  gravity. 
Stimulation  of  the  sympathetic  causes  in  the  rabbit  a  scanty 
flow  of  saliva  free  from  mucin,  but  containing  more  proteins 
and  ferment  than  the  cerebro-spinal  secretion.  In  the  dog 
stimulation  of  the  sympathetic  causes  no  secretion,  although 
the  changes,  that  we  have  already  described  as  accompanying 
activity,  take  place  in  the  cells. 

Reflex  secretion. — The  secretion  of  saliva  is  normally 
brought  about  reflexly  by  stimulation  of  the  branches  of  the 
fifth  and  glosso-pharyngeal  nerves  distributed  to  the  mucous 
membrane  of  the  mouth  and  tongue,  the  stimulus  being 
furnished  by  the  presence  of  food  in  the  mouth,  by  acids,  or 
by  the  masticatory  movements. 

The  centre  for  the  secretion  of  saliva  is  located  in  the 
medulla,  since  from  this  part  of  the  central  nervous  system 
arise  both  the  aft'erent  and  efferent  nerves  by  which  the 
secretion  is  regulated.  The  peripheral  nerve-cells,  such  as 
the  collection  that  goes  by  the  name  of  the  submaxillary 
ganglion,  cannot  act  as  reflex  centres,  and  probably  their  sole 
function  is  to  preside  over  the  nutrition  of  the  nerve -fibres 
distributed  to  the  glands. 

Paralytic  secretion. — If  the  chorda  tympani  nerve  be  cut  on  one  side,  the 
submaxillary  gland  enters  into  a  condition  of  what  we  might  term  '  overflow 
activity.'  If  a  cannula  be  placed  in  Wharton's  duct,  a  constant  slight  dribbling 
of  saliva  occurs  (about  1  drop  in  20  minutes).  This  is  known  as  'paralytic 
secretion,'  Sections  of  the  gland  present  the  typical  appearance  of  a  resting 
gland.  This  condition  is  attended  by  a  gradual  atrophy  of  the  gland,  which 
may  lose  half  its  weight  in  a  few  weeks.  Section  of  the  sympathetic  does  not 
give  rise  to  any  analogous  phenomenon,  nor  does  it  stop  the  paralytic  secretion 
caused  by  section  of  the  chorda. 


316  PHYSIOLOGY 


Section  3 
DIGESTION   IN  THE   STOMACH 

The  food  after  thorough  mastication  and  admixture  with 
the  saliva  passes  down  the  oesophagus  into  the  stomach. 
Here  sahvary  digestion  continues  until  the  reaction  of  the 
food  is  rendered  acid  by  the  secretion  of  gastric  juice,  a 
change  which  does  not  occur  for  twenty  to  thirty  minutes  or 
even  longer.  Thus  the  greater  part  of  salivary  digestion 
takes  place  in  the  stomach. 

The  stomach  is  a  saccular  dilatation  of  the  alimentary 
canal,  lying  obliquely  in  the  abdomen,  with  the  oesophagus 
opening  into  its  larger  cardiac  end  or  fundus,  while  its  oppo- 
site right  extremity  gradually  narrows  to  the  pyloric  orifice 
which  opens  into  the  duodenum.  The  wall  is  composed  of 
four  coats,  the  serous,  muscular,  submucous,  and  mucous 
coats.  The  mucous  membrane  in  the  contracted  condition  is 
thrown  into  folds,  the  submucous  coat,  which  carries  the  large 
vessels  and  nerves,  being  very  loose  in  texture.  The  mucous 
membrane  consists  of  a  delicate  connective  tissue  rich  in 
lymphoid  elements,  covered  by  a  continuous  layer  of  hyaline 
columnar  cells,  and  presenting  a  number  of  minute  pits  which 
are  the  orifices  of  tubular  glands  set  closely  together  so  as  to 
make  up  the  greater  mass  of  the  mucous  membrane.  The 
gastric  juice  is  formed  by  the  activity  of  these  glands. 

Two  varieties  of  glands  may  be  distinguished.  At  the 
cardiac  end  of  the  stomach  the  glands  are  simple  tubules, 
with  short  necks  or  ducts.  The  secreting  part  of  the  tubule 
is  lined  with  a  single  layer  of  small  granular  cubical  cells 
(chief  cells)  ;  between  these  and  the  basement  membrane  are 
a  number  of  larger  oval  cells — parietal  or  oxyntic  cells — which 
stain  differently  from  the  central  cells. 

In  the  pyloric  region  the  glands  consist  of  tubules  which 
are  branched  at  the  end,  and  have  a  comparatively  long  neck 
or  duct.  In  this  region  we  find  only  chief  cells,  no  oxyntic 
cells  being  present.  The  necks  of  all  the  glands  are  lined 
with  columnar  epithelium,  similar  to  that  covering  the  free 
surface  of  the  mucous  membrane. 


THE   MECHANISJMS   OF   DIGESTION 


317 


The  best  method  of  obtaining  pure  gastric  juice  is  that 
devised  by  Pawlow,  in  which  the  secretion  is  evoked  by  letting 
a  dog  eat  meat  after  making  openings  in  the  oesophagus  and 
stomach,  so  that  the  food  eaten  cannot  enter  the  stomach. 
Or   a   diverticulum   of   the   stomach  may  be  established  by 

Ficx.  169. 


Fig.  ir.8. 


Fig,  168. — A  gland  from  the  cardiac  end  of  the  stomach  (after 
Klein),  d,  duct  of  the  gland ;  6,  base  of  one  of  its  tubules ; 
c,  central  cell ;  p.  parietal  or  oxyntic  cell. 

Fig.  169. — A  pyloric  gland  (Ebstein).  m,  mouth ;  n,  neck ; 
tr,  deep  portion  of  one  of  the  tubules  cut  transversely. 


making  an  incision  as  shown  in  Fig.  170,  a,  and  then  reflect- 
ing and  suturing  the  mucous  membrane,  so  that  a  little  sac 
of  the  cardiac  end  is  produced  in  nervous,  vascular,  and  mus- 
cular continuity  with  the  rest  of  the  stomach,  and  separated 
from  the  latter  only  by  a  diaphragm  consisting  of  a  double 
layer  of   mucous  membrane    (Fig.    170,  b).     A    secretion  of 


318 


PHYSIOLOGY 


gastric  juice  may  then  be  evoked  at  any  time  by  allowing  the 
animal  to  eat  in  the  ordinary  way.  Thus  obtained  it  is  a 
clear,  colourless,  acid  liquid,  with  a  specific  gravity  varying 
from  1001  to  1010.  Its  chief  constituents  are  two  ferments, 
pepsin  and  rennet  ferment ;  and  free  hydrochloric  acid.  The 
amount  of  free  HCl  in  the  gastric  juice  varies  from  0" 2-0*3 
per  cent,  in  man  to  0*6  per  cent,  in  the  dog  and  cat.  It  also 
contains  salts,  and  a  large  amount  of  water  which  constitutes 
about  98  per  cent,  of  its  bulk. 

The  hydrochloric  acid  is  shown  to  exist  in  a  free  condition 

Fig.  170. 


Diagram  to  show  Pawlow's  method  of  making  a  cul-de-sac  of  the 
cardiac  end  of  the  stomach,  with  vascular  and  nerve  supply  intact. 
In  A  the  line  of  the  incision  into  the  stomach  wall  is  shown. 
B  represents  the  operation  as  completed.  In  A :  O,  a?sophagus ; 
B.V.,  L.V.,  right  and  left  vagus  nerves;  P,  pylorus;  C,  cardiac 
portion  of  stomach;  A,  B,  line  of  incision.  In  B:  V,  main  portion 
of  stomach  ;  S,  cardiac  cul-de-sac  ;  A,  abdominal  wall ;  e,  e,  mucous 
membrane  reflected  to  form  diaphragm  between  the  two  cavities. 

from  the  fact  that  on  elementary  analysis  the  amount  of 
chlorine  present  is  more  than  sufficient  to  saturate  the 
bases. 

It  is  often  of  importance  to  be  able  to  determine  the  presence  of  free  mineral 
acid  (HCl)  in  the  gastric  contents.  Mere  acidity  to  litmus  may  be  due  to  lactic 
or  fatty  acids  produced  by  fermentation  of  the  contents.  The  following  reactions 
may  be  used  to  test  this  point : 

a.  Paper  stained  with  congo-red  turns  blue  in  the  presence  of  free  inorganic 
or  organic  acids,  but  is  not  altered  by  acid  salts. 

b.  To  some  of  the  stomach-contents  an  equal  quantity  of  Gunzberg's 
reagent  (a  solution  of  phloroglucin  and  vanillin  in  alcohol)  is  added  and  the 
mixture  evaporated  on  a  water-bath.  A  delicate  rose  tint  shows  the  presence  of 
hydi'ochloric  acid. 


THE   MECHANISMS   OF  DIGESTION  319 

c.  A  drop  of  saturated  solution  of  tropaeolin  00  is  allowed  to  evaporate  on 
a  porcelain  slab  at  40°  C,  and  while  at  this  temperature  a  drop  of  the  liquid 
to  be  tested  is  added.  On  evaporation  a  violet  stain  is  left  if  free  HCl  be 
present. 

An  artificial  gastric  juice  may  be  prepared  by  digesting 
the  fresh  mucous  membrane  of  the  stomach  with  0*2  per  cent. 
HCl  at  40°  C.  for  some  hours  and  then  filtering  from  the 
undissolved  nucleins  of  the  cells.  The  fluid  thus  obtained 
contains  a  large  proportion  of  albumoses  mixed  with  the  pep- 
sin. In  order  to  get  this  latter  approximately  pure,  the  diges- 
tion is  continued  for  three  weeks,  by  which  time  the  greater 
proportion  of  the  proteins  are  converted  into  peptones.  On 
saturating  the  fluid  with  ammonium  sulphate,  the  whole  of 
the  pepsin  is  thrown  down  contaminated  only  with  a  slight 
trace  of  unconverted  albumoses.  This  precipitate  mixed  with 
0'4  per  cent.  HCl  gives  a  powerful  artificial  gastric  juice. 
The  simplest  method  is  however  to  extract  the  fresh  mucous 
membrane  with  glycerin.  A  few  drops  of  the  glycerin  extract 
added  to  dilute  HCl  makes  an  effective  digestive  mixture. 

Action  of  Gastric  Juice  on  the  Food 

The  chief  action  of  gastric  juice  is  on  proteins,  which  it 
converts  into  albumoses  and  peptones.  This  action  is  easily 
studied  if  we  take  some  washed  fibrin  and  put  it  in  0*2  per  cent, 
hydrochloric  acid.  In  this  acid  the  fibrin  swells  up  and 
becomes  transparent,  but  does  not  dissolve,  even  though  kept 
at  40"  C.  for  some  time.  If  now  to  the  swollen-up  mass  we 
add  some  gastric  juice,  or  a  few  drops  of  glycerin-extract  of 
gastric  mucous  membrane,  the  fibrin  is  speedily  dissolved 
and  a  clear  solution  results.  Neutralisation  of  the  fluid  with 
alkali  throws  down  nearly  the  whole  of  the  protein  present  as 
acid  albumen.  If  however  the  action  be  long  continued,  the 
neutralisation  precipitate  becomes  less  and  less,  and  the  fluid 
contains  chiefly  albumoses  with  a  little  peptone.  These  may 
be  shown  to  be  present  by  the  following  tests. 

Nitric  acid  gives  a  precipitate  which  dissolves  on  heating 
and  reappears  on  cooling. 

Caustic  potash  and  a  trace  of  copper  sulphate  give  a  pink 
colour,  which  turns  to  violet  on  the  addition  of  more  copper 
sulphate — biuret  reaction. 


320  PHYSIOLOGY 

Saturation  with  ammonium  sulphate  gives  a  copious 
precipitate  of  albumoses.  If  tliis  be  filtered  off,  the  filtrate 
contains  a  small  amount  of  peptone.  To  produce  any  large 
quantity  of  peptone  the  gastric  juice  must  act  for  a  consider- 
able length  of  time. 

The  stages  in  the  action  of  gastric  juice  on  proteins  are 
therefore — 

Coagulable  protein. 

Syntonin  or  acid  albumen. 

Albumoses. 

Peptones. 

The  action  of  gastric  juice  on  proteins  resembles  that 
of  ptyalin  on  starch  in  that  it  consists  of  a  series  of  hydro- 
lytic  changes,  converting  the  large  indiffusible  coagulable  or 
coagulated  molecule  into  a  number  of  smaller  non-coagulable 
more  soluble  and  more  diffusible  molecules.  The  study  of 
the  disintegration  products  of  proteins  (p.  33)  has  shown 
however  that  they  are  much  more  complex  in  constitution 
than  the  starches,  and  hence  we  find  a  greater  variety  among 
the  first  products  of  their  hydrolysis.  The  number  of 
albumoses  and  peptones  is  probably  large,  but  we  are  able  to 
group  them  into  fairly  definite  classes  according  to  the  stage 
in  digestion  at  which  they  are  produced.  In  the  process  of 
digestion  the  proteins  undergo  progressive  disintegration,  so 
that  the  first  products,  the  primary  albumoses,  are  large 
molecules  reacting  to  saline  precipitants  like  globulins,  while 
the  latest  stages,  the  peptones,  are  small  molecules,  not 
precipitated  by  neutral  salts,  and,  relatively  to  proteins, 
difl'usible.  The  stages  in  the  peptic  digestion  of  fibrin  are 
shown  in  the  following  table : 

Fibrin. 

I 
Soluble  globulin,  coagulating  at  56°  C. 

I 
Acid  albumen. 

Primary  albumoses  (proto-  and  hetero-albumose). 

Secondary  albumoses  (deutero-albumoses). 

I 
Peptones. 


THE   MECHANISMS   OF   DIGESTION  321 

Kiihne's  method  for  the  separation  of  these  bodies  from  a  gastric  digest  of 
fibrin  can  be  carried  out  in  the  following  way : 

Neutralise — Acid  albumen  precipitated. 

Boil  in  slightly  acid  reaction — Coagulable  proteins  precipitated. 

The  filtrate  contains  only  albumoses  and  peptones. 

Saturate  filtrate  with  solid  NaCl  ;  all  the  hetero-albumose  and  the  greater 
part  of  the  proto-albumose  are  precipitated.  This  precipitate  is  collected, 
dissolved  in  water,  and  dialysed.  As  the  salt  passes  out  the  hetero-albumose 
is  precipitated,  being  insoluble  in  pure  water,  while  the  proto-albumose  remains 
in  solution  and  can  be  precipitated  by  concentration  and  addition  of  absolute 
alcohol. 

The  filtrate  from  the  i^rimary  albumoses  is  treated  with  a  little  glacial 
acetic  acid  previously  saturated  with  NaCl.  This  throws  down  the  last  traces 
of  proto-albumose  and  a  little  deutero-albumose.  The  precipitate  is  filtered 
off,  and  the  filtrate  saturated  at  boiling  temperature  with  ammonium  sulphate, 
which  precipitates  all  the  deutero-albumoses.  This  precipitate  is  filtered  off 
and  concentrated  to  separate  off  the  greater  portion  of  the  Am.,SO,.  The 
last  traces  of  Am^SO^  are  got  rid  of  by  the  addition  of  baryta,  and  after  filtering 
off  the  BaSO^  precipitate,  and  boiling  off  the  ammonia,  the  peptones  are 
precipitated  by  adding  a  large  excess  of  absolute  alcohol,  or  by  means  of 
phosphomolybdic  acid. 

By  means  of  fractional  precipitation  with  ammonium  sulphate,  Pick  has 
separated  three  deutero-albumoses,  which  are  designated  as  A,  B,  and  C 
deutero-albumose.  These  can  be  further  subdivided  by  treatment  with  alcohol, 
which  dissolves  certain  of  the  albumoses  and  leaves  others.  Among  the  pro- 
ducts so  separated  may  be  mentioned  a  thio-albumose,  rich  in  sulphur,  and 
a  glyco-albumose,  containing  the  greater  part  of  the  carbohydrate  moiety  of  the 
original  protein.  These  various  bodies  also  differ  according  to  the  nature  of 
the  monamino-acids  and  the  amount  of  basic  nitrogen  they  yield  on  hydro- 
lysis. The  peptone  obtained  by  Kiihne's  method  may  also  be  divided  into 
a  peptone  insoluble  in  alcohol  which  contains  carbohydrate,  and  a  peptone 
soluble  in  alcohol  which  yields  no  carbohydrate  radicle  on  hydrolysis,  and 
therefore  does  not  give  the  Molisch  reaction. 

The  later  stages  of  gastric  digestion  are  by  no  means 
rapid,  so  that  even  after  twenty-four  hours'  digestion  the 
greater  portion  of  the  proteins  are  found  in  the  form  of 
albumoses. 

Gastric  juice  acts  in  the  same  way  on  insoluble  coagu- 
lated proteins,  dissolving  these  and  converting  them  into  acid 
albumen,  albumoses,  and  peptone. 

Gelatin  is  converted  by  gastric  juice  into  bodies  known  as 
gelatin  peptones,  and  in  this  conversion  loses  the  power  of 
forming  a  jelly  when  cold. 

Collagen,  the  constituent  of  the  connective  tissues  from 
which  gelatin  is  obtained  on  prolonged  boiling,  is  also  digested 
by  gastric  juice,  giving  rise  to  the  same  end-products  as 
gelatin. 

21 


322  PHYSIOLOGY 

In  this  way  the  connective  tissue  binding  together  the  fat- 
cells  of  adipose  tissue  is  broken  up  and  dissolved,  and  the 
fat  is  set  free  in  a  liquid  form,  ready  to  be  acted  on  by  the 
pancreatic  juice. 

Another  important  function  of  the  gastric  juice  depends 
on  the  fact  that  dilute  hydrochloric  acid  acts  as  an  anti- 
septic. Meat  and  fibrin  may  be  kept  for  several  days  in 
gastric  juice  without  undergoing  decomposition.  If  however 
the  acid  be  neutralised,  decomposition  sets  in  rapidly  on 
exposure  to  air,  and  at  the  end  of  twenty-four  hours  the 
mixture  has  a  foetid  odour,  and  is  found  to  be  swarming  with 
bacteria.  This  action  is  of  great  importance  in  the  normal 
life  of  the  individual.  The  microbes  which  have  been  shown 
to  be  the  causes  of  typhoid  and  cholera  are  destroyed  by 
gastric  juice.  Hence  there  is  little  likelihood  of  contracting 
these  diseases  unless  the  secretion  of  gastric  juice  be  insuf- 
ficient, or  the  acid  neutralised  by  the  presence  of  alkalies  or 
rendered  inert  by  too  much  dilution. 

On  starches  and  fats  gastric  juice  has  no  action.  Inges- 
tion of  large  amounts  of  cane-sugar  gives  rise  to  a  free 
secretion  of  mucus  on  the  surface  of  the  gastric  mucous 
membrane,  and  this  mucus  is  said  to  contain  an  invert  fer- 
ment which  has  the  power  of  converting  cane-sugar  into 
dextrose  and  laevulose.  A  certain  amount  of  inversion 
must  also  be  caused  by  the  action  of  the  dilute  HCl  at 
the  body  temperature.  The  same  factor  would  convert 
some  of  the  maltose  formed  by  the  salivary  digestion  into 
dextrose. 

Circumstances  affecting  activity  of  gastric  juice. — Gas- 
tric juice  is  most  active  at  about  40°  C.  At  0°  its  action  is 
indefinitely  suspended.  If  boiled  the  ferment  is  destroyed. 
The  action  goes  on  most  rapidly  when  the  proportion  of  HCl 
present  is  0*4  per  cent.  ;  larger  amounts  of  acids  hinder  its 
action.  Neutralisation  stops  the  action  altogether  ;  and  if 
the  juice  be  rendered  slightly  alkaline  and  be  kept  at  the 
temperature  of  the  body  for  a  short  time,  its  activity  is  per- 
manently destroyed.  Its  action  is  also  hindered  if  the  pro- 
ducts of  its  activity  be  allowed  to  accumulate  to  a  large  extent. 
In  the  stomach  this  is  guarded  against  by  the  continual 
absorption  through  the  gastric  mucous  membrane  of  the 
albumoses  and  peptones  as  they  are  formed. 


THE   MECHANISMS   OF  DIGESTION  323 


Action  of  Gastric  Juice  on  Milk 

Milk,  which  is  the  sole  diet  of  the  infant,  is  in  itself  a 
whole  food,  and  contains  representatives  of  all  five  classes 
of  foodstuffs — proteins,  fats,  carbohydrates,  salts,  and  water. 
The  chief  protein  of  milk — caseinogen — is  a  body  allied  to 
the  nucleo-albumens.  From  the  gastric  mucous  membrane, 
especially  in  young  animals,  a  ferment  may  be  extracted 
known  as  rennet  ferment  or  rennin.  On  adding  a  few  drops 
of  rennet  solution  to  milk,  and  warming  the  mixture  to  about 
40"  C,  it  sets  into  a  solid  mass,  so  that  the  vessel  may  be 
inverted  without  spilling  the  contents.  On  allowing  the  clot 
or  curd  to  stand  it  shrinks,  enclosing  in  its  meshes  the 
greater  part  of  the  fat-globules  of  milk,  so  that  the  clot  floats 
in  an  almost  transparent  fluid  (whey).  This  clotting  depends 
on  a  change  induced  in  the  caseinogen  of  the  milk  under  the 
action  of  the  ferment. 

Pure  caseinogen  may  be  prepared  in  the  following  way. 
A  litre  of  '  separated '  milk  {i.e.  milk  that  has  been  freed  from 
cream  by  the  use  of  a  centrifuge)  is  diluted  with  9  litres  of 
distilled  water  in  a  tall  glass  vessel  and  10  c.c.  of  glacial  acetic 
acid  added  and  the  mixture  stirred.  A  flocculent  precipitate 
of  caseinogen  is  produced,  and  rapidly  sinks  to  the  bottom  of 
the  vessel.  The  precipitate  is  collected,  pressed  between 
linen,  and  rubbed  up  in  a  mortar  with  20  c.c.  of  ammonia,  and 
water  added  to  5  litres.  The  solution  is  allowed  to  stand  for 
some  hours,  any  fat  which  may  rise  to  the  top  being  skimmed 
off.  Acetic  acid  is  added  as  before,  and  the  resultant  pre- 
cipitate is  washed  several  times  by  deeantation  with  distilled 
water,  and  finally  collected  on  a  filter  and  pressed  free  of 
fluid.  This  precipitate  may  be  dissolved  in  weak  soda  or 
potash,  forming  a  clear  solution.  If  it  be  dissolved  in  lime-water 
an  opalescent  solution  is  formed.  If  rennet  be  added  and  the 
mixture  allowed  to  stand  at  40°  C,  a  clear  colourless  clot  of 
casein  is  produced.  This  act  of  clotting,  just  as  the  clotting  of 
blood,  is  intimately  dependent  on  the  presence  of  a  neutral  salt 
of  lime.  If  the  precipitate  of  caseinogen  be  washed  till  all  lime 
salts  are  removed,  addition  of  rennet  causes  no  clotting ;  but 
the  mixture  of  caseinogen  and  rennet  clots  at  once  on  addition 
of  calcium  phosphate  or  chloride. 


324  PHYSIOLOGY 

Thus  in  clotting  of  milk  two  processes  are  concerned  : 

1.  A  conversion  of  caseinogen  into  some  other  body  which 
may  be  called  soluble  casein. 

2.  A  combination  of  this  soluble  casein  with  a  lime  salt 
to  form  insoluble  casein,  which  is  precipitated  in  a  gela- 
tinous form. 

These  facts  are  well  shown  by  the  following  experiment 
of  Ringer,  Two  test-tubes  are  taken,  a  and  h,  containing 
a  solution  of  pure  caseinogen  free  from  lime.  To  a  rennet 
ferment  is  added,  and  to  h  a  solution  of  calcium  chloride ; 
and  the  two  tubes  are  kept  at  40°  C.  for  some  time.  No 
visible  reaction  takes  place.  If  a  be  now  boiled,  so  as  to 
destroy  the  rennet  ferment  present,  and  on  cooling  a  few 
drops  of  calcium  chloride  be  added,  clotting  occurs.  In 
this  experiment  the  rennet  ferment  has  evidently  produced 
some  change  in  the  caseinogen  although  no  clotting  took 
place,  since  the  boiled  fluid  needs  only  the  addition  of  lime 
salt  to  make  it  clot.  The  fact  that  h  did  not  clot  shows 
that  lime  salts  are  without  effect  on  a  solution  of  caseinogen 
which  has  not  been  previously  exposed  to  the  action  of  rennet 
ferment,' 

The  Secretion  of  Gastric  Juice 

The  functions  of  the  two  different  kinds  of  gastric  glands 
have  been  determined  by  cutting  out  a  portion  of  the  cardiac 
or  pyloric  parts  of  the  stomach,  and  sewing  its  edges  to  the 
margins  of  the  abdominal  wound.  The  gap  in  the  gastric 
wall  thus  produced  is  closed  by  suturing  the  edges  together, 
so  that  the  final  result  of  the  operation  is  that  the  stomach  is 
rather  smaller,  and  there  is  a  little  cul-de-sac  consisting  of 
either  cardiac  or  pyloric  mucous  membrane  communicating 
with  the  exterior.     Secretion  may  be  excited  by  mechanical 

'  The  student  must  be  careful  to  distinguish  between  the  curdling  of  milk 
by  rennet  and  its  curdling  by  addition  of  acid,  or  when  it  becomes  sour  in  con- 
sequence of  the  development  in  it  of  lactic  acid.  In  the  former  case  the 
curdling  is  a  true  clotting,  and  is  due  to  the  conversion  of  the  soluble 
caseinogen  into  the  insoluble  casein,  in  the  same  way  as  when  fibrinogen  is 
converted  into  fibrin.  When  acid  is  added  to  milk  the  caseinogen  is  merely 
precipitated,  just  as  uric  acid  is  precipitated  by  addition  of  HCl  to  a 
sglution  of  a  urate.  And  this  precipitate  can  be  dissolved  up  again  as  caseinogen 
and  ma4G  to  clot,  {.e,  caij  be  ppnverted  into  casein  by  the  agency  of  rennet 
fernjent, 


THE  MECHANISMS   OF  DIGESTION  325 

irritation  of  this  mucous  membrane  by  the  introduction  of  a 
sponge  or  some  food,  and  the  juice  may  be  collected.  It  is 
better  however  to  make  the  cul-de-sac  as  described  on  p.  317, 
leaving  the  nervous  connections  of  the  mucous  membrane 
intact.  Under  these  conditions  the  isolated  portion  of  the 
stomach  secretes  whenever  food  is  taken  by  the  mouth  into 
the  main  stomach,  and  the  secretion  may  be  regarded  as 
practically  normal  in  all  respects.  It  is  then  found  that  a 
cardiac  cul-de-sac  contains  free  HCl  and  pepsin,  and  so  has 
the  power  of  digesting  proteins.  A  pyloric  cul-de-sac,  on  the 
other  hand,  yields  a  secretion  which  is  neutral  or  slightly 
alkaline,  but  which  is  shown  to  contain  pepsin  from  the  fact 
that,  on  adding  0*2  per  cent,  of  hydrochloric  acid,  the  juice 
is  able  to  digest  proteins.  This  shows  that  both  cardiac  and 
pyloric  glands  yield  pepsin,  but  only  the  cardiac  glands 
yield  free  hydrochloric  acid.  Since  in  both  sets  of  glands 
the  central  cells  are  the  same,  it  is  concluded  that  these  cells 
give  rise  to  the  pepsin,  while  the  large  oval  parietal  cells  in 
the  cardiac  end  form  the  free  hydrochloric  acid. 

Coincident  with  activity,  changes  take  place  in  the  central 
cells  analogous  to  those  which  we  studied  in  the  case  of  the 
salivary  glands.  The  central  cells  of  the  glands  from  a 
fasting  stomach  {i.e.  of  an  animal  that  has  not  taken  food  for 
eighteen  hours)  are  swollen  and  filled  with  granules.  When 
secretion  occurs  two  zones  can  be  distinguished,  an  outer 
protoplasmic  zone,  free  from  granules,  and  an  inner  granular 
zone,  which  becomes  less  and  less  marked  as  secretion  pro- 
ceeds. These  granules  consist  of  a  zymogen,  pepsinogen.  If 
the  fresh  mucous  membrane  be  extracted  with  glycerin,  much 
less  ferment  is  obtained  than  if  the  extraction  be  performed 
after  treatment  of  the  mucous  membrane  with  dilute  acid. 
The  pepsinogen  is  further  distinguished  from  the  pepsin  by 
the  fact  that  it  is  only  slowly  affected  by  a  solution  of  sodium 
carbonate,  which  very  rapidly  destroys  pepsin.  Thus  if  an 
acid  extract  of  the  gastric  mucous  membrane  be  made  alka- 
line to  the  extent  of  1  per  cent,  sodium  carbonate,  the  whole  of 
the  pepsin  is  destroyed  within  thirty  seconds.  On  the  other 
hand  a  neutral  watery  extract  of  the  membrane  may  be  made 
alkaline  for  two  or  three  minutes,  and  on  then  adding  HCl 
to  0*2  per  cent,  it  is  found  that  the  mixture  has  a  strong 
digestive  action.     The  acid  extract  contains  pepsin  which  is 


826  PHYSIOLOGY 

destroyed  by  the  carbonate.  The  watery  extract  contains 
chiefly  pepsinogen  and  is  therefore  only  slowly  altered  by  the 
treatment. 

The  secretion  of  gastric  juice  may  be  excited  by  direct 
stimulation  of  the  mucous  membrane  by  the  presence  of 
food,  etc.,  in  the  stomach.  We  have  evidence  that  a  copious 
flow  of  gastric  juice  may  be  excited  through  nervous  channels, 
either  reflexly  through  the  mouth  or  in  consequence  of  events 
occurring  in  the  brain.  This  reflex  secretion  is  well  shown 
in  the  following  experiment : — The  oesophagus  of  a  dog  is 
divided  in  the  neck,  and  the  two  ends  stitched  to  the  wound 
80  that  they  open  exteriorly.  At  the  same  time  a  gastric 
fistula  is  made.  The  dog  is  fed  and  kept  in  good  condition 
by  the  introduction  of  milk  into  the  lower  end  of  the  oeso- 
phagus, or  by  the  direct  introduction  of  food  into  the  stomach. 
When  the  animal  has  quite  recovered,  he  is  starved  for  nine 
hours  and  is  then  allowed  to  eat  meat.  The  dog  eats  greedily, 
and,  since  the  food  cannot  reach  the  stomach  but  tumbles 
out  by  the  opening  of  the  oesophagus  in  the  neck,  will  go 
on  eating  for  a  very  long  time.  Directly  the  dog  begins 
to  eat,  a  copious  secretion  of  gastric  juice  is  obtained,  as 
much  as  300  c.c.  of  pure  gastric  juice  being  poured  out  in 
one  hour,  clear  and  colourless  like  water.  The  same  eftect 
may  be  produced  by  simply  showing  the  dog  a  piece  of 
meat,  and  it  is  stated  that  the  flow  ceases  as  soon  as  the 
dog  realises  that  he  is  not  intended  to  have  the  meat. 
Pawlow,  to  whom  we  owe  the  above  experiment,  has  shown 
that  the  efferent  nerve  in  the  reflex,  that  is  to  say,  the 
secretory  nerve  to  the  stomach,  is  the  vagus.  If  proper 
precautions  be  observed,  stimulation  of  the  vagus  causes 
invariably  a  secretion  of  gastric  juice. 

In  testing  the  action  of  the  vagus  on  the  gastric  secretion,  we  have  to  bear 
in  mind  that  the  activity  of  the  gastric  glands  is  reflexly  inhibited  by  al 
painful  stimuli,  and  is  abolished  by  any  stoppage  of  the  blood-flow  or  by  the 
action  of  anaesthetics.  The  experiment  is  therefore  carried  out  in  the  follow- 
ing way : — On  one  day  the  gastric  fistula  is  established,  and  one  vagus  in  the 
neck  divided  and  ligatured,  the  ligature  being  allowed  to  hang  out  of  the 
wound.  Four  days  later  the  animal  is  placed  in  an  upright  position,  the 
vagus  drawn  out  of  the  wound  and  stimulated  with  induction-shocks  at  the 
rate  of  one  or  two  a  second.  The  animal  is  not  anaesthetised  and  the  experi- 
ment is  unattended  with  pain.  The  cardio-inhibitory  fibres  are  degenerated  in 
consequence  of  the  section,  so  that  the  stimulation  does  not  alter  the  blood- 
pressure  in  any  way.     After  a  latent  period   of  four  or  five  minutes,  gastric 


THE   MECHANISMS   OF  DIGESTION  327 

juice  begins  to  drop  from  the  cannula  in  the  fistula,  and  the  secretion 
can  be  obtained  as  often  as  the  nerve  is  stimulated.  Section  of  both  vagi  stops 
the  reflex  secretion  of  gastric  juice  normally  evoked  by  the  sham  feeling 
described  above. 

There  are  however  two  sets  of  factors  involved  m  the 
normal  secretion  of  gastric  juice.  The  more  important  is 
the  nervous  mechanism  described  above,  set  into  action  from 
the  mouth  or  the  cerebral  cortex  under  the  influence  of  appetite, 
and  having  as  its  efferent  channel  the  vagus  nerve.  This 
mechanism  is  destroyed  by  section  of  both  vagi.  It  is  then 
observed  that  introduction  of  food  into  the  stomach  evokes 
a  secretion  of  juice,  which  comes  on  one  to  two  hours  later, 
and  lasts  a  considerable  time.  If  a  diverticulum  of  the  stomach 
has  been  made,  introduction  of  food  into  the  main  stomach 
excites  in  this  way  secretion  in  the  isolated  cul-de-sac. 
Edkins  has  recently  shown  that  the  mechanism  of  this  latter 
secretion  is  chemical.  Under  the  influence  of  the  food,  some 
substance,  a  chemical  messenger  or  '  hormone,'  is  produced  in 
the  pyloric  mucous  membrane,  is  absorbed  into  the  blood,  and 
carried  to  all  parts  of  the  mucous  membrane,  in  which  it 
acts  as  a  specific  excitant  of  the  gastric  glands,  evoking  a 
secretion  of  gastric  juice. 


828 


PHYSIOLOGY 


Section  4 
PANCKEATIC    JUICE 

The  food,  after  being  acted  on  by  the  gastric  juice,  is 
gradually  passed  on  as  the  fluid  chyme  into  the  duodenum, 
the  first  part  of  the  small  intestine.  Here  it  is  subjected  to 
the  action  of  three  secretions,  the  pancreatic  juice,  the  bile, 
and  the  succus  entericus. 

The  pancreas  lies  in  the  curve  formed  by  the  duodenum, 
and  pours  its  secretions  into   that   viscus  by  a  duct  which 

Fm.  171. 


O 


^,{ 


k 


&j 


-  d 


'"d 


-■^     f 


p.<^/" 


Section  of  human  pancreas  (Bohm  and  v.  Davidoft').  x  450.  a,  cell- 
nest  between  acini  ;  b,  connective  tissue ;  c,  large  duct ;  d,  d, 
alveoli;  e,  duct  passing  into  alveoli;  /,  inner  granular  zone  of 
acinus. 

joins  the  duct  from  the  liver  and  opens  at  a  pomt  about  two 
or  three  inches  below  the  pylorus.  It  is  a  tubulo-racemose 
gland,  somewhat  similar  to  a  salivary  gland.  Its  tubules 
however  are  much  longer  than  those  of  the  parotid.  The 
tubules  are  lined  with  polyhedral  cells,  the  inner  two-thirds  of 
which  are  granular,  while  the  outer  third  is  free  from  granules 
and  stains  darkly  with  baematoxylin.  Activity  is  associated 
with  a  discharge  of  these  zymogen  granules,  so  that  the  cells 


THE   MECHANISMS   OF  DIGESTION  829 

shrink  and  become  clearer.  At  the  same  time  there  is  a  growth 
in  the  protoplasm,  so  that  in  a  discharged  gland  nearly  the 
whole  of  the  cell  stains  with  haematoxylin. 

On  examining  a  stained  section  of  pancreas  under  the  microscope,  we  rarely 
fail  to  see  here  and  there  small  circumscribed  patches  of  epithelioid  cells  which 
have  very  little  athnity  for  dyestuffs,  and  therefore  appear  as  lighter  areas  in 
the  section  (a,  fig.  171).  The  cells  are  often  arranged  in  short  columns  or 
clusters,  and  but  little  trace  of  an  alveolar  structure  can  be  made  out.  These 
structures,  which  are  known  as  the  '  islets  of  Langerhans,'  have  often  been 
regarded  as  a  special  tissue  planted  in  the  midst  of  the  ordinary  secreting  tissue 
of  the  pancreas.  It  has  been  shown  by  Dale  however  that  the  number  of 
these  islets  is  markedly  increased  by  activity,  and  that,  in  a  gland  which  has 
been  thoroughly  exhausted,  the  larger  part  of  the  secreting  substance  loses  its 
staining  properties  and  becomes  converted  into  islet  tissue.  One  must  there- 
fore regard  these  islets  as  a  phase  in  the  life  history  of  the  secreting 
cells,  and  as  representing  an  extreme  stage  of  exhaustion  of  these  cells. 
Whether  they  recover  their  staining  properties  and  resume  their  secretory 
activity,  or  whether  they  are  removed  altogether  to  make  room  for  newly 
formed  glandular  tissue,  is  not  yet  thoroughly  made  out,  though  the  fact  that 
at  a  certain  period  of  foetal  life  all  the  pancreatic  cells  have  the  appearance  of 
islet  cells  is  certainly  in  favour  of  the  former  alternative. 

In  order  to  collect  pancreatic  juice  for  examination  and  to 
investigate  the  conditions  of  its  formation,  it  is  necessary  to 
make  a  pancreatic  fistula.  To  make  a  permanent  fistula,  the 
abdomen  (of  a  dog)  is  opened  in  the  middle  line,  the  first  part 
of  the  duodenum  drawn  up  to  the  surface,  and  a  lozenge- 
shaped  piece  of  the  duodenal  wall  cut  out,  in  such  a  position 
as  to  include  the  orifice  of  the  chief  pancreatic  duct.  The 
margins  of  the  wound  in  the  gut  are  then  stitched  together  so 
as  to  restore  its  continuity,  and  the  little  piece  of  excised 
duodenal  wall  is  stitched  into  the  opening  in  the  abdominal 
wall.  When  the  wound  is  healed,  the  nipple-like  orifice  of 
the  duct  is  seen  on  the  surface  of  the  abdomen  in  the 
middle  of  a  small  red  patch  of  mucous  membrane.  To 
make  a  temporary  fistula,  it  is  only  necessary  to  expose 
the  duct  at  its  entry  into  the  duodenum  and  insert  into  it  a 
small  cannula,  connected  with  a  rubber  tube  through  which 
the  juice  is  allowed  to  flow  into  a  suitable  vessel. 

In  an  animal  with  a  permanent  fistula  no  flow  of  juice 
occurs  so  long  as  it  is  in  a  fasting  condition.  At  a  variable 
period  after  a  meal  has  been  taken,  a  flow  of  juice  begins  and 
continues  for  several  hours.  It  can  be  shown  that  the  flow 
of  juice  follows  immediately  the  entrance  of  the  acid  chyme 


380  PHYSIOLOGY 

into  the  duodenum,  no  flow  being  excited  so  long  as  the 
food  remains  in  the  stomach.  The  flow  of  juice  can  in 
fact  be  evoked  at  any  time  by  the  introduction  of  weak  acid 
{e.g.  0*4  per  cent.  HCl)  into  the  duodenum  or  the  upper  part  of 
the  small  intestine.  No  effect  follows  the  introduction  of  acid 
into  the  lower  part  of  the  ileum  or  into  the  large  intestine. 
The  effective  stimulus  to  pancreatic  activity  is  therefore  the 
presence  of  acid  in  the  upper  part  of  the  small  intestine. 

How  is  this  stimulus  conveyed  to  the  pancreatic  cells  ? 
It  was  thought  by  Pawlow  and  other  physiologists  that  the 
medium  of  transmission  here,  as  in  the  case  of  the  salivary 
glands,  was  the  nervous  system— either  the  central  nervous 
system  by  means  of  the  vagi,  or  peripheral  collections  of 
ganglion  cells  round  the  abdominal  vessels  and  in  the 
pancreas  itself  through  the  fine  nervous  filaments  which  pass 
between  these  and  the  intestinal  wall.  This  idea  however 
had  to  be  abandoned  when  it  was  found  that  introduction  of 
acid  into  a  loop  of  small  intestine,  entirely  freed  from  nervous 
connection  with  the  rest  of  the  body,  produced  a  flow  of  pan- 
creatic juice  as  profuse  as  that  obtained  from  a  loop  of  normal 
intestine.  In  the  former  case  the  only  connection  of  the 
intestine  with  the  pancreas  was  by  means  of  the  blood. 

We  must  therefore  conclude  that  the  stimulus  is  a  chemical 
one,  produced  in  the  mucous  membrane  by  the  action  of  the 
acid  and  carried  thence  to  the  pancreas  in  the  blood  stream. 
If  the  mucous  membrane  be  scraped,  and  the  scrapings 
rubbed  up  with  dilute  hj^drochloric  acid,  and  the  mixture  be 
then  boiled,  neutralised,  and  filtered  to  separate  the  proteins, 
a  clear  colourless  filtrate  is  obtained,  which,  on  injection  in 
small  quantities  into  the  blood  stream,  evokes  a  copious 
flow  of  pancreatic  juice.  A  watery  or  normal  saline  extract  of 
mucous  membrane  has  no  effect.  These  facts  prove  that  the 
mucous  membrane  of  the  upper  part  of  the  small  intestine 
contains  a  body,  which  may  be  cQ\\e([  prosecretin.  Under  the 
influence  of  dilute  acids  this  body  is  converted  into  or  gives 
off  another  substance,  secretin,  which  is  a  specific  stimulus 
for  the  pancreatic  cells.  This  secretin  is  discharged  by  the 
cells,  not  into  the  intestine,  but  into  the  blood  stream.  By 
the  blood  it  is  carried  to  all  parts  of  the  body,  and  on  passage 
through  the  pancreas,  excites  secretion  of  pancreatic  juice. 

Secretin  is   apparently  a  body  of  relatively  simple  con- 


THE  MECHANISMS   OF  DIGESTION  331 

stitution.  It  is  soluble  in  acid,  neutral,  or  alkaline  solutions, 
or  in  alcohol.  It  is  diffusible,  and  is  not  d-estroyed  by  boiling. 
It  is  not  a  ferment.     It  has  not  yet  been  isolated. 

Characters  and  Actions  of  the  Pancreatic  Juice. 

The  juice  obtained  from  a  permanent  fistula  after  a  meal, 
or  from  a  temporary  fistula  as  the  result  of  introduction  of 
acid  into  the  duodenum  or  the  injection  of  a  solution  of  secre- 
tin into  the  blood  stream,  is  a  clear  colourless  liquid,  strongly 
alkaline  from  the  presence  of  sodium  carbonate.  It  contains 
from  1-5  to  3-5  per  cent,  total  solids,  of  which  about  0*5  to  2*5 
are  coagulable  proteins.  It  has  a  digestive  action  on  all  three 
classes  of  foodstuffs.  It  converts  starches  into  dextrin  and 
sugar  by  means  of  a  ferment,  amylopsin  ;  it  hydrolyses  fats  by 
means  of  its  ferment,  steapsin  ;  and  after  entry  into  the  gut  it 
contains  a  strong  proteolytic  ferment,  trypsin.  If  however 
the  juice  be  collected  by  a  cannula  in  the  duct,  so  as  to  pre- 
vent any  contamination  by  the  intestinal  mucous  membrane, 
it  has  only  slight  proteolytic  powers,  taking  twelve  hours  to 
dissolve  fresh  fibrin,  and  l)eing  without  action  on  coagulated 
egg  albumen  or  gelatin.  The  addition  of  a  few  drops  of 
succus  entericus  or  a  fragment  of  intestinal  mucous  membrane 
into  the  fresh  juice,  rapidly  converts  it  into  one  of  the  most 
powerful  proteolytic  agents  with  which  we  are  acquainted. 

The  proteolytic  ferment  contained  in  the  juice  under  these 
circumstances  is  known  as  trypsin.  The  juice  as  secreted 
contains  a  precursor  of  this  ferment,  known  as  trypsinogen, 
which,  under  the  action  of  a  ferment,  cnterokinase,  secreted 
by  the  intestinal  mucous  membrane,  is  converted  into 
trypsin. 

Glycerin  extracts  of  a  fresh  pancreas  have  a  marked 
amylolytic  action,  but  have  practically  no  proteolytic  action 
unless  the  extract  has  been  contaminated  with  mtestinal  con- 
tents containing  enterokinase,  the  smallest  trace  of  which  will 
in  time  convert  almost  any  quantity  of  trypsinogen  into 
trypsin.  The  action  of  pancreatic  extracts  on  fats  is  only 
observed  in  perfectly  fresh  extracts,  the  steapsin  apparently 
undergoing  rapid  spontaneous  destruction. 


332  PHYSIOLOGY 


Action  on  Proteins 


Proteins  are  converted  by  trypsin  into  albumoses  and 
peptones,  as  in  the  case  of  the  gastric  juice.  Trypsin  how- 
ever is  only  active  in  an  alkahne  or  neutral  medium,  and 
is  inert  in  the  presence  of  an  acid.  If  fresh  fibrin  be 
digested  with  this  juice,  it  does  not  swell  up  but  is  gradually 
eroded  and  dissolved  at  the  edges.  The  first  product  of  its 
action  is  not  acid-  but  alkali-albumen.  Another  important 
difference  of  this  ferment  from  pepsin  lies  in  the  fact  that  it 
carries  on  the  process  of  decomposition  still  further,  causing 
a  profound  disintegration  of  the  protein  molecule,  resembling 
that  brought  about  by  the  action  of  boiling  hydrochloric  acid. 
The  final  results  therefore  of  tryptic  digestion  are  a  number 
of  bodies  of  comparatively  small  molecular  weight,  belonging  to 
the  series  of  bodies  described  as  amino-acids  and  hexone  bases. 

In  consequence  of  the  much  greater  activity  of  trypsin  as 
compared  with  pepsin,  the  first  stages  of  proteolysis  are  much 
more  rapid  with  the  first-named.  At  no  period  of  the  process 
is  it  possible  to  detect  the  presence  of  primary  albumoses, 
the  alkali-albumen  being  directly  converted  into  deutero- 
albumoses  and  peptones. 

The  end-products  of  pancreatic  digestion  include  nearly 
the  whole  series  of  monamino-  and  diamino-acids  which  can 
be  obtained  by  the  action  of  acids  on  proteins. 

These  may  be  enumerated  as  follows  : — 

A.  Mono-amino-acids. 

I.  Monobasic  acids  of  fatty  series. 
Glycine  (amino-acetic  acid). 
Alanine  (amino -propionic  acid). 
Serine  or  oxyalanine  (oxyamino-propionic  acid). 
Amino-valerianic  acid. 
Leucine  (amino-isobutyl-acetic  acid). 
Isoleucine  (amino-caproic  acid). 

II.  Dibasic  acids. 

Aspartic  acid  (amino-succinic  acid). 
Glutamic  acid. 

III.  Benzene  derivatives. 
Phenylalanine. 
Tyrosine  (oxyphenyl-alanine). 


THE   MECHANISMS   OF  DIGESTION  333 

B.  Heterocyclic    compounds     {i.e.    ring    compounds  like 
benzene,  but  containing  both  C  and  N    in   the 
closed  ring. 
Proline'  (pyrrolidine  carboxylic  acid^. 
Oxyproline  (oxypyrrolidine  carboxylic  acid). 
Tryptophane  (indolamino-propionic  acid), 

C.  Diamino-acids  and   allied  bodies,    strongly  basic    in 
character  (the  *  hexone  bases  '). 
Lysine  (diamino-caproic  acid). 
Arginine  (guanidine  amino-valerianic  acid). 
Histidine  (imidazol  amino-propionic  acid). 
Diamino-trioxydodecoic  acid   (derived  from  a   12- 
carbon  acid). 

D.  Sulphur-containing  body. 

Cystine  (derived  from  amino-thio-lactic  acid). 

Not  all  of  these  are  to  be  obtained  from  every  protein. 
All  proteins  however  yield  a  large  number  of  different  amino- 
acids. 

Two  of  these  substances,  namely,  leucine  and  tyrosine,  have 
long  been  known,  owing  to  the  ease  with  which  they  may  be 
separated  from  a  pancreatic  digest. 

Leucine  belongs  to  the  fatty  series,  being  amino-isobutyl- 
acetic  acid — 

CH3CH, 
\/ 

CH 

i 
CH2 

I 
CH.NH, 

I 
COOH. 

When  pure  it  crystallises  in  white  transparent  plates,  but 
in  the  impure  state,  as  it  separates  out  on  concentrating 
a  pancreatic  digest,  it  forms  microscopic  globules  with  radial 
or  concentric  striation,  often  spoken  of  as  *  leucine  cones  ' 
(Fig.  172).  About  10  to  20  per  cent,  of  leucine  may  be 
obtained  from  most  proteins. 

'  Phenylalanine  and  proline  a,i'e  not  found  frge  in  a  pancreatic  digest,  being 
combined  to  form  a  polypeptide. 


334  PHYSIOLOGY 

Tyrosine  is  an  amino-fatty  acid  in  combination  with  an 
aromatic  group.     Its  constitution  is  shown  by  its  formula,  its 

C.,H,NH.,COOH 
/\ 

\/ 
OH 

full  name  being  para-oxyphenyl-a-amino-propionic  acid. 

aromatic  moiety  amino-fatty  acid. 

It  crystallises  in  long  slender  needles  aggregated  into 
sheaves  and  rosettes  (Fig.  173).  It  is  very  sparingly  soluble 
in  water  and  alcohol,  and  so  is  easily  separated  from  pancreatic 

Fig.  172.  Fig.  173. 


Leucine  'cones'  (imperfect  crystals)  Tyrosine  crystals  (Frey). 

(Frey) 

digests,  often  crystallising  out  spontaneously  at  the  bottom 
of  the  liquid.  It  forms  only  2-3  per  cent,  of  the  original 
protein. 

Aspartic  acid  (amino-succinic  acid  CoH,(NH.J.(COOH).^)  is  also  formed  in 
small  quantity  in  the  pancreatic  digestion  of  proteins.  It  forms,  however,  the 
greater  part  (35  per  cent.)  of  the  amino-acids  obtained  by  digestion  of  the 
proteins  of  wheat. 

Antipeptone  is  a  name  which  has  been  given  to  the 
precipitate  obtained  by  treating  the  digest  with  phospho- 
molybdic  acid,  after  separation  of  the  chief  amino-acids. 
Although  it  gives  the  biuret  test,  it  does  not  give  any  colora- 
tion with  Millon's  reagent,  and  is  not  properly  included 
in  the  class  of  peptones.  It  is  probably  a  mixture  of  bodies, 
including  the  bases,  lysine  and  arginine,  the  former  being 
diamino-caproic  acid  C,r,H,,  (NH^).^  COOH,  and  the  latter  a 
body  analogous  to  creatine  and  giving  rise,  like  creatine,  to 


THE   MECHANISMS   OF  DIGESTION  335 

urea  on  heating  with  baryta  water.  In  addition  to  these 
bases  a  small  amount  of  ammonia  is  set  free  during  the 
proteolysis. 

A  certain  proportion  of  the  nitrogen  is  in  the  form  of  a  polypeptide  which 
on  further  hydrolysis  yields  several  amino-acids  of  the  fatty  series  as  well  as 
the  greater  part  of  the  phenyl -alanine  and  proline  contained  in  the  original 
protein. 

Another  constant  product  of  the  pancreatic  digestion  of  proteins  is  trijpto- 
pliane,  a  body  belonging  to  the  aromatic  group,  and  giving  a  typical  red 
colour  with  bromine.  To  the  presence  of  this  group  in  proteins  is  due  the 
reaction  of  Adamkiewicz  and  Hopkins,  viz.  a  purple  colour  on  treatment  with 
strong  sulphuric  acid  and  glyoxylic  acid,  or  glacial  acetic  acid  contaminated  with 
the  latter  substance. 

The  changes  involved  in  the  pancreatic  digestion  of  pro- 
teins may  therefore  be  represented  as  follows  : 

Fibrin 

Soluble  protein  (globulins) 

Alkali-albumen 

I 
Deutero-albumoses 

Peptones 


Amino-acids  Hexone  bases       Tryptophane,      Polypeptide 

(leucine,  tyrosine,  etc.)       (lysine,  arginine,  etc.)         etc. 

Cystine 

In  the  case  of  coagulated  protein,  there  is  apparently 
no  formation  of  soluble  proteins  or  alkali-albumen,  the  first 
product  of  proteolysis  which  can  be  separated  being  deutero- 
albumose. 

Until  recently,  it  was  thought  that  the  small  molecules, 
which  constitute  the  end  products  of  pancreatic  digestion, 
must  be  practically  valueless  for  the  purposes  of  nutrition, 
and  that  the  main  use  of  the  energetic  action  of  the  trypsin 
was  the  gain  of  time  in  the  first  stages  of  proteolysis.  It  must 
be  remembered  however  that  we  employ  as  food  numbers  of 
different  kinds  of  proteins,  whose  constitution  diverges  con- 
siderably from  that  of  the  proteins  building  up  our  tissues. 
A  man  can  make  muscle  protein  out  of  the  most  various  kinds 
of  vegetable  protein,  whether  from  peas,  beans,  or  wheat.  For 
this  conversion  to  take  place  the  protein  molecule  must  be  first 
resolved  into  its  constituent  parts,  out  of  which  a  new  and 
different  protein  can  be  built  up.     A  thorough  disintegration 


336  PHYSIOLOGY 

is  therefore  a  necessary  preliminary  to  the  assimilation  of 
any  foreign  form  of  protein,  and  it  is  this  thorough  dis- 
integration that  is  effected  by  the  pancreatic  juice,  working 
with  the  succus  entericus.  Loewi  has  succeeded  in  keeping 
dogs  in  health,  and  in  a  state  of  nitrogenous  equilibrium,  for 
some  time  on  a  diet  consisting,  so  far  as  its  nitrogen  was 
concerned,  entirely  of  the  ultimate  products  of  pancreatic 
digestion. 

Since  the  bacteria  of  putrefaction  thrive  readily  in  a 
slightly  alkaline  solution  of  protein  such  as  pancreatic  juice, 
care  must  be  taken  in  all  experiments  with  this  juice  to  pre- 
vent putrefaction.  To  this  end  thymol  or  1  per  cent,  sodium 
fluoride  may  be  mixed  with  the  solution.  If  this  precaution 
be  omitted,  the  mixture  at  the  end  of  twenty-four  hours  has 
a  foul  odour,  and  is  found  to  be  swarming  with  bacteria,  under 
the  agency  of  which  the  proteins  are  further  split  up,  with 
the  formation  of  many  amino-acids,  free  fatty  acids,  and 
aromatic  bodies  such  as  phenol,  indol,  and  skatol.  To  the 
presence  of  the  last  two  bodies  is  due  the  horrible  faecal  odour 
of  a  pancreatic  digestion -mixture  which  has  been  allowed  to 
stand  without  the  addition  of  an  antiseptic.  These  aromatic 
substances  are  produced  from  the  tryptophane  (indoxyl  amino- 
propionic  acid),  which  is  one  of  the  primary  decomposition 
products  of  proteins. 

Gelatin  is  affected  by  pancreatic  juice  in  the  same  way  as 
by  gastric  juice,  being  converted  into  gelatin-peptones,  which 
do  not  gelatinise  on  cooling.  This  juice  however  is  unable 
to  dissolve  collagen,  the  chief  constituent  of  the  connective 
tissues.  Hence  if  the  stomach  of  a  dog  be  cut  out,  and  the 
lower  end  of  the  cesophagus  sewn  to  the  upper  end  of  the 
duodenum,  it  is  found  that  considerable  quantities  of  fat  pass 
undigested  through  the  alimentary  canal,  since  the  connective 
tissue  binding  fat-cells  together  can  no  longer  be  dissolved  by 
a  stomachless  dog. 

Action  on  Carbohydrates 

The  action  of  the  pancreatic  juice  on  starch  is  similar  to 
that  of  ptyalin,  but  is  more  rapid.  It  is  still  further  accele- 
rated by  the  addition  of  small  quantities  of  bile.  The  stages 
of  conversion  are  the  same  as  in  the  case  of  ptyalin,  the  end- 


THE   MECHANISMS   OF  DIGESTION  337 

products  being  acliroodextrin  and  maltose.  If  the  action  be 
long  continued,  the  process  of  hydrolysis  may  go  on  to  the 
formation  of  dextrose,  but  the  amount  of  this  body  formed  is 
in  all  cases  very  slight,  and  is  probably  due  to  the  presence  of 
traces  of  the  special  ferment,  maltase,  in  the  pancreatic  juice. 

Action  on  Fats 

Fresh  pancreatic  juice  contains  a  ferment  which  has  a 
hydrolytic  action  on  neutral  fats,  splitting  them  up  into 
glycerin  and  a  free  fatty  acid,  thus  : 

C3H,(C„H3,02)3  +  3H,0  =  C3H,(OH)3  +  3HC„H3,0,. 

Tripalmitin  (neutral  fat).  Glycerin.  Palmitic  acid. 

This  decomposition  takes  place  in  the  presence  of  the 
alkaline  salts  of  the  pancreatic  juice  and  bile.  The  free 
fatty  acid  formed  by  the  ferment  action  combines  with 
the  alkali  present,  displacing  the  CO.  to  form  a  soap.  The 
presence  of  soap  in  the  solution  enables  it  to  hold  all  the 
rest  of  the  neutral  fat  in  suspension.  Thus  if  a  drop  of 
rancid  oil  {i.e.  one  containing  free  acid)  be  allowed  to  drop  on 
to  the  surface  of  a  1  per  cent,  solution  of  sodium  carbonate, 
the  acid  at  the  exterior  of  the  drop  unites  with  the  alkali  to 
form  a  soap,  which  is  immediately  dissolved.  The  chemical 
change  and  solution  going  on  at  the  surface  of  the  drop  set 
up  in  the  surrounding  fluid  diffusion-currents  which  carry  off 
little  particles  of  the  neutral  fat.  These  immediately  become 
coated  with  a  layer  of  soap  which  prevents  them  running 
together  again.  So  we  see  a  white  cloud  appearing  round 
the  drop  of  rancid  oil,  and  under  the  microscope  the  cloud 
is  observed  to  consist  of  innumerable  tiny  droplets  of  fat 
suspended  in  the  alkaline  liquid.  A  single  shake  causes  the 
whole  drop  to  break  up  into  these  droplets,  the  milky  fluid 
thus  formed  being  spoken  of  as  an  emulsion.  In  this  way  the 
pancreatic  juice  has  the  power  of  emulsifying  neutral  fats. 

This  fat-splitting  action  may  go  on  in  a  neutral  or  slightly 
acid  medium,  and  so  is  not  subject  to  such  restrictions  as  are 
the  proteolytic  or  amylolytic  functions  of  the  pancreatic  juice. 

This  juice  also  contains  a  ferment  similar  to  rennet,  which 
has  the  property  of  curdling  milk.  It  is  probably  of  no 
physiological  importance. 

22 


338  PHYSIOLOGY 


Section  5 
THE   BILE 


The  bile  is  the  product  of  secretion  of  the  Hver.  This 
organ  differs  in  structure  from  all  other  glands  of  the  body, 
the  cells  being  so  numerous  and  pressed  together  around 
the  capillary  meshwork  that  their  primitive  arrangement 
as  secreting  tubules  is  no  longer  to  be  made  out  in  the  adult 
liver. 

The  liver  has  a  double  blood-supply :  the  portal  vein, 
which  supplies  a  rich  capillary  anastomosis  round  every  liver- 
cell,  and  carries  venous  blood  from  the  alimentary  canal ;  and 
the  hepatic  artery,  which  carries  oxygenated  arterial  blood, 
and  supplies  chiefly  the  connective  tissue  surrounding  the 
bile-ducts  and  blood-vessels  in  the  divisions  between  the 
lobules,  known  as  Glisson's  capsule. 

The  lobules  are  about  1-5  mm.  in  diameter  and  consist 
of  the  liver-cells,  blood-vessels,  and  bile-capillaries,  and  are 
bounded  by  the  connective  tissue  forming  Glisson's  capsule. 
The  branches  of  the  portal  vein  run  in  this  connective  tissue 
at  the  periphery  of  the  lobules,  where  they  are  termed  inter- 
lobular veins.  From  these  veins  a  rich  plexus  of  capillaries 
runs  towards  the  centre  of  the  lobule,  where  they  join  a  small 
vessel  known  as  the  intralobular  vein.  The  intralobular  veins 
unite  to  form  the  sublobular  veins,  which  pour  their  contents 
into  the  hepatic  veins.  These  at  the  hinder  border  of  the 
liver  run  into  the  inferior  vena  cava,  which  carries  their  blood 
into  the  heart. 

Corresponding  with  the  semi-arterial  functions  of  the 
portal  vein,  we  find  the  muscular  tissue  in  its  walls  better 
developed  than  is  the  case  with  most  veins.  The  portal  vein 
is  also  well  supplied  w^ith  vaso-motor  nerves,  which  leave  the 
cord  by  the  lower  dorsal  roots  and  run  in  the  great  splanchnic 
nerves.  Since  the  pressure  in  the  portal  vein  is  low  (about 
10  mm.  Hg),  any  constriction  of  the  portal  vein  will  help  to 
raise  the  arterial  blood-pressure,  not  so  much  by  increasing 
the  peripheral  resistance  as  by  lessening  the  total  capacity  of 
the  vascular  system. 


THE   MECHANISMS   OF  DIGESTION 


339 


The  whole  of  the  space  between  the  capillaries  is  filled  with 
liver-cells,  which  are  polygonal  in  shape,  and  about  0'02  mm. 
across.     They  consist  of  granular  protoplasm  with   a    large 

Fig.  174. 


Section  of  injected  liver,  showing  the  division  into  lobules.  The 
interlobular  branches  of  the  portal  vein  (F  P)  are  connected  with 
the  intralobular  branches  of  the  hepatic  vein  {H  V)  by  numerous 
radiating  capillaries.  Below  is  a  portion  of  the  same,  more  highly- 
magnified,  o,  liver  cell  ;  n,  nucleus ;  6,  blood-capillaries ;  c,  bile- 
capillaries. 


rounded  nucleus  in  the  centre  and  may  often  contain  hyaline 
masses  of  glycogen  (Fig.  175).  The  bile-ducts,  which  run  at 
the  periphery  of  the  lobules,  receive  the  bile  secreted  by  the 
liver-cells,  by  means  of   a   number   of   very  fine  branching 


340 


PHYSIOLOGY 


canals,  known  as  the  bile-capillaries.  The  latter  may  in 
section  be  seen  to  be  situated  between  the  adjacent  flat  sides 
of  the  liver-cells,  as  far  removed  from  a  blood-capillary  as 
possible,  their  walls  being  formed  by  the  liver-cells  themselves. 
The  bile-ducts  join  to  form  the  hepatic  duct,  which  leaves  the 
liver  at  the  transverse  fissure  and  passes  to  the  duodenum, 
giving  off  on  its  way  a  diverticulum,  the  gall-bladder,  con- 
nected with  the  main  duct  by  a  special  canal,  the  cystic  duct. 
This  subordination  of  the  glandular  to  the  vascular  arrange- 
ments of  the  liver  betokens  a  similar  subordination  of  the 
secreting  function  of  the  liver.  Although  the  bile  is  of  im- 
portance both  as  an  excretion  and  as  a  digestive  secretion,  its 

Fig.  175. 


a,  hepatic  cells  containing  glycogen ;  b,  hepatic  cells  from  which 
the  glycogen  has  been  removed;  c,  a  pi ejiaration  similar  to  b, 
but  from  a  liver  containing  originally  much  less  glycogen. 
(Heidenhain.) 


formation  represents  but  a  small  part  of  the  total  activities  of 
the  liver,  which  is  the  great  chemical  factory  of  the  body, 
modifying  in  the  direction  of  assimilation  and  dissimilation 
the  various  products  of  digestion  brought  to  it  by  the  portal 
blood.  An  animal  may  survive  complete  obstruction  of  the 
bile-ducts  for  many  months,  wheieas  abolition  of  the  total 
liver  functions  is  followed  by  death  within  a  few  hours. 

The  secretion  of  bile  is  a  continuous  process,  but  it  does 
not  flow  directly  into  the  intestine,  being  stored  up  during 
fasting  in  the  gall-bladder,  whence  it  is  discharged  by  the 
contraction  of  this  viscus  when  the  acid  chyme  passes  the 
orifice  of  the  common  bile-duct. 

The  discharge  of  bile  into  the  intestine  is  greatest  about 


THE   MECHANISMS   OF  DIGESTION  841 

three  to  five  hours,  and  agam  about  thirteen  hours,  after  the 
ingestion  of  food.  The  secretion  of  bile  is  much  quickened  by 
the  injection  of  secretin  into  the  blood  stream  :  a  fact  that 
will  account  for  the  increased  secretion  observed  within  the 
first  few  hours  after  the  taking  of  food.  Since  bile  is  an 
important  adjuvant  to  the  pancreatic  juice,  it  is  evidently  of 
advantage  that  its  secretion  should  be  increased  by  the  same 
mechanism  that  evokes  the  secretion  of  the  pancreatic  juice. 

Bile  as  obtained  from  the  gall-bladder  is  dark  brown  or 
greenish  in  colour.  It  is  alkaline  and  slimy  from  the  presence 
of  mucin.  Its  specific  gravity  varies  from  1010  to  1040. 
The  following  table  represents  the  average  composition  of 
human  bile  taken  from  the  gall-bladder. 

100  parts  contain — 

Water 85  parts. 

Bile  salts 10     ,> 

Fats,  lecithin,  and  cholesterin  .         .  1  part. 

Mucus  and  pigment          ...  3  parts. 

Inorganic  salts         .         .          about  1  part. 

Besides  these  constituents,  bile  contains  gases,  especially 
carbon  dioxide,  and  traces  of  soaps. 

If  the  bile  be  collected  as  it  is  secreted  by  the  liver,  by 
inserting  a  cannula  in  the  hepatic  duct,  it  is  found  to  contain 
a  larger  percentage  of  water  and  little  or  no  mucin.  It  is 
evident  therefore  that  during  its  stay  in  the  gall  bladder  the 
bile  loses  its  water,  and  acquires  mucin,  which  is  secreted  by 
the  mucous  membrane  of  the  gall-bladder.  • 

The  bile  salts  are  two  in  number — glycocholate  and  tauro- 
cholate  of  soda.  The  relative  amounts  of  the  two  salts  vary 
in  difierent  animals,  the  taurocholate  being  as  a  rule  most 
abundant  in  carnivora,  and  the  glycocholate  in  herbivora. 
In  human  bile,  glycocholate  forms  nearly  the  whole  of  the 
bile  salts  present. 

The  bile  salts  may  be  extracted  in  the  following  way  : — 
Bile  is  mixed  into  a  paste  with  animal  charcoal,  and  the 
mixture  dried,  pounded  up,  and  extracted  with  absolute 
alcohol   and   filtered.      On   adding   ether   to    the    alcoholic 

'  The  greater  part  of  the  muein-like  substance  occurring  in  ox-bile,  and  pre- 
cipitated by  acetic  acid,  is  really  a  nucleo-albumen.  Human  bile  on  the  other 
hand  contains  true  mucin. 


34'2  PHYSIOLOGY 

filtrate,  and  allowing  it  to  stand,  a  crystalline  precipitate  is 
produced,  consisting  of  the  two  bile  salts.  These  have  a 
bitter  taste,  and  are  easily  soluble  in  water.  Their  presence 
in  a  fluid  may  be  shown  by  Pettenkofer's  reaction. 

Addition  of  a  drop  of  cane-sugar  solution  and  excess  of  concentrated 
sulphuric  acid  to  a  solution  of  bile  salts  gives  a  purple  colour,  due  to  the  inter- 
action oifurfurol,  produced  by  the  action  of  the  sulphuric  acid  on  the  cane-sugar, 
with  cholalic  acid,  the  common  constituent  of  both  bile  acids.  This  colour 
may  be  interfered  with  by  the  dark-brown  colour  produced  by  the  charring  of 
the  sugar  with  the  sulphuric  acid.  Either  of  the  following  ways  may  be  adopted 
to  obtain  a  good  purple  reaction  : 

A.  The  bile  and  sugar  are  shaken  up  in  a  test-tube  until  the  upper  part  of  the 
tube  is  filled  with  froth.  If  the  concentrated  sulphuric  acid  be  no\v  poured  down 
the  side  of  the  tube,  the  froth  is  stained  a  purple  colour  where  it  comes  in 
contact  with  the  acid. 

B.  A  porcelain  capsule  is  rinsed  out  successively  with  solutions  of  bile  salts, 
cane  sugar,  and  dilute  sulphuric  acid  (25  per  cent.).  On  warming  the  capsule 
gently  over  a  flame,  water  is  driven  off  from  the  thin  film  of  dilute  acid,  and 
the  concentrated  acid  thus  produced  acts  on  the  thin  film  of  sugar  and  bile 
salts,  causing  a  brilliant  purple  colour  of  the  whole  of  the  inner  surface  of  the 
capsule. 

Glycocholic  acid  is  a  conjugated  acid,  which  on  hydro- 
lysis   splits  up  into  glycine  (amido-acetic  acid)  and  cholalic 

acid  (C,,H,oO,). 

Taurocholic  acid  is  also  a  conjugated  acid,  which  can 
be  split  up  into  taurine,  an  amido-acid  containing  a  large 
proportion  of  oxidised  sulphur  (amino-isethionic  acid),  and 
cholalic  acid. 

The  glycocholic  acid  can  be  obtained  from  the  bile  of  some  herbivora  in  a 
free  condition  by  adding  a  little  hydrochloric  acid  and  ether  and  shaking,  when 
the  acid  crystallises  out  in  fine  needle-shaped  crystals. 

(CHOH 
Cholalic  acid  is  a  monobasic  acid  C.,,,!!^,  -I  (CH.,OH).,,   and  is  obtained  from 

( COOH 
either  bile   acid   by  prolonged  boiling  with   strong  solution  of  caustic   soda. 
Under  the  influence  of  acids  or  putrefaction  (as  in  the  intestine)  it  is  converted 
into  an  insoluble  anhydride,  known  as  dyslysin. 
CH,.NH2 
Glycine  orglycocoll  |  is  also  obtained  by  the  hydrolytic  dissociation 

COOH 
of  some  proteins  and  of  certain  albuminoids  such  as  gelatin  and  elastin.     It 
occurs  in  the  urine  conjugated  with  benzoic  acid  as  hippuric  acid. 
/C,H,.NH2 
Taurine  BO^'C  i^  found  nowhere  else  in  the  body,  and  is  the  only 

\0H 
known  example  of  a  sulphonic  acid  which  plays  any  part  in  the  normal  functions 
of  the  body.    It  can  be  formed  artificially  from  cystine,  a  normal  constituent  of 
all  proteins. 


THE   IVIECHANISMS   OF  DIGESTION  343 

The  mucin  and  nucleo-albumen  present  may  be  preci- 
pitated by  acetic  acid.  The  precipitate  is  sokible  in  dilute 
alkalies. 

The  bile-pigments  are  hiliruhin  (brown)  and  hiliverdin 
(green),  and  occur  in  the  bile  in  combination  with  calcium. 
The  colour  of  the  bile  depends  on  the  relative  amounts  of 
these  two  pigments  present.  Biliverdin  (CicHi^N204)  may 
be  obtained  on  oxidation  of  bilirubin  (CigHi^N.jOy). 

The  presence  of  bile-pigments  may  be  proved  by  Gmelin's 
test.  A  drop  of  bile  on  a  white  plate  is  treated  with  a  drop 
of  yellow  nitric  acid.  Where  the  two  drops  come  in  contact 
a  play  of  colours  is  produced,  due  to  the  formation  of  various 
oxidation-products  of  bilirubin.  These  colours  occur  in  the 
following  order — brown,  green,  blue,  red,  yellow.  The  end- 
product  of  the  reaction  which  gives  the  yellow  colour  is  known 
as  choletelin. 

The  cholesterin  present  is  probably  kept  in  solution  by 
the  bile  salts.  Under  abnormal  conditions,  cholesterin  may 
be  precipitated  and  may  form  concretions  in  the  gall-bladder 
(gall-stones).  More  rarely  we  meet  with  gall-stones  con- 
sisting of  the  bile-pigments  in  combination  with  alkaline 
earths. 

Actions  of  Bile 

Bile  contains  small  quantities  of  an  amylolytic  ferment 
which  has  a  feeble  digestive  action  on  starch.  If  added  to  a 
mixture  of  starch  and  pancreatic  juice,  it  materially  hastens 
the  action  of  the  latter. 

On  adding  bile  to  an  acid  solution  of  albumoses  and 
peptones,  such  as  the  products  of  gastric  digestion  which  come 
through  the  pylorus  into  the  first  part  of  the  duodenum,  a 
precipitate  is  produced,  consisting  of  glycocholic  acid,  syntonin, 
and  albumoses. 

One  function  of  the  bile  then  is  to  neutralise  the  gastric 
juice  and  prepare  the  way  for  pancreatic  secretion. 

The  alkaline  salts  of  the  bile  can  combine  with  the  fatty 
acids  set  free  by  the  pancreatic  juice  to  form  soaps,  and  so 
aid  in  the  digestion  and  emulsification  of  fats.  Bile  also 
assists  in  the  absorption  of  fats  by  virtue  of  the  bile  salts  it 
contains.  Oil  will  not  run  through  a  filter  moistened  with 
water,  but  will  do  so  if  it  be  moistened  with  a  solution  of  bile 


344  PHYSIOLOGY 

salts.  The  presence  of  bile  salts  lowers  the  surface-tension 
between  the  oil  and  the  water,  so  that  in  the  intestine  the  drop- 
lets of  fat  are  able  to  come  into  intimate  contact  with  the  absorb- 
ing surface  of  the  epithelium,  and  with  the  digestive  fluids. 

The  main  importance  of  the  bile  however  is  as  a  fat 
solvent.  In  a  slightly  acid  medium  the  bile  acids  will  dis- 
solve 4  to  5  per  cent,  of  free  fatty  acid.  Bile  has  no  solvent 
action  on  neutral  fats,  but  will  dissolve  soaps,  including  the 
insoluble  soaps  of  the  alkaline  earths.  The  importance  of 
bile  for  the  absorption  of  fats  is  shown  by  the  occurrence  of 
a  large  amount  of  undigested  fat  in  the  fasces  on  shutting  off 
the  bile  from  the  intestine.  The  fat-solvent  action  of  the  bile 
salts  in  the  bile  is  much  aided  by  the  simultaneous  presence 
in  this  fluid  of  lecithin  and  cholesterin  in  solution. 

The  presence  of  bile  in  the  intestine  is  said  to  excite 
contractions  of  the  muscular  walls,  and  so  act  as  a  natural 
purgative.  In  the  same  way  the  muscular  fibres  of  the 
absorbent  villi  are  stimulated  by  the  presence  of  bile,  and 
contract,  forcing  the  contents  of  the  villus  into  the  subjacent 
lacteal. 

Bile  is  often  spoken  of  as  an  antiseptic,  but  this  state- 
ment must  be  qualified.  The  free  bile  acids,  especially 
taurocholic  acid,  have  a  pronounced  antiseptic  action.  The 
action  is  however  lost  when  the  acids  are  combined  with 
alkalies,  as  in  the  bile  itself,  which  decomposes  extremely 
readily. 

The  Origin  and  Fate  of  the  Biliary  Constituents 

The  bile  is  to  be  regarded  partly  as  a  secretion,  having  an 
important  function  in  the  digestion  of  fats,  and  partly  as  an 
excretion— a  means  by  which  the  eft'ete  colouring  matter  of 
the  blood  is  got  rid  of. 

The  bile  salts  are  formed  by  the  liver,  as  is  shown  by  the 
fact  that,  after  extirpation  of  the  liver  in  frogs  or  birds,  no 
accumulation  of  these  salts  takes  place  in  the  body.  If 
however  the  bile-ducts  be  ligatured,  bile  salts  are  found  in 
the  blood  and  in  the  urine.  The  glycine  and  taurine  are 
probably  derived  from  protein  disintegration,  but  we  know 
nothing  concerning  the  precursors  of  cholalic  acid.  In  the 
intestine  the  bile  salts  play  their  part  in    the   digestion  of 


THE   MECHANISMS   OF  DIGESTION  345 

fats,  and  are  then  for  the  most  part  reabsorbed,  passing 
along  the  portal  vessels  to  the  liver,  where  they  are  again 
secreted,  so  that  they  can  exert  their  functions  over  and 
over  again,  A  certain  amount  is  split  up  in  the  intestine 
into  the  amino-acids  and  cholalic  acid,  the  former  being 
reabsorbed,  and  the  latter  being  excreted  with  the  faeces. 

The  bile-pigments  are  the  products  of  disintegration  of  the 
haemoglobin  of  the  blood.  They  play  no  further  part  in  the 
body,  and  are  excreted  with  the  faeces  in  a  slightly  altered 
form.  It  was  long  debated  whether  they  were  formed  by  the 
liver,  or  whether  some  might  not  be  formed  in  the  blood  itself 
or  in  the  other  tissues  from  the  disintegrated  red  corpuscles. 
It  is  found  that,  after  blood  has  been  extravasated  into  the 
tissues,  the  htemoglobin  undergoes  certain  modifications,  and 
is  converted  into  the  body  named  haematoidin.  Now  hfema- 
toidin  is  isomeric  and  probably  identical  with  bilirubin,  and 
this  fact  was  looked  upon  as  furnishing  strong  evidence  for 
the  haematogenous  origin  of  bile-pigments.  Experiment  has 
shown  however  that  when  blood-corpuscles  are  broken  up 
in  the  circulation  (a  process  which  is  normally  taking  place 
on  a  small  scale)  no  bile-pigment  is  formed  except  by  the 
agency  of  the  liver.  A  great  breakmg-up  of  blood-corpuscles 
and  setting  free  of  haemoglobin  may  be  caused  in  animals  by 
the  inhalation  of  arseniuretted  hydrogen.  If  the  liver  be 
present,  this  disintegration  of  blood-corpuscles  causes  a 
greatly  increased  formation  of  bile-pigment,  which  is  elimi- 
nated with  the  bile,  or  partly  reabsorbed  by  the  lymphatics 
from  the  biliary  passages,  giving  rise  to  jaundice.  If  in  a 
goose  the  liver  be  shut  out  from  the  circulation  or  extirpated, 
and  arseniuretted  hydrogen  administered,  not  a  trace  of  bile- 
pigment  is  produced. 

The  cholesterin  of  the  bile  is  sometimes  looked  upon  as  a 
product  of  nerve-disintegration,  since  this  substance  is  found 
abundantly  in  the  central  nervous  system  ;  but  we  have  no 
evidence  for  or  against  this  view. 

Bile  is  secreted  at  a  very  low  pressure— 15  mm.  Hg.  If 
the  pressure  in  the  bile-ducts  rises  above  this  point,  as  may 
easily  happen  when  the  flow  is  obstructed  in  consequence  of 
inflammatory  thickening  of  the  mucous  membrane,  or  by  the 
presence  of  a  gall-stone,  or  even  by  a  very  viscid  bile,  the  bile 
is  reabsorbed  by  the  lymphatics  and  reaches  the  blood,  and 


346  PHYSIOLOGY 

nearly  all  the  tissues  of  the  body  are  stained  yellow  by  the 
pigments,  giving  rise  to  jaundice.  This  pressure  however  is 
higher  than  the  pressure  in  the  portal  vein,  which  is  only 
about  10  mm.  Hg ;  for  we  must  remember  that  the  blood  in 
the  portal  vein  has  already  passed  through  a  system  of  capil- 
laries, so  that  its  pressure  is  extremely  low.  The  fact  that 
the  pressure  in  the  bile-ducts  may  exceed  that  in  the  portal 
vein  shows  that  the  secretion  of  the  water  of  the  bile  is  not 
effected  by  a  mere  process  of  filtration. 


THE   MECHANISMS   OF   DIGESTION  347 

Section  6 
SUCCUS   ENTEEICUS   OE   INTESTINAL  JUICE 

The  secretion  of  the  tubular  glands  (Lieberkiihn's  follicles), 
which  beset  the  mucous  membrane  of  the  intestine,  may  be 
obtained  in  a  pure  condition  in  the  following  way.  An  opening 
is  made  into  the  abdomen  of  an  animal,  and  a  piece  of  the 
small  intestine,  ten  or  twelve  inches  long,  is  separated  from 
the  rest,  its  attachment  to  the  mesentery  with  its  blood-vessels 
and  nerves  being  left  intact.  The  two  ends  of  the  remaining 
piece  of  intestine  are  sutured  together,  so  that  the  animal  is 
left  with  a  continuous  but  shortened  alimentary  canal.  One 
end  of  the  excised  piece  is  closed  by  sutures,  and  the  margins 
of  the  other  end  sewn  to  the  margins  of  the  abdommal  wound. 
An  intestinal  fistula  is  thus  produced,  from  which  the  juice 
may  be  collected  free  from  contamination  by  the  other  diges- 
tive juices.  Secretion  is  normally  excited  by  the  mechanical 
stimulation  of  the  food  as  it  passes  down  the  intestine,  and 
may  be  evoked  in  the  isolated  loop  by  the  introduction  of  a 
rubber  balloon.  The  presence  of  pancreatic  juice  is  said  also 
to  act  as  a  specific  stimulus  for  the  intestinal  glands.  The 
amount  of  succus  entericus  to  be  obtained  in  either  of  these 
ways  varies  very  much  according  to  the  position  in  the 
intestine  of  the  isolated  loop.  Thus  a  large  secretion  is 
obtained  from  an  isolated  loop  of  the  upper  part  of  the  intes- 
tine including  the  duodenum,  and  it  is  stated  that  in  this 
situation  secretion  may  be  excited  by  the  intravenous  injection 
of  secretin.  If,  however,  a  loop  be  made  from  the  lower 
end  of  the  ileum,  it  is  difficult  to  obtain  even  a  few  drops 
of  intestinal  juice,  whatever  form  of  stimulation  be  employed. 

Intestinal  juice  is  a  clear,  limpid  fluid,  with  a  specific 
gravity  of  1010,  containing  a  trace  of  protein,  and  salts, 
of  which  sodium  carbonate  is  the  most  abundant.  In 
consequence  of  the  presence  of  this  salt  it  has  a  strong 
alkaline  reaction. 

Actions 

On  coagulable'proteins,  fats,  and  starch,  succus  entericus 
has  no  action.  It  contains  however  invert  ferments,  by  the 
agency  of  which  cane-sugar  is  converted  into  dextrose  and 


348  PHYSIOLOGY 

Isevulose,  and  maltose  is  converted  into  dextrose.  Lactase, 
which  converts  milk  sugar  into  galactose  and  dextrose,  is 
often  present,  and  is  always  found  in  animals  on  a  milk  diet. 
The  important  '  ferment  of  ferments  '  enterohinase,  which 
awakens  the  proteolytic  activity  of  the  pancreatic  juice,  has 
already  been  mentioned.  The  alkaline  reaction  of  the  succus 
is  probably  important  in  neutralising  the  free  acids  (lactic, 
butyric,  etc.)  produced  by  the  action  of  putrefactive  micro- 
organisms on  the  foodstuffs.  Although  intestinal  juice  has 
no  action  on  coagulable  proteins,  it  contains  a  ferment, 
erepsin,  which  converts  the  albumoses  and  peptones,  produced 
higher  up  in  the  gut,  into  the  amino-acids  and  hexone  bases 
which  are  the  final  products  of  the  action  of  trypsin  on 
proteins. 

The  following  experiment  is  supposed  to  show  the  influence 
of  the  intestinal  nerves  on  the  secretion.  The  abdomen  being 
opened,  the  small  intestine  is  ligatured  in  four  places,  so  as 
to  shut  oft"  three  equal  lengths  of  bowel  ;  all  the  nerves  going 
to  the  middle  segment  are  divided  and  the  abdomen  closed. 
At  the  end  of  two  or  three  hours  the  wound  is  opened,  and  it 
is  found  that  the  middle  segment  is  distended  with  fluid, 
whereas  the  other  two  segments,  which  have  their  nerves 
intact,  are  comparatively  empty.  This  secretion  has  been 
regarded  as  analogous  to  the  paralytic  secretion  of  saliva, 
which  continues  for  some  weeks  after  section  of  all  the  nerves 
going  to  the  submaxillary  gland.  We  do  not  know  however 
how  far  this  phenomenon  is  to  be  ascribed  to  vascular  changes 
taking  place  in  the  loop  of  intestine  in  consequence  of  the 
section  of  its  nerves. 


THE   MECHANISMS   OF  DIGESTION 


349 


Section  7 
ABSOEPTION   OF   FOODSTUFFS 

We  must  now  consider  the  ways  in  which  the  foodstuffs, 
that  have  been  digested  and  rendered  soluble  in  the  ali- 
mentary canal,  pass  into  the  circulation  to  be  distributed  to 
all  the  cells  of  the  body.  There  are  two  main  paths  of 
absorption— the  blood-vessels  and  lymphatics.  The  blood- 
vessels form  a  dense  capillary  anastomosis  immediately 
under  the  epithelial  layer  covering  the  inner  surface  of  the 


Fig.  176. 


Epithelium  of 

villus 


Artery 


Lieberkiilni's 

follicle 


Central  lacteal 


^^•m^^^^^^^MMMMM 


—  Mucosa 

—  Muscularis  muc. 


Lymphatic  plexus 

Circular  muscle 

Lymphatic  plexus 
Longitudinal  muse. 


Diagramniiitic  section  thiougli  wall  of  small  intestine  to  show 
vascular  and  lymphatic  arrangements  of  mucous  membrane. 
(From  Bohm  and  Davidoff,  after  IMall.) 

mucous  membrane.  In  order  to  increase  the  absorbing  sur- 
face, the  mucous  membrane  of  the  small  intestine  in  man  is 
thrown  into  transverse  folds  — the  valvules  conniventes,  which 
are  thickly  covered  with  finger-like  elevations  or  villi.  The 
body  of  a  villus  is  made  up  of  a  reticular  tissue  composed  of 
branching  cells,  the  meshes  of  which  may  contain  leucocytes 
of  various  forms.  In  the  centre  of  the  villus  is  a  wide 
lymphatic  vessel,  the  central  lacteal.     The  endothelial  cells 


350  PHYSIOLOGY 

forming  the  wall  of  the  lacteal  are  continuous  with  the 
l)ranched  cells  of  the  reticular  tissue,  but  there  is  no  free  com- 
munication between  the  spaces  of  this  tissue  and  the  begin- 
ning of  the  lymphatic.  Below,  the  cavity  of  the  lacteal  is 
continued  into  a  plexus  of  lymphatics  lying  in  the  mucous 
membrane,  which  communicates  by  vertical  branches  with 
a  large  plexus  lying  in  the  submucosa.  The  intestinal  surface 
of  the  villus  is  covered  with  a  single  layer  of  columnar 
epithelial  cells,  which  have  a  hyaline  border  presentmg 
delicate  vertical  striation,  apparently  due  to  the  presence  in 
the  border  of  a  number  of  cilia-like  processes.  The  capillary 
network  lies  outside  the  lacteal  immediately  under  the 
epithelium.  The  blood-vessels  pour  their  contents  into  the 
radicles  of  the  portal  vein,  which  carry  them  thence  to  the 
liver.  The  lymphatics  in  the  submucosa  join  to  form  larger 
trunks,  which  run  between  the  two  layers  of  the  mesentery 
to  a  collection  of  lymphatic  glands  at  the  back  of  the  peri- 
toneal cavity.  The  lymph,  after  flowing  through  these 
glands,  is  collected  into  a  large  vessel — the  receptaculum 
chyli,  from  which  it  is  carried  in  the  thoracic  duct  to  be 
discharged  into  the  blood-stream  at  the  junction  of  the  left 
jugular  and  subclavian  veins. 

Absorption  of  Fats 

During  fasting  the  lymph  contained  in  these  vessels  is 
exactly  similar  to  that  contained  in  any  other  part  of  the 
body.  If  a  cannula  be  inserted  in  the  thoracic  duct  of  a 
fasting  dog,  and  the  animal  be  given  a  meal  rich  in  fat,  it  is 
found  that  the  amount  of  lymph  flowing  from  the  cannula 
is  the  same  as  before,  but  the  lymph  has  changed  its  appear- 
ance, being  now  white  like  milk.  On  microscopic  examina- 
tion this  milky  appearance  is  found  to  be  due  to  the  presence 
of  small  fatty  globules  similar  to  those  in  milk,  and  of  a 
number  of  very  fine  particles— much  finer  than  any  of  the 
globules  met  with  in  milk— which  may  exhibit  Brownian 
movement.  These  constitute  the  '  molecular  basis '  of  the 
chyle.  If  the  abdomen  of  the  animal  be  opened,  the  course 
of  the  lymphatics  along  the  mesentery  is  evident  from  the 
milky  character  of  their  contents.  It  is  on  account  of  this 
milky  appearance  during  digestion  that  the  name  of  lacteal 


THE   MECHANISMS   OF  DIGESTION  351 

has  been  given  to  the  lymphatics  of  the  ahmentary  canal ; 
and  chyle  is  simply  lymph  with  fatty  globules,  and  molecular 
basis. 

Since  the  whole  of  the  chyle  is  poured  by  the  thoracic 
duct  into  the  blood,  the  plasma  or  serum  of  the  latter, 
obtained  after  a  meal  containing  fat,  is  also  found  to  present 
a  milky  appearance,  in  consequence  of  the  presence  of  fat 
globules  derived  from  the  chyle.  If  however  the  thoracic 
duct  be  ligatured,  a  fatty  meal  is  not  followed  by  any  increase 
in  the  amount  of  fat  in  the  blood. 

It  is  apparent  then  that  the  greater  part  of  the  fat  is 
absorbed  by  the  chyle,  and  60  per  cent,  of  the  absorbed  fat 
can  be  obtained  from  the  chyle  through  a  cannula  placed  in 
the  thoracic  duct.  Comparative  analyses  of  portal  and 
carotid  blood  during  digestion  show  that  the  amounts  of  fat 
contained  in  the  two  are  the  same ;  hence  it  is  concluded 
that  no  fat  is  absorbed  through  the  intermediation  of  the 
blood-vessels.  It  has  not  yet  been  found  possible  to  trace 
the  mechanism  of  absorption  of  that  portion  of  the  fat  which 
does  not  enter  the  blood  by  way  of  the  lacteals.  It  is 
possible  that  it  may  be  utilised  or  built  up  into  more  com- 
plex compounds  in  the  tissues  of  the  intestinal  mucous 
membrane. 

We  must  now  inquire  how  the  fat  gets  into  the  lacteals. 
If  sections  be  made  of  the  villus  during  the  digestion  of  fat, 
and  stained  with  osmic  acid,  the  epithelial  cells  are  seen  to 
be  full  of  black  fatty  granules  of  various  sizes  (Fig.  177). 
These  granules  are  also  to  be  observed  in  the  spaces  sur- 
rounding the  central  lacteal.^  The  lacteal  itself  is  full  of 
lymph  with  fat-globules,  but  the  latter  are  here  much  more 
minute,  and  correspond  to  the  molecular  basis  of  the  chyle. 

We  must  conclude  that  the  fat  is  taken  up  by  the 
epithelial  cells  covering  the  villus.  But  in  what  form '? 
We  have  already  seen  that  the  effect  of  the  pancreatic  juice 
on  neutral  fats,  which  form  the  great  proportion  of  the  fat 
of  food,  is  to  split  them  up  to  a  certain  extent  into  free 
fatty  acid  and  glycerin.  The  rancid  fat  thus  produced  in  an 
alkaline  medium  at  once  forms  an  emulsion,  and  since  the 

'  Many  of  the  leucocytes  also  present  black  granules,  but  these  are  supposed 
not  to  be  of  a  fatty  nature,  since  they  are  not  dissolved  on  treating  the  section 
with  ether. 


352 


PHYSIOLOGY 


chyle  also  contains  fat  in  a  finely  divided  condition,  it  was 
thought  that  the  digestion  of  fat  essentially  consisted  in 
a  splitting  up  into  particles  fine  enough  to  be  taken  up  by 
the  epithelial  cells.  On  examining  sections  of  the  villi 
during  fat-absorption,  it  will  be  seen  however  that  the 
striated  border  is  always  free  from  fat-globules,  and  that 
the  globules  grow  in  size  from  the  inner  to  the  attached 
border  of  the  cell.  Moreover  by  such  an  explanation  it  is 
difficult  to  account  for  the  extreme  importance  of  bile  for 
the  absorption  of  fat.  It  seems  practically  certain  that  there 
is  no  fundamental  difference  between  the  absorption  of  fats 

Fig.  177. 


■••*V 


•    ••  • 


*     2«     t 


Columnar  epithelium  from  small  intestine  of  frog  stained  with 
osmic  acid  to  show  fat-absorption.  A,  live  hours  after  a 
meal  of  olive  oil ;  B,  three  hours  later.  It  should  be  noticed 
that  the  fat  globules  first  formed  grow  in  size  in  the  course 
of  digestion,  pointing  to  a  gradual  deposition  of  fat  on  the 
globules  from  solution  in  the  protoplasm. 


and  that  of  carbohydrates  and  proteins,  and  that  they  are 
all  alike  absorbed  in  a  state  of  solution.  If  fatty  acids  are 
administered  to  an  animal  they  are  absorbed,  but  appear 
in  the  chyle  as  neutral  fat,  showing  that  a  synthesis  of 
the  fatty  acid  and  glycerin  has  taken  place  on  the  passage  of 
the  fatty  acid  from  the  intestine  to  the  lacteal  through  the 
epithelium.  In  the  same  way,  soaps  taken  with  the  food 
appear  in  the  chyle  as  neutral  fats,  and  in  both  cases  the 
assimilation  is  improved  by  the  addition  of  the  necessary 
amount  of  glycerin.  We  may  conclude  that  the  formation  of 
an  emulsion  in  the  intestine  is  not  the  end  of  the  digestive 


THE  MECHANISMS   OF  DIGESTION  353 

process,  but  serves  merely  to  bring  a  larger  surface  of  the 
fat  in  contact  with  the  digestive  juices.  In  the  digestion 
and  absorption  of  fat  there  is  a  concerted  action  of  the 
pancreatic  juice  and  the  bile.  The  pancreatic  juice  splits 
about  6  per  cent,  of  the  neutral  fat  into  fatty  acid  and 
glycerin.  In  the  presence  of  excess  of  alkalies,  the  fatty 
acid  forms  a  soluble  soap  which,  together  with  the  glycerin, 
is  absorbed  by  the  epithelial  cells,  and  recombined  in  the 
body  of  the  cell  into  neutral  fat.  If,  as  is  often  the  case 
especially  with  a  highly  fatty  diet,  the  reaction  of  the  small 
intestine  be  acid,  the  formation  of  soaps  can  no  longer  go  on. 
The  fat-splitting  action  of  the  pancreatic  juice  however  con- 
tinues, and  the  fatty  acids  set  free  are  dissolved  by  the  bile 
acid  and  taken  up  by  the  epithelial  cells.  Here  the  synthesis 
of  the  neutral  fat  once  more  occurs,  and  the  bile  acid  is 
carried  by  the  portal  blood  to  the  liver  to  be  re-secreted  with 
the  bile  into  the  intestine,  where  it  may  aid  the  absorption  of 
a  further  amount  of  fat.  As  the  free  fatty  acids  and  soaps 
are  absorbed,  the  pancreatic  juice  is  able  to  split  up  a  further 
portion  of  the  neutral  fat,  until  the  whole  of  the  neutral  fat 
of  the  food  has  been  absorbed  by  the  epithelial  cells  of  the 
intestine  in  a  state  of  solution  as  soaps  or  fatty  acids. 
It  is  partly  on  this  account  that,  after  extirpation  of  the 
pancreas,  fats  are  not  absorbed  even  if  administered  to  the 
animal  in  the  form  of  a  fine  emulsion  containing  neutral  fat 
suspended  in  a  solution  of  soap.  If  however  to  this  emulsion 
chopped-up  pancreas  be  added,  a  large  proportion  of  the  fat 
is  absorbed. 

The  epithelial  cells  extrude  the  fat-granules  into  the 
spaces  of  the  reticular  tissue.  Here  a  certain  amount  may 
be  taken  up  by  the  leucocytes  and  carried  into  the  beginning 
of  the  lacteal.  The  greater  proportion  however  is  probably 
pressed  forcibly  through  the  wall  of  the  central  lacteal  by  the 
contractions  of  the  muscular  tissue  of  the  villus.  A  further 
contraction  of  these  fibres  empties  the  lacteal  into  the 
submucous  plexus  of  lymphatics,  where  the  presence  of  valves 
prevents  any  reflux,  so  that,  on  relaxation  of  the  muscle- 
fibres,  there  is  no  hindrance  to  the  further  passage  of  fat  and 
lymph  into  the  flaccid  central  lacteal.  This  is  the  so-called 
pumping  action  of  the  villus. 

23 


354  PHYSIOLOGY 

Absorption  of  Carbohydrates 

The   other    constituents   of    the   foodstuffs    seem   to    be 
absorbed    chiefly,    if   not    completely,   by   the   blood-vessels. 
Blood  normally  contains  a  small  amount  of  dextrose  (O'l  to 
0*2  per  cent.),  and  the  proportion  of  sugar  in  the  lymph  is 
the    same  as  in   the  blood.     After   a   meal   rich    in    carbo- 
hydrates, the  proportion  of  sugar  in  the  chyle  flowing  from 
the  thoracic  duct  is  the  same  as  that  in  the  blood  from  the 
carotid  artery  ;  but  it  is  found  that  there  is  slightly  more 
sugar  in  the  blood  from  the  portal  vein  than  in  that  from 
the  hepatic  vein  or  carotid  arter}'.     It  is  inferred  therefore 
that  the  blood  in  the  capillaries  of  the  intestinal  wall  takes 
up  sugar  in  the  form  of  dextrose  and  carries  it  to  the  liver. 
Here  the  excess  of    sugar  is  taken  up  by  the  hepatic  cells, 
and  converted  by  them  into  the  colloid    carbohydrate,  gly- 
cogen, which  is  deposited  in  the  substance  of  the  cell.     In 
this   way    the   liver    acts    as   a  storehouse   of  carbohydrate 
material,  and  prevents  the  sugar  in  a  rich  carbohydrate  meal 
from  escaping  into  the  general  circulation.     This  function  is 
important,  since  it  is  found  that  if  the  amount  of  sugar  in  the 
blood  be  raised  above  the  normal,  the  excess  is  immediately 
excreted  by  the  kidney  ;  so  that  without  such  an  economising 
organ  as  the  liver  the  greater  part  of  a  carbohydrate  meal 
would  at  once  be  wasted. 

We  have  already  seen  (p.  336)  that  the  end-product  of  the 
action  of  the  salivary  and  pancreatic  ferments  on  starch 
is  a  mixture  of  maltose  and  achroodextrin.  In  the  blood 
of  the  portal  vein  however,  we  find  only  dextrose  ;  and  it 
appears  that  the  dextrin  and  maltose  must  undergo  some 
further  change  before  reaching  the  blood.  We  know  for 
certain  that  the  succus  entericus  contains  a  ferment  which 
can  convert  maltose  into  dextrose ;  but  it  is  possible  also 
that  the  epithelial  cells  lining  the  intestine  are  able  to  effect 
a  transformation  of  both  dextrin  and  maltose  into  dextrose. 
It  must  be  remembered  too  that  normal  blood-plasma,  serum, 
and  lymph  contain  a  ferment  which  quickly  converts  boiled 
starch,  glycogen,  dextrin,  or  maltose  into  dextrose.  At  any 
rate  under  normal  circumstances  no  dextrin  or  maltose  is  to 
be  found  in  the  blood  of  the  portal  vein  or  in  the  chyle, 
although  these  substances  are  absorbed  from  the  intestine. 


THE   MECHANISMS   OF   DIGESTION  355 

Absorption  of  Proteins 

Proteins  are  for  the  most  part  converted  into  peptones 
and  albumoses  before  absorption.  This  absorption  takes 
place  by  means  of  the  blood-vessels.  Thus  a  large  protein 
meal  is  as  readily  absorbed  in  a  dog  whose  thoracic  duct  is 
ligatured  as  in  a  normal  dog,  A  large  protein  meal  always 
gives  rise  to  a  marked  increase  in  the  amount  of  urea 
excreted  in  the  urine,  and  this  increase  is  found  also  in  a 
dog  whose  thoracic  duct  has  been  ligatured,  showing  that 
the  protein  has  been  absorbed  and  distributed  through  the 
whole  system. 

If  however  we  analyse  the  blood  of  the  portal  vein 
during  active  protein  digestion,  not  a  trace  of  peptone  is 
found.  Injection  of  even  small  quantities  of  albumoses  or 
peptone  into  the  portal  vein  or  any  part  of  the  blood-stream 
gives  rise  at  once  to  peptonuria,  and  the  greater  part  of  the 
peptone  injected  reappears  in  the  urine,  from  which  it  can 
be  collected.  Thus  it  is  impossible  that  the  proteins  can 
reach  the  blood-stream  in  the  form  of  peptone;  and  the 
following  experiments  have  been  interpreted  as  showing  that 
peptone  is  regenerated  into  coagulable  protein  in  its  passage 
through  the  epithelial  cells  of  the  alimentary  canal. 

A  piece  of  the  mucous  membrane  of  the  stomach  during 
active  protein  digestion  is  excised  and  divided  into  two 
pieces.  One  piece  (a)  is  thrown  at  once  into  boiling  water, 
and  the  other  piece  (b)  is  allowed  to  remain  for  three  hours 
in  a  warm  moist  chamber  at  40-'  C,  and  is  then  plunged 
into  boiling  water.  On  analysing  the  two  pieces  a  large 
amount  of  peptone  is  found  in  a,  whereas  in  b  the  merest 
trace  or  none  is  present.  During  the  stay  in  the  warm 
chamber,  all  the  peptone  in  b  has  been  converted  into  some- 
thing else,  probably  coagulable  protein.  That  this  action 
presumably  depends  on  the  vital  activity  of  the  epithelial 
cells  and  not  on  unorganised  ferments  present  in  the  cells,  is 
shown  by  the  fact  that  plunging  the  membrane  into  water  at 
60°  C.  is  as  efficacious  in  stopping  the  action  as  when  water 
at  100°  C.  is  used.  At  60°  C.  all  living  cells  in  the  body  are 
destroyed,  but  not  all  unorganised  ferments. 

The  following  experiment  also  points  to  a  total  disappear- 
ance of  the  peptone  during  absorption  : — A  loop  of  a  dog's 


356  PHYSIOLOGY 

intestine  is  excised,  its  contents  washed  out  with  a  normal 
saline  fluid,  one  gramme  of  peptone  placed  in  it,  and  the  ends 
ligatured.  Dilute  defibrinated  blood  is  now  passed  for  two  or 
three  hours  through  the  vessels  supplying  the  loop  in  order 
to  keep  it  alive.  At  the  end  of  this  time,  on  cutting  open  the 
loop,  all  the  peptone  is  found  to  have  disappeared,  and  on 
analysis  of  the  blood  that  has  passed  through  the  vessels 
of  the  loop,  no  peptone  can  be  found.  It  was  therefore  con- 
cluded that  peptone  had  been  converted  into  a  coagulable 
protein  in  its  passage  through  the  absorbing  epithelium. 

More  recent  investigations  by  Leathes  and  others  have, 
however,  tended  to  put  a  different  interpretation  on  these 
results.  The  mucous  membrane  of  the  whole  alimentary  canal 
contains  erepsin,  and  the  disappearance  of  peptone  can  be 
equally  well  explained  on  the  assumption  that  it  has  been 
entirely  converted  by  this  ferment  into  amino-acids  and  other 
substances  which  no  longer  give  the  biuret  reaction  typical  of 
albumoses  and  peptones.  It  is  true  that  so  far  observers  have 
failed  to  obtain  any  increase  in  the  nitrogenous  extractives  of 
the  portal  blood  during  digestion,  but  the  circulation  through 
the  vessels  of  the  gut  is  so  rapid  that  the  whole  of  a  large 
protein  meal  might  be  absorbed  in  the  form  of  these  nitro- 
genous extractives  without  giving  such  a  rise  in  the  amino- 
nitrogen  of  the  blood  as  to  be  detectable  by  our  means  of 
analysis. 

On  the  other  hand,  it  is  not  necessary  that  the  protein 
should  be  all  peptonised  before  being  taken  up  by  the 
epithelial  cells.  It  has  been  shown  that  protein,  such  as  egg 
albumen  or  acid  albumen,  may  be  absorbed  by  an  isolated 
loop  of  bowel  or  by  the  lower  end  of  the  large  intestine,  which 
has  been  washed  free  from  any  trace  of  proteolytic  ferment 
that  may  have  been  carried  down  to  it  from  the  pancreatic 
juice.  Peptonisation  however  helps  the  work  of  the  epithe- 
lial cells,  and  materially  hastens  the  process  of  absorption. 
It  is  probable  that  a  large  proportion  of  the  proteins  of  the 
food  may  undergo  complete  disintegration  into  amino-acids 
and  bases,  being  absorbed  in  these  forms  into  the  blood,  and 
-built  up  again  into  proteins  in  the  living  cells  of  the  tissues, 
the  extent  to  which  any  individual  protein  is  broken  down 
during  digestion  being  determined  by  the  degree  in  which  its 
composition  approximates  to  that  of  the  normal  proteins  of 
the  digesting  animal. 


THE   MECHANISMS   OF   DIGESTION 


357 


Absolution  of  Water  and  Salts 

Owing  to  the  simple  character  of  the  factors  involved, 
the  study  of  the  absorption  of  water  and  salt  solutions  from 
the  intestine  is  of  considerable  importance  for  the  problem  of 
the  mechanism  of  absorption  generally,  since  it  enables  us  to 
determine  whether  the  absorption  is  effected  along  purely 
mechanical  lines  or  whether  the  cells  lining  the  intestine 
take  an  active  part  in  the  process.  On  the  latter  hypothesis 
we  should  regard  the  process  as  one  of  inverted  secretion. 
Whereas  in  glands  the  secretory  epithelium  takes  up  sub- 
stances from  the  lymph  and  turns  them  out  with  more  or  less 
modifications  into  the  lumen  of  the  alveoli,  in  the  intestine 
the  epithelium  would  take  up  the  food  material  and  products 
of  digestion  and  excrete  them  in  a  more  or  less  changed  con- 
dition on  the  other  side  into  the  lymph-  and  blood-spaces  of 
the  villus. 


Fk;.  178. 


We  may  first  consider  the  physical  factors  which  would 
influence  the  passage  of  water  and  dissolved  substances 
across  the  intestinal  epithelium.  In  the  case  of  two  com- 
partments A  and  B,  separated  by  a  permeable  membrane  c, 
an  interchange  of  material  will  go  on  so  long  as  there  is  any 
difference  in  the  chemical  composition  on  the  two  sides.  If 
the  proportion  of  a  dissolved  substance  such  as  sodium 
chloride,  be  greater  in  a  than  in  b,  the  movement  of  salt 
from  A  to  B  will  preponderate  over  that  from  b  to  a.  If  the 
total  number  of  dissolved  molecules  be  greater  in  a  than  in  b, 
the  osmotic  pressure  in  a  will  be  greater  than  the  osmotic 
pressure  in  b,  and  there  will  consequently  be  a  flow  of  water 
from  B  to  A.  In  the  case  of  substances  of  different  diffusi- 
bility,  the  less  diffusible  substance  will  exert  at  first  more 
osmotic   pressure   than  the  more  diffusible,  so  that  if  equi- 


358  PHYSIOLOGY 

molecular  solutions  of  sugar  and  sodium  chloride  be  taken, 
the  fluid  will  move  from  the  sodium  chloride  side  on  to  the 
sugar  side  at  a  greater  rate  than  in  the  reverse  direction ; 
and  the  volume  of  the  sugar-solution  will  increase  at  the 
expense  of  the  sodium  chloride.  It  is  evident  that  if  the 
membrane  be  partially  or  completely  impermeable  to  certain 
substances,  these  substances  will  have  a  greater  effective 
osmotic  pressure  and  so  be  instrumental  in  determining  an 
absorption  towards  their  side.  Where  however  the  fluids  on 
the  two  sides  are  identical,  no  property  of  the  membrane  will 
effect  a  transference  from  one  side  to  the  other,  nor  can  any 
specific  permeability  cause  the  passage  of  a  dissolved  sub- 
stance, so  to  speak,  uphill,  i.e.  from  a  position  of  lower  to 
a  position  of  higher  partial  tension.  On  investigating  the 
absorption  of  saline  fluids  from  the  intestines  in  the  light  of 
these  principles,  we  find  that,  although  absorption  is  influ- 
enced by  the  concentration  and  composition  of  the  fluids 
within  the  intestine,  yet  these  physical  factors  are  not  the 
only  nor  the  chief  determinants  in  the  process,  Init  that  the 
living  cells  of  the  mucous  membrane  play  an  important  part 
in  choosing  and  actively  passing  on  certain  constituents  of 
the  food.  If  we  take  a  solution  of  a  salt,  such  as  sodium 
sulphate,  the  question  seems  at  first  sight  a  purely  physical 
one,  a  hypertonic  '  solution  being  increased  in  amount,  while 
a  hypotonic  solution  is  concentrated  by  the  absorption  of 
water,  until  its  molecular  concentration  is  the  same  as  that 
of  the  blood  serum.  If  however  we  make  use  of  the  more 
physiological  solutions,  such  as  dextrose  or  sodium  chloride, 
we  find  that  these  substances  are  absorbed  rapidly  from 
weak  or  from  strong  solutions,  the  rate  of  absorption  being 
apparently  very  little  influenced  by  the  dift'usibility  of  the 
substance  in  question.  We  can  in  fact  represent  absorption 
as  made  up  of  two  factors,  the  physical  and  the  physiological. 
Where  they  are  in  opposition,  absorption  goes  on  slowly 
and  represents  work  done  on  the  fluid  l)y  the  absorbing  cells. 
We  may  abolish  this  physiological  factor  by  the  addition  of 
such  substances  as  sodium  fluoride  to  the  solutions,  by  which 
means  we   poison  the  cells  without  apparently  altering  the 

'  A  hypertonic  solution  is  one  that  is  more  concentrated  than  normal  fluids, 
such  as  serum  or  normal  NaCl  solution.  A  hypotomc  fluid  is  less  concentrated 
than  a  normal  fluid. 


THE   MECHANISMS  OF  DIGESTION  359 

consistence  of  the  membrane.  By  a  twisting  of  hypotheses, 
it  might  be  possible  to  exphxin  even  these  facts  without 
recourse  to  a  physiological  factor,  but  it  is  impossible  to 
explain  in  this  way  the  absorption  of  substances  such  as 
serum  or  solutions  of  egg  albumen,  containing  indiffusible 
colloids  in  solution,  and  in  the  former  case  identical  in  all 
respects  with  the  serum  circulating  in  the  blood-vessels  of 
the  villi. 

We  have  already  met  so  many  examples  of  an  activity  of 
the  intestinal  epithelium,  such  as  the  re-synthesis  of  fats,  the 
regeneration  of  peptone,  and  the  passage  by  some  means  or 
other  of  the  regenerated  coagulable  protein  into  the  blood- 
vessels of  the  villi,  that  it  is  not  strange  to  find  the  cell  also 
caring  for  the  absorption  of  such  necessary  constituents  of 
the  food  as  salts  and  sugars.  Where  the  salts  are  of  small 
or  no  physiological  importance,  the  cell  in  many  cases 
appears  to  have  no  power  of  physiological  absorption,  and 
the  salts  exert  their  full  physical  influence,  absorbing  water 
or  retaining  that  by  which  they  are  dissolved,  so  that  they 
act  as  saline  purges,  e.g.  sodium  or  magnesium  sulphate. 

The  rate  of  absorption  of  water  by  the  alimentary  canal 
increases  from  above  downwards.  No  water  or  dilute  saline 
solutions  appear  to  be  absorbed  by  the  stomach.  If  a 
duodenal  fistula  be  established,  as  much  fluid  will  be  dis- 
charged by  the  fistula  as  is  administered  to  the  animal  by  the 
mouth.  In  the  small  intestine,  the  process  of  absorption  is 
much  more  rapid  in  the  ileum  than  in  the  jejunum.  In 
consequence  however  of  the  continual  secretion  of  the  succus 
entericus,  the  intestinal  contents  reach  the  ileo-colic  valve  in 
a  fluid  condition,  and  the  excess  of  water  is  only  absorbed  in 
the  large  intestine. 

This  difference  in  the  absorbing  power  of  different  parts  of 
the  gut  has  reference  chiefly  to  water.  Dissolved  substances, 
such  as  alcohol,  dextrose,  alkaloidal  poisons,  are  absorbed  with 
extreme  rapidity  from  the  stomach  as  from  all  parts  of  the  gut. 

Water  and  salts  are  absorbed  almost  entirely  by  means 
of  the  blood-vessels.  If  a  cannula  be  placed  in  the  thoracic 
duct,  little  or  no  increase  of  the  flow  is  obtained  even  during 
the  absorption  of  large  quantities  of  salt  solutions  from  the 
intestine. 


360  PHYSIOLOGY 


Section  8 

SUMMAKY   OF  THE   CHANGES   UNDEEGONE   BY 
THE   FOOD   IN   THE   ALIMENTAEY   CANAL 

In  the  mouth  the  food  is  broken  up  into  small  particles  bj' 
mastication,  and  moistened  with  alkaline  saliva,  in  order  to 
fit  it  for  deglutition.  A  small  part  of  the  starch  is  converted 
into  dextrin  or  maltose. 

On  reaching  the  stomach  the  action  of  the  saliva  may  go 
on  for  thirty  to  sixty  minutes.  At  the  end  of  this  time 
the  secretion  of  gastric  juice,  excited  by  the  presence  of 
food  and  of  alkaline  saliva  in  the  stomach,  is  sufficiently 
abundant  to  neutralise  and  render  acid  all  the  gastric 
contents,  and  so  stop  the  action  of  the  ptyalin.  Under  the 
action  of  the  gastric  juice  the  greater  part  of  the  protein  is 
dissolved,  and  converted  into  syntonin,  albumoses,  or  pep- 
tones. The  connective  tissues  are  also  dissolved,  setting  free 
the  fat,  which  floats  about  in  a  free  state.  At  the  same  time 
some  of  the  salts  and  sugar  which  have  been  swallowed, 
and  the  peptones  formed  from  the  food,  are  being  absorbed 
by  the  gastric  mucous  membrane.  For  the  first  half-hour 
after  ingestion  of  a  meal  of  solid  food,  the  pylorus  is  firmly 
closed.  At  the  end  of  this  time  it  relaxes  at  intervals  to 
allow  the  passage  of  the  fluid  parts  of  the  gastric  contents, 
which  are  spoken  of  at  this  period  as  chyme.  The  passage 
of  food  through  the  pylorus  goes  on  for  seven  or  eight  hours 
after  the  ingestion  of  food,  and  towards  the  end  of  this  time 
larger  lumps  of  undigested  material  are  allowed  to  pass  on 
into  the  duodenum. 

As  the  acid  chyme  is  squirted  through  the  pylorus  into 
the  duodenum,  it  comes  into  contact  with  the  cells  of  the 
mucous  membrane,  and  in  them  causes  a  conversion  of  pro- 
secretin into  secretin.  The  latter  is  absorbed  by  the  blood 
stream  and  carried  to  the  pancreas  and  liver,  causing  a  flow 
of  pancreatic  juice  and  an  increased  secretion  of  bile.  The 
strongly  alkaline  pancreatic  juice  neutralises  the  acid  chyme. 
So  long  as  the  duodenal  contents  are  acid,  the  pylorus  remains 
closed,  but  opens  as  soon  as  sufficient  pancreatic  juice  has 


THE   MECHANISMS   OF  DIGESTION  361 

been  secreted  to  neutralise  the  chyme  that  has  ah'eady 
passed  through.  When  this  happens  the  pylorus  relaxes,  a 
fresh  quantity  of  acid  gastric  contents  is  forced  through, 
more  secretin  is  formed,  and  the  pancreatic  secretion  is  again 
excited  until  the  second  amount  of  chyme  is  neutralised  ;  and 
this  self- steering  regulation  of  the  activities  of  the  stomach, 
pancreas,  and  intestines  goes  on  until  all  the  food  has  passed 
through  the  pylorus.  At  the  same  time  the  pancreatic  juice 
poured  into  the  gut  evokes  a  secretion  of  succus  entericus, 
which  supplies  the  enterokinase  necessary  for  the  activation 
of  the  proteolytic  ferment  of  the  pancreatic  juice. 

In  the  duodenum  the  chyme  comes  in  contact  with  the 
bile,  the  flow  of  which  is  also  quickened  under  the  influ- 
ence of  the  secretin  which  is  being  poured  into  the  circu- 
lation. This  causes  a  precipitate  in  the  chyme,  consisting 
of  bile  acids,  syntonin,  and  albumoses.  This  precipitate  is 
dissolved  later  on  by  the  further  operation  of  the  pancreatic 
juice.  Here  the  remaining  digestive  processes  take  place ; 
the  undigested  proteins  are  dissolved,  and  the  acid  albumen 
and  albumoses  resulting  from  gastric  digestion  are  converted 
into  peptone  and  partially  into  leucine,  tyrosine,  and  similar 
nitrogenous  bodies.  Starches  are  changed  into  maltose  and 
dextrin,  and,  under  the  further  agency  of  the  intestinal  juice, 
into  dextrose.     The  fats  are  partially  split  up  and  emulsified. 

Throughout  the  whole  of  the  small  intestine  active 
secretion  and  absorption  are  taking  place,  so  that  the  amount 
of  water  in  the  intestinal  contents  in  the  lower  part  of  the 
small  intestine  is  about  the  same  as  in  the  upper  part.  The 
contents  of  the  lower  part  acquire  a  distinct  faecal  odour  from 
the  indol  and  skatol  produced  by  the  action  of  putrefactive 
bacteria  on  the  tryjitophane  contained  in  the  proteins  of  the 
food.  In  the  large  intestine  the  processes  of  absorption 
predominate  over  those  of  secretion  ;  hence  that  part  of  the 
intestinal  contents  which  has  not  been  absorbed  becomes  less 
and  less  watery,  and  acquires  the  character  of  faeces,  in  which 
form  it  is  periodically  expelled  from  the  body. 

The  fteces  consist  mainly  of  the  indigestible  residue  of 
the  food,  or  of  substances  which  have  been  taken  in  too  large 
quantities  to  be  digested,  and  contain  — 

(rt)  Cellulose,  woody  fibre,  elastic  tissue,  keratin,  and 
remains  of  muscle-fibres,  starch-grains,  and  fat. 


362  PHYSIOLOGY 

They  also  contain — 

{h)  The  unabsorl)able  part  of  the  digestive  juices,  such  as 
mucin,  altered  cholalic  acid  {dijslysi)!),  bile-pigments,  clioles- 
terin. 

(c)  Indol  and  skatol,  various  forms  of  bacteria,  and  disinte- 
grated epithelial  cells  from  the  intestinal  mucous  membrane. 

(d)  Certain  products  of  excretion  of  the  intestinal  mucous 
membrane.  Many  metallic  poisons  {e.g.  iron,  mercury)  are 
excreted  into  the  cavity  of  the  intestine  and  leave  the  body  in 
the  ffeces  as  sulphides  of  the  metals. 


THE   MECHANISMS   OF  DIGESTION  36B 


Section  9 

MUSCULAE  MECHANISMS   OF  DIGESTION 

Mastication 

By  movements  of  the  lower  against  the  upper  jaw,  the 
food  is  crushed  ])etween  the  teeth  and  reduced  to  a  finely 
subdivided  condition  to  fit  it  for  the  action  of  the  various 
digestive  fluids.  The  lumps  of  food  are  continually  pushed 
between  the  teeth  by  movements  of  the  tongue,  cheek,  and 
lips.  The  whole  act  is  voluntary,  although  it  is  associated 
with  and  rendered  easier  by  the  saliva  which  is  poured  out 
into  the  mouth  at  the  same  time,  and  the  secretion  of  which 
is  excited  reflexly. 

The  nerves  supplying  the  muscles  engaged  in  mastication 
are  the  fifth  nerve  (to  jaw  muscles),  facial,  and  hypoglossal. 

Deglutition 

When  the  food  is  sufficiently  subdivided,  it  is  gathered 
by  movements  of  the  tongue  against  the  hard  palate  into 
a  bolus  which  rests  on  the  dorsal  surface  of  the  tongue, 
whence  it  is  propelled  through  the  fauces  into  the  oeso- 
phagus. 

The  movements  of  deglutition  may  be  divided  into  three 
stages. 

In  the  first  stage  the  bolus  is  carried  by  the  tongue 
through  the  isthmus  faucium.  This  act  is  voluntary.  As 
soon  as  the  bolus  has  passed  the  isthmus,  it  is  in  a  region 
common  to  the  food  and  respiratory  processes. 

Here,  by  a  series  of  rapid  reflex  movements,  constituting 
the  second  stage,  it  is  sent  on  into  the  beginning  of  the 
cesophagus.  The  movements  are  as  follows  : — The  levator 
palati  draws  the  soft  palate  upwards  and  l)aekwards,  and  this 
with  the  contracted  palato-pharyngei  entirel}'  closes  the  nasal 
cavities.  At  the  same  time  the  intrinsic  muscles  of  the 
larynx  contract,  closing  the  rima  glottidis  by  approximating 
the  vocal  cords  while  the  entire  larynx  is  drawn  up  behind 
the  hyoid  bone  by  the  thvro-hyoid  muscle,  and  the  superior 


364 


PHYSIOLOGY 


opening  of  the  larynx  is  closed  by  the  approximation  of  the 
arytenoid  cartilages  to  the  l)ase  of  the  tongue  and  the  epi- 
glottis. Then  by  the  contraction  of  stylo-pharyngei  and 
palato-pharyngei  the  upper  part  of  the  pharynx  is  drawn  like 
a  glove  on  a  finger  over  the  bolus  of  food,  which  is  grasped  by 

Fm.  179. 


Dissection  to  show  muscles  employed  in^deglutition.  b,  styloid 
process,  from  which  arise  1,  the  styloglossus,  2,  the  stylohyoid, 
3,  the  stylo-pharyngeus  muscles  ;  c,  section  of  lower  jaw  ;  d,  hyoid 
bone ;  e,  thyroid  cartilage ;  g,  isthmus  of  thyroid  gland  ;  4,  cut 
edge  of  mylohyoid  muscle  ;  5,  6,  7,  8,  muscles  of  tongue  ;  9,  10,  11, 
supei'ior,  middle,  and  inferior  constrictors  of  pharynx  ;  12,  oeso- 
phagus.    (Allen  Thomson.) 


the  superior  constrictor,  and  passed  on  from  this  to  the  middle 
and  inferior  constrictors  of  the  pharynx. 

The  third  or  oesophageal  stage  is  slow  and  entirely 
involuntary.  The  bolus  is  forced  down  the  oesophagus  by 
a  peristaltic  wave  of  contraction  passing  down  the  muscular 
walls  of  this  viscus. 

The  propagation  of  this  contraction  from  one  segment  of 


THE   MECHANISMS   OF  DIGESTION  365 

the  cesophagus  to  the  next  is  a  reflex  act.  Section  of  the 
vagus  branches  to  the  oesophagus  arrests  the  wave,  although 
it  is  not  checked  by  section  of  the  cesophagus  itself.  A 
peristaltic  contraction  is  not  necessary  in  most  cases  to 
secure  the  carrying  of  food  to  the  stomach.  If  a  series  of 
acts  of  deglutition  be  made  at  intervals  of  a  second,  no 
peristaltic  wave  of  contraction  takes  place  till  after  the  last 
mouthful  has  been  swallowed.  It  seems  that  each  act  of 
deglutition  inhibits  the  third  stage  of  the  preceding  one, 
so  that  the  food  slides  easily  through  a  relaxed  a-sophagus. 

The  cardiac  end  of  the  stomach  is  normally  contracted, 
but  relaxes  in  the  last  stage  of  deglutition. 

Nervous  Mechanism  of  Deglutition 

Deglutition  is  a  complex  reflex  act,  which  is  started  by 
impulses  from  the  mucous  membrane  of  the  fauces  or  upper 
part  of  the  larynx.  These  travel  up  to  the  medulla  through 
branches  of  the  fifth  nerve  and  the  su]ierior  laryngeal  branches 
of  the  vagus.  A  movement  of  deglutition  may  often  be 
excited  by  stimulation  of  the  central  end  of  the  superior 
laryngeal  nerve. 

Stimulation  of  the  central  end  of  the  giosso-pharyngeal 
nerve  checks  any  movements  of  deglutition  that  are  in  progress 
and  may  excite  vomiting. 

The  efferent  channels  are  the  hypoglossal  nerve  (to  the 
tongue),  the  fifth  (to  the  mylo-hyoid),  the  giosso-pharyngeal, 
vagus,  and  spinal  accessory  (to  the  muscles  of  the  soft  palate, 
pharynx,  and  oesophagus).  The  vagus  carries  both  motor  and 
inhibitory  fibres  for  the  lower  part  of  the  ctsophagus  and 
the  cardiac  sphincter  of  the  stomach.  Stimulation  of  the 
peripheral  end  of  the  vagus  in  the  normal  animal  causes 
tonic  contraction  of  the  whole  cesophagus.  If  however  curare 
and  atropine  be  given,  excitation  of  the  vagus  generally 
inhibits  the  cardiac  sphincter,  so  allowing  of  a  free  flow  of 
fluid  from  cesophagus  to  stomach. 

Movements  of  the  Stomacli 

The  stomach,  which  serves  at  once  as  a  digestive  organ 
and  as  a  reservoir  which  allows  only  small  amounts  of  the 
food  at  a  time  to  pass  into  the  intestines,  is  divided    into 


366 


PHYSIOLOGY 


cardiac  and  pyloric  portions.  Its  muscular  coat  consists  of 
three  layers,  an  outer  longitudinal,  a  middle  circular,  and  an 
internal  oblique  layer,  the  first  being  continuous  with  the 
longitudinal  fibres,  and  the  two  latter  with  the  circular  fibres 
of  the  oesophagus.  About  the  middle  of  the  pyloric  half  the 
circular  fibres  are  slightly  thickened  to  form  the  '  transverse 

Fig.  180. 


Shadow  sketches  of  the  outlines  of  the  stomach  of  a  cat,  immediately 
after  a  meal  (11.0),  at  various  intervals  afterwards  (12.0,  2.0, 
3.30,  4.30) ;  c,  situation  of  oesophageal  opening ;  y2,  '  transverse 
band;'  ivx,  junction  of  cardiac  and  pyloric  portions.  (W.  B. 
Cannon.) 

band,'  and  at  the  pyloric  orifice  there  is  a  very  pronounced 
thickening  of  the  same  fibres,  forming  the  strong  pyloric 
sphincter.  The  movements  of  the  stomach  in  the  normal 
animal  have  been  studied  by  Cannon,  by  the  administration 
of  subnitrate  of  bismuth  with  the  food  (bread  and  milk).  On 
examining  the  animal  under  the  Eontgen  rays,  the  opaque 


THE   MECHANISMS   OF   DIGESTION  367 

bismuth  salt  caused  a  shadow  of  the  outlines  of  the  stomach 
on  the  platino-cyanide  screen.  Figures  obtained  in  this  way 
are  shown  in  Fig.  180.  Within  five  minutes  after  the  end  of 
a  meal  slight  constrictions  appear  near  the  middle  of  the 
stomach  and  travel  slowly  towards  the  i)yloric  end.  The  con- 
tractions succeed  one  another  at  intervals  of  ten  seconds,  so 
that  the  pyloric  half  is  occupied  with  two  or  three  such  waves. 
Ten  or  fifteen  minutes  after  the  first  constriction  has 
appeared,  the  arrival  of  a  wave  at  the  pylorus  is  attended  by 
a  relaxation  of  the  pyloric  sphincter  and  a  squirting  of  some 
of  the  contents  into  the  duodenum.  After  this  time  the 
pylorus  opens  at  regular  intervals  to  allow  some  of  the  gastric 
contents  to  pass.  The  relaxation  of  the  sphincter  is  at  once 
inhibited  by  the  arrival  of  any  solid  or  indigestible  particle  in 
the  region  of  the  pylorus. 

During  this  time,  although  free  from  distinct  waves  of 
contraction,  the  cardiac  part  of  the  stomach  has  not  been 
idle.  As  the  pyloric  half  empties  itself  into  the  duodenum, 
the  cardiac  half  contracts  steadily  on  its  contents,  so  com- 
pelling ever  fresh  portions  of  food  to  enter  the  pyloric  mill. 
At  the  end  of  digestion  the  cardiac  part  is  contracted  to  the 
shape  of  a  tube  (Fig.  180,  4.30).  The  period  elapsing  from 
the  taking  of  food  to  the  complete  emptying  of  the  stomach 
may  vary,  according  to  the  meal  taken,  from  four  to  sixteen 
hours. 

Nervous  Mechanism  of  the  Stomach  Movements 

The  manner  in  which  these  co-ordinated  muscular  con- 
tractions are  mitiated  and  carried  out  is  quite  unknown. 
Since  somewhat  similar  movements  have  been  observed  in 
the  excised  stomach,  it  has  been  supposed  that  they  are  due 
to  a  local  nervous  mechanism  ;  but  whether  this  mechanism 
is  situated  in  the  plexus  of  Auerbach  between  the  muscular 
coats,  or  m  the  microscopic  ganglia  which  are  found  at 
various  places  under  the  serous  coat,  is  quite  unknown. 

From  the  central  nervous  system  the  stomach  receives 
fibres  by  two  ways,  viz.  from  the  vagi  and  from  the  splanchnic 
nerves.  It  is  generally  stated  that  the  effect  of  the  vagus  is 
to  increase,  and  that  of  the  splanchnic  to  diminish  the  stomach 
movements.     It   is    certain  however  that  the  vagi  may  act 


368 


PHYSIOLOGY 


either  as  motor   or   inhibitory  nerves   to   the  walls  of    the 
stomach  as  well   as    to  the  cardiac  and  pyloric  sphincters. 


Fig.  181. 


LSyCk 


G.SpW 


LSp.N 


Distribution  of  the  vagus  in  the  abdomen  of  the  dog  (M.  H.  Naylor). 
R.V.,  L.V.  Hight  and  left  vagi.  The  right  vagus  runs  behind 
the  oesophagus  (Oe)  and  stomach  {St),  and  in  those  places  is 
represented  by  a  discontinuous  line.  Cb.  Connecting  branch  be- 
tween right  and  left  vagi.  P.  Pancreas.  Dd.  Duodenum.  F.D.J. 
Flexura  duodeno-jejunalis.  I,  I,  1,  Intestine.  L.  Liver.  K. 
Kidney.  A.  Suprarenal  capsule.  E.G.,  L.G.  Eight  and  left  crura 
of  diaphragm.  L.Sy.Ch.  Left  sympathetic  chain.  12  D,  13  D. 
Twelfth  and  thirteenth  dorsal  ganglia.  3  //.  Third  lumbar  gan- 
glion. G.Sp.N.,  L.Si^N.  Great  and  small  splanchnic  nerves.  S.G. 
Left  semilunai  and  superior  mesenteric  ganglia.  D.A.  Dorsal 
aorta. 


The  most  usual  effect  of  stimulating  the  vagus,  especially  after 
atropine,  is  contraction  of  the  j)ylorus  with  relaxation  of  the 
cardiac  sphincter  ;  but  in  other  cases  there  may  be  an  equally 


THE   MFX'HANISMS   OF   DIGESTION  369 

definite  contraction  of  the  cardiac  sphincter,  or  an  equally 
definite  relaxation  of  the  pyloric  sphincter.  It  is  doubtful 
whether  in  the  higher  animals  the  splanchnics  have  any 
direct  influence  on  the  muscular  wall  of  the  stomach. 

Vomiting 

Vomiting  is  a  reflex  act  which  lies  on  the  borderland 
between  physiological  and  pathological  processes.  It  is  at 
any  rate  the  normal  reaction  of  the  stomach  to  an  irritant. 
The  act  of  vomiting  is  generally  preceded  by  a  feeling  of 
nausea,  copious  salivation,  and  retching.  Retching  is  a 
violent  inspiration  while  the  glottis  is  kept  firmly  closed,  so 
that  air  is  drawn  into  the  oesophagus,  and  distends  it.  This 
stage  is  followed  by  contraction  of  the  fibres  radiating  from 
the  cardiac  end  of  the  oesophagus,  which  opens  and  allows 
gas  to  escape.  The  head  being  bent  forward  and  the  mouth 
widely  opened,  so  as  to  straighten  the  oesophagus  as  much 
as  possible,  and  the  glottis  kept  closed,  a  forcible  contraction 
of  the  abdominal  muscles  occurs,  attended  by  contraction  of 
the  muscular  wall  of  the  stomach  itself,  which  forces  out  its 
contents. 

Vomiting  can  be  accomplished  by  contraction  of  the 
stomach  alone,  or  of  the  abdominal  muscles  alone  ;  for  it 
may  be  excited  in  an  animal  by  injection  of  apomorphine  even 
after  its  stomach  has  been  replaced  by  a  bladder,  or  its 
abdominal  muscles  and  diaphragm  paralysed  by  section  of 
the  intercostal  and  phrenic  nerves.  Vomiting  may  be  ex- 
cited reflexly  by  irritation  of  the  palate,  fauces,  stomach, 
peritoneum,  or  indeed  of  any  abdominal  organ.  It  may  also 
be  excited  from  the  brain,  in  consequence  of  emotions  or  evil 
smells.  The  co-ordination  of  the  movements  of  vomiting  is 
dependent  on  a  centre  in  the  medulla — '  vomiting  centre  ' — 
not  far  from  the  respiratory  centre.  The  various  emetics 
may  cause  vomiting,  either  reflexly  by  irritation  of  the 
stomach  (mustard,  salt  water,  zinc  sulphate),  or  directly  by 
their  action  on  the  centre,  e.g.  apomorphine. 


24 


370  PHYSIOLOGY 

Movements  of  the  Small  Intestine 

The  muscular  wall  of  the  small  intestine  is  composed  of 
two  continuous  layers  of  longitudinal  unstriated  fibres  ex- 
ternally and  circular  fibres  internally.  Between  the  two 
layers  we  find  a  peculiar  nerve-plexus,  '  the  plexus  of  Auer- 
bach,'  composed  of  crossing  strands  of  non-medullated  nerve- 
fibres  with  collections  of  ganglion  cells  at  their  nodal  points. 
Fine  fibres  given  off  from  this  plexus  ramify  among  the 
muscle-cells  on  each  side.  A  finer  plexus,  the  plexus  of 
Meissner,  lies  in  the  submucosa  and  sends  nerves  up  into 
the  villi  to  supply  the  muscles  of  the  muscularis  mucosae  and 
the  glandular  epithelium. 

On  examining  the  small  intestines  in  a  warm  saline  bath, 
after  division  of  all  nerves  connecting  them  with  the  central 
nervous  system,  they  are  seen  to  be  in  a  state  of  continual 
movement.  The  movements  are  of  two  kinds.  In  the  first 
place,  all  the  coils  of  intestine  are  in  a  condition  of  swaying 
motion,  the  movement  from  side  to  side  being  caused  by  slight 
waves  of  constriction  affecting  both  coats  simultaneously  and 
travelling  rapidly  down  the  intestine  at  the  rate  of  2  to  5  cm. 
per  second.  They  recur  at  regular  intervals  of  five  or  six 
seconds,  so  that  if  a  rubber  balloon  be  placed  in  the  intestine 
and  connected  with  a  recording  tambour,  we  get  a  tracing 
as  regular  as  that  of  a  beating  heart.  These  movements 
are  apparently  myogenic,  i.e.  they  are  due  to  an  inherent 
rhythmicity  of  the  muscle-fibres  themselves  and  are  propa- 
gated down  the  intestine  from  one  muscle-fibre  to  another. 
They  are  increased  in  amplitude  by  local  distension  and  in 
frequency  by  rise  of  temperature.  They  are  incompetent  to 
push  a  mass  of  food  onwards  ;  their  chief  use  seems  to  be  to 
effect  a  continuous  movement  and  mixing  of  the  intestinal 
contents.  In  animals  which  have  been  fed  with  food  con- 
taining bismuth  subnitrate,  these  contractions  may  be  seen 
under  the  Eontgen  rays  to  cause  the  intestinal  contents  to 
break  up  into  segments,  which  fuse  again  to  be  broken  up 
by  fresh  constrictions.  The  movements  are  therefore  often 
called  the  '  segmenting  movements.' 

In  strong  contrast  to  these  segmenting  contractions  are  the 
true  peristaltic  waves.  Where  not  present  spontaneously  in 
the  exposed  intestine,  they  may  be  easily  evoked  by  intro- 


THK   MECHANISMS   OF   DIGESTION  371 

ducing  a  bolus  of  cotton  wool  and  vaseline  into  the  gut.  The 
introduction  of  the  bolus  is  at  once  followed  by  a  twofold 
effect.     Immediately  above  the  bolus,  a  strong  ring  of  con- 

FiG.  182. 


Passage  of  bolus.  Contractions  of  longitudinal  coat  (enteiograph). 
The  bolus  (of  soap  and  cotton  wool)  was  inserted  into  the  intes- 
tine four  inches  above  the  recorded  spot  at  a.  The  figures  below 
the  tracing  indicate  the  distance  of  the  middle  of  the  bolus  from 
the  recording  levers.  As  the  bolus  arrived  two  inches  above  the 
levers  there  is  cessation  of  the  rhythmic  contractions  and  inhibi- 
tion of  the  tone  of  the  muscle.  This  is  followed,  as  the  bolus  is 
forced  past,  by  a  strong  contraction  in  the  rear  of  the  bolus. 

striction    is    produced,    and    this    travels    slowly   down     the 
intestine,    driving    the    bolus    before  it.       The   rate  of   pro- 

FiG.  iH'd. 


Intestinal  contractions  (balloon  method).  In  this  dog  all  the  abdominal 
ganglia  had  been  excised,  and  botli  vagi  cut.  Showing  propagated 
effects  of  mechanical  stimulation  above  and  below  the  balloon. 
(1)  pinch  above,  (2)  pinch  below,  (.3)  pinch  below  balloon. 

gress  of  the  wave  is  very  variable  and  may  be  as  little  as  a 
centimetre  in  a  minute.  The  passage  of  the  bolus  downwards 
is  rendered  possible  by  an  inhibition  of  all  the  spontaneous 


372  PHYSIOLOGY 

contractions  of  the  intestinal  wall  for  some  distance  below 
the  bolus,  so  that  the  advancing  wave  drives  the  bolus  always 
into  a  relaxed  portion  of  the  gut.  A  peristaltic  wave  there- 
fore is  a  co-ordinated  act  involving  two  factors,  an  excitation 
above  and  an  inhibition  below  the  excited  spot.  This  two- 
fold effect  of  local  excitation  can  be  easily  demonstrated  by 
pinching  the  intestine  above  and  below  a  balloon  connected 
with  a  tambour.  A  pinch  above  causes  instant  relaxation 
of  the  gut  at  the  balloon,  whereas  stimulation  just  below 
causes  increased  tone  and  increased  rhythmic  contraction 
(Fig.  183). 

These  peristaltic  contractions,  as  well  as  the  double  re- 
action to  local  stimulus,  can  be  abolished  by  painting  the 
intestine  with  cocain  or  by  injection  of  nicotine.  Under  these 
circumstances  the  rhythmic  swaying  movements  continue  as 
actively  as  before,  but  the  intestine  is  powerless  to  move  on 
a  bolus  or  mass  of  food.  We  must  therefore  regard  them 
as  co-ordinated  reflexes  carried  out  by  the  local  nervous 
mechanism  — the  only  example  known  so  far  of  a  true  co- 
ordinated reflex  dependent  entirely  on  peripheral  nervous 
structures. 

Fig.  184.1 


Excitation  of  both  splanchnic  nerves.     Balloon  method. 
Intestine  returned  to  abdomen. 


Innervation  of  Small  Intestine 

The  small  intestine  receives  fibres  (1)  from  the  greater  and 
smaller  splanchnic  nerves  which  run  through  the  semilunar 
and  superior  mesenteric  ganglia  and  arrive  at  their  destination 


THE   :\[ECHANISMR    OF   DIGESTION 


873 


along  the  mesenteric  arteries,  and  (2)  from  the  continuation 
of  the  right  vagus.  The  hitter  however  contains  fibres  from 
both  vagi  in  the  neck.  The  sphmchnic  nerves  convey  purely 
inhibitory  impulses  (Fig.  184),  the  inhibition  being  independent 
of  the  simultaneous  effect  on  the  blood-vessels  of  the  intestines. 
Both  longitudinal  and  circular  fibres  are  involved  in  the  inhi- 
bition.    Any  painful  stimulation  of  a  sensory  nerve,  especially 


Effect  of  stiinuliitiou  of  riirht  va^us  un  intcotiual  coutraction> 


in  the  abdomen,  excites  a  retiex  inhibition,  so  that  it  is 
necessary  to  cut  all  the  splanchnic  fibres  if  we  wish  to  study 
by  operation  the  normal  activities  of  the  mtestine. 

The  vagus  is  a  motor  nerve  to  the  intestme.  Stimulation 
of  the  vagus  after  the  administration  of  atropine  (to  paralyse 
the  cardio-inhibitory  fibres)  gives,  after  a  prolonged  latent 
period,  increased  amplitude  of  the  rhythmic  contractions 
accompanied  by  augmentation  of  tone  (Fig.  185). 

In  the  dog  there  is  a  preliminary  temporary  inhibition, 
but  this  inhibition  is  possibly  determined  by  an  immediate 
motor  effect  on  the  stomach  or  duodenum,  since  it  is  absent 
in  such  animals  as  the  rabbit,  where  a  descending  inhibition 
as  the  result  of  local  stimulation  is  also  absent  or  little 
marked. 


B74  PHYSIOLOGY 

Movements  of  Large  Intestine 

Just  as  at  the  upper  end  of  the  Hhmentary  canal  there  is 
a  gradual  transition  from  the  co-ordinated  cerebro-spinal 
reflex  of  deglutition  to  the  local  automatic  movements  of  the 
stomach,  so  in  the  large  intestine  we  find  a  gradual  transition 
in  the  opposite  direction,  the  activity  of  the  gut  becoming  less 
automatic  and  more  dependent  on  its  extrinsic  innervation  as 
we  proceed  from  ileo-colic  valve  to  anus.  The  muscular  wall 
consists,  as  in  the  small  intestine,  of  two  coats,  the  outer 
longitudinal  fibres  however  being  especially  condensed  in 
man  and  some  animals  to  form  three  bands  of  muscular  fibre. 
The  colon  separated  from  its  nervous  connections  shows  slow 
rhythmic  contractions,  much  more  irregular  and  less  frequent 
than  those  of  the  small  intestine.  A  peristaltic  wave  may 
often  be  produced  in  the  upper  part  of  the  colon  or  be  trans- 
mitted onwards  from  the  ileum,  but  in  nearly  all  cases  dies 
away  before  arriving  at  the  sigmoid  flexure. 

In  the  upper  part  of  the  large  intestine  in  many  animals 
we  find  peristaltic  movements  alternating  with  antiperistaltic 
movements,  so  that  the  onward  movement  of  the  intestinal 
contents  is  cut  short  by  a  reverse  contraction  which  drives 
them  up  against  the  ileo-caecal  valve.  The  to  and  fro  move- 
ment of  the  intestinal  contents  seems  specially  adapted  for 
enabling  the  organism  to  extract  the  last  traces  of  nutriment 
from  the  contents  of  the  gut  before  these  are  passed  on  as 
faeces  into  the  lower  bowel. 

Innervation  of  Large  Infesfiiie 

The  colon  receives  fibres  from  two  sources — from  the 
lower  dorsal  and  upper  lumbar  nerves,  and  from  the  second 
and  third  sacral  nerves.  The  upper  set  of  nerves  pass  by  the 
rami  communicantes  into  the  sympathetic  chain,  thence  into 
the  inferior  mesenteric  ganglion,  where  many  of  them  make 
connections  with  the  ner^'e-cells.  From  the  ganglion  they 
pass  as  non-medullated  nerve-fibres  along  the  branches  of 
the  inferior  mesenteric  artery  in  the  walls  of  the  gut. 
(Fig.  30G.)  The  sacral  fibres  pass  in  the  pelvic  visceral 
nerves  or  nervi  erigentes  to  the  hypogastric  plexus,  whence 
branches  are  given  off,  which  run  up  in  the  walls  of  the  colon. 


THE   MECHANISMS   OF   DIGESTION  375 

The  action  of  these  two  sets  of  nerves  is  exactly  analogous  to 
that  of  the  nerves  supplying  the  small  intestine,  the  part  of 
the  vagus  being  taken  by  the  pelvic  visceral  nerve.  The 
branches  of  the  inferior  mesenteric  ganglia  are  purely  inhi- 
bitory to  both  longitudinal  and  circular  coats,  whereas  the 
sacral  nerves  cause  a  strong  contraction  of  both  coats,  which 
may  in  some  cases  take  the  form  of  a  descending  peristaltic 
wave,  but  is  in  all  cases  accompanied  by  a  strong  contraction 
of  the  thickened  collection  of  longitudinal  fibres  forming 
the  recto-coccygeal  muscle.  This  contraction  is  in  the  dog 
often  preceded  by  a  preliminary  inhibition  analogous  to  that 
produced  in  the  small  intestine  by  the  vagus. 

A  well-marked  band  of  circular  muscles  forms  a  sphincter  at  the  entry  of 
ileum  into  colon.  It  is  interesting  to  note  that  excitation  of  the  splanchnics, 
which  inhibits  all  the  rest  of  the  gut,  causes  a  strong  contraction  of  the  ileo- 
colic sphincter. 

Defaecation 

The  residue  of  the  undigested  food  and  other  matters 
forming  the  faeces  are  driven  on  by  the  peristaltic  contractions 
of  the  upper  part  of  the  large  intestine,  until  they  reach 
the  sigmoid  flexure.  The  mass  of  faeces  accumulates  in  the 
sigmoid  flexure,  and  is  added  to  after  each  meal. 

The  anus  is  closed  by  two  distinct  muscles — the  external 
sphincter,  a  thin  sheet  of  striated  muscle  ;  and  the  internal 
sphincter,  a  thick  ring  of  unstriated  muscle  surrounding  the 
last  three  inches  of  the  rectum,  and  about  half  an  inch  thick. 
The  internal  sphincter  is  normally  in  a  condition  of  tonic 
contraction.  This  contraction  however  is  not  usually  needed 
to  keep  back  the  faeces,  since  any  that  have  escaped  past  the 
sigmoid  flexure  are  retained  in  the  upper  part  of  the  rectum 
by  a  transverse  fold  of  mucous  membrane.  They  are  also 
kept  back  by  the  acute  angle  that  the  last  part  of  the  rectum 
makes  wdth  the  preceding  part,  and  by  the  contractions  of  the 
perinaeal  muscles  which  maintain  this  curvature  and  empty 
the  lower  part  of  the  bowel. 

Defaecation  is  normally  started  by  a  voluntary  act,  although 
it  may  take  place  involuntarily,  as  is  shown  by  the  fact  that 
it  occurs  in  a  dog  whose  spinal  cord  has  been  divided  in  the 
dorsal  region. 


376  PHYSIOLOGY 

The  steps  of  normal  defsecation  are  as  follows  : — The 
glottis  being  closed,  a  forcible  expiratory  effort  of  the  abdo- 
minal muscles  is  made.  The  perinceal  muscles  being  relaxed 
at  the  same  time,  the  lower  part  of  the  rectum  is  straightened, 
and  a  portion  of  the  contents  of  the  sigmoid  flexure  is  forced 
down  into  the  lower  part  of  the  rectum.  The  presence  of  a 
foreign  body  in  the  lower  part  of  the  rectum  irritates  the 
mucous  membrane,  and  excites  reflexly  the  rest  of  the  act. 
Strong  peristaltic  contractions  take  place  along  the  whole  of 
the  descending  colon  (sigmoid  flexure  and  rectum),  while  both 
sphincters  are  relaxed,  thus  forcing  out  the  contents  of  the 
bowel.  The  recto-coccygeal  muscle  contracts  forcibly  at  the 
same  time,  ])ulling  down  and  straightening  the  lowest  part  of 
the  bowel  and  causing  some  eversion  of  the  mucous  membrane. 
The  last  section  of  the  rectum  at  the  close  of  the  act  is  emptied 
by  a  forcible  contraction  of  the  levator  ani  and  the  other 
perinfieal  muscles,  and  this  contraction  also  serves  to  restore 
the  everted  mucous  membrane. 

The  carrying  out  of  this  reflex  act  is  dependent  on  the 
integrity  of  a  certain  part  of  the  lumbar  spinal  cord.  If  this 
'  centre  '  be  destroyed,  the  tonic  contraction  of  the  sphincter 
muscles  disappears.  This  centre  may  be  either  excited  to 
increased  action,  or  be  inhibited  by  peripheral  stimulation  of 
various  nerves,  or  by  emotion,  such  as  fear.  Application  of 
warmth  to  the  region  of  the  anus  causes  reflex  relaxation 
of  the  sphincter  ;  application  of  cold  increases  its  tonic  con- 
traction. 


377 


CHAPTER   IX 

RESPIRATION 

Section  1 
THE   EESPIRATOEY   MOVEMENTS 

The  processes  of  external  respiration,  namely,  the  taking  up 
of  oxygen  and  the  giving  off  of  carbon  dioxide— the  product 
of  the  union  of  the  oxygen  with  the  carbon  of  the  foodstuffs — 
are  effected  in  the  lungs,  which  are  built  up  in  the  following 
way.  The  trachea  or  windpipe,  a  wide  tube  about  4i  inches 
long,  divides  below  into  two  main  branches— iro?ic/ri  ; 
and  these  subdivide  again  and  again,  becoming  gradually 
smaller.  The  terminal  ramifications  or  hroncliioles  open  into 
rather  wider  parts — the  infundihula,  the  walls  of  which  are 
beset  with  a  number  of  minute  cavities,  the  alveoli.  The 
larger  tubes  are  kept  patent  by  rings  or  plates  of  cartilage  in 
their  walls.  The  smaller  tubes  have  no  cartilage,  their  walls 
being  composed  of  fibrous  and  elastic  tissue  and  a  coating  of 
unstriated  muscular  fibres,  which  are  able  by  their  contraction 
to  occlude  the  passage.  The  whole  system  of  tubes  is  lined 
with  a  layer  of  epithelium — ciliated  columnar  in  the  trachea, 
bronchi,  and  bronchioles,  and  cubical  over  the  parts  of  the 
infundibulum  not  occupied  by  air-cells. 

The  alveoli  are  the  special  respiratory  parts  of  the  lung. 
Their  walls  are  composed  of  connective  tissue  containing  a 
large  number  of  elastic  fibres,  and  are  covered  internally  by 
a  single  layer  of  extremely  thin  large  flattened  cells.  The 
alveoli  are  closely  packed  together,  so  that  in  a  section  of 
the  lung  an  alveolus  is  seen  to  be  in  contact  with  others  on 
all  sides.  Immediately  below  the  squamous  epithelium  ramify 
blood -capillaries  derived  from  the  pulmonary  artery.  These 
form  a  close  network,  and  the  blood  in  them  is  in  close 
proximity  to  air  on  all  sides,  being  separated  from  the  air  in 


378 


PHYSIOLOGY 


the  alveoli  only  by  the  thin  endothelial  cells  of  the  capillary 
wall  and  the  flattened  cells  lining  the  alveoli. 

The  lungs  in  their  development  grow  out  from  the  front 
part  of  the  alimentary  canal  into  the  front  part  of  the  body- 
cavity  on  each  side — the  pleural  cavity.  The  surrounding 
body-walls  become  strengthened  by  the  formation  of  the  ribs, 
so  that  the  lungs  are  suspended  in  a  bony  cage-work,  the 
thorax.  Their  outer  surface  is  covered  with  a  special 
membrane,  the  pleura,  which  is  reflected  on  to  the  wall  of 
the  thorax  from  the  roots  of  the  lungs,  and  completely  lines 
the   cavity  in   which    they  lie.     The    surface  of  the  pleura 

Fig.  186. 


Diagrammatic  representation  of  the  structure  of  the  lungs.  Tlie 
trachea  branches  into  two  bronchi,  which  subdivide  again  and 
again  before  ending  in  the  infundibula  (from  Yeo). 


facing  the  pleural  cavity  is  lined  with  a  continuous  layer  of 
flattened  endothelial  cells,  and  is  kept  constantly  moist  by 
the  secretion  of  lymph  into  the  cavity.  Thus,  being  attached 
to  the  thorax  only  where  the  bronchi  and  great  vessels 
enter,  the  lungs  are  able  to  glide  easily  over  the  inner  surface 
of  the  thorax,  with  which  under  normal  circumstances  they 
are  in  intimate  contact. 

A  constant  renewal  of  the  air  in  the  lungs  is  secured  by 
movements  of  the  thorax,  which  constitute  normal  breathing. 
With   inspiration  the  cavity  of  the  thorax  is  enlarged,  and 


RESPIRATION  379 

the  lungs  swell  up  to  fill  the  increased  space.  The  capacity 
of  the  air-passages  of  the  lungs  heing  thus  increased,  air  is 
sucked  in  through  the  trachea.  The  movement  of  inspiration 
is  followed  by  that  of  expiration,  which  causes  diminution  of 
the  capacity  of  the  thorax  and  expulsion  of  air. 

The  expiration  follows  immediately  upon  inspiration.  At 
the  end  of  expiration  there  is  normally  a  slight  pause.  The 
number  of  respirations  in  the  adult  is  about  17  or  18  a 
minute.  This  is  however  much  influenced  by  various  con- 
ditions of  the  body,  and  also  by  the  age  of  the  individual. 
Thus  a  new-born  child  breathes  about  44  times  a  minute,  a 
child  of  five  about  26  times,  a  man  of  twenty-five  about  16, 
and  of  fifty  about  18.  The  frequency  is  increased  by  any 
muscular  effort,  so  that  even  standing  up  increases  the 
number  of  respirations.  These  movements  are  much  affected 
hy  psychical  activity  ;  they  are  to  a  certain  extent  under 
the  control  of  the  will,  although,  as  we  shall  see  later,  they 
can  occur  in  an  animal  deprived  of  its  brain,  and  we  know 
they  are  normally  carried  out  without  any  special  act  of 
volition.  We  can  breathe  fast  or  slow  at  pleasure,  and  can 
even  cease  breathing  for  a  time.  It  is  impossible  however 
to  prolong  this  respiratory  standstill  for  more  than  a  minute  ; 
the  need  of  breathing  becomes  imperative,  and  against  our 
will  we  are  forced  to  breathe. 

With  every  inspiration  the  cavity  of  the  thorax  is  enlarged 
in  all  dimensions,  from  above  downwards  by  the  contraction 
of  the  diaphragm,  and  in  its  transverse  diameters  by  the 
movements  of  the  ribs. 

The  diaphragm  is  a  sheet  of  muscle  separating  the  cavity 
of  the  chest  from  that  of  the  abdomen.  This  sheet,  which  is 
tendinous  at  the  centre,  is  arched,  the  convex  side  protruding 
up  into  the  thorax,  forming  thus  a  dome-like  boundary  of 
the  peritoneal  cavit3\  In  expiration  the  lateral  muscular 
parts  of  the  diaphragm  lie  in  contact  with  the  lower  part  of 
the  thoracic  wall.  During  inspiration  the  muscle-fibres  con- 
tract and  draw  the  central  tendon  downwards,  so  that 
the  lower  border  of  the  lungs  descends  (Fig.  188).  The 
enlargement  of  the  lungs  at  the  lower  part  of  the  thorax 
is  aided  by  the  abduction  of  the  floating  ribs,  produced  by 
the  contraction  of  the  quadratus  lumborum  and  deep  costal 
muscles.     In  this  contraction  the  diaphragm  presses  on  the 


380  PHYSIOLOGY 

contents  of  the  abdomen,  so  that  the  abdomen  swells  up  with 
each  inspiratory  movement. 

In  forced  inspiration  the  descent  of  tlie  tendinous  part  of  the  diaphriigni 
may  also  involve  the  heart  and  inferior  vena  cava. 

The  enlargement  in  the  other  diameters  is  effected  by  an 
elevation  of  the  ribs.  Each  pair  of  corresponding  ribs,  which 
are  articulated  behind  with  the  spinal  column  and  in  front 
with  the  sternum,  forms  a  ring  directed  obliquely  from 
behind  downwards  and  forwards.  With  each  inspiratory 
movement  the  ribs  are  raised,  the  obliquity  becomes  less, 
and  the   horizontal    distance   between    sternum    and   spinal 

Fig.  187. 


Diagi'am  showing  movements  of  diaphragm  in  respiration,     i  i, 
inspiratory  position  ;  e  e,  expiratory  position.     (Yeo.) 

column  is  therefore  increased.  Moreover  the  ribs  from  the 
first  to  the  seventh  increase  in  length  from  above  downwards, 
so  that  when  they  are  raised,  the  sixth  rib,  for  instance, 
occupies  the  situation  previously  taken  by  the  fifth,  and  the 
transverse  diameters  of  the  thorax  at  this  height  are  in- 
creased. With  each  inspiration  there  is  a  rotation  of  the 
ribs.  In  the  expiratory  condition  they  are  so  situated  that 
their  outer  surfaces  are  directed  not  only  outwards,  but  also 
downwards.  As  they  are  raised  by  the  inspiratory  move- 
ments, they  rotate  on  an  axis  directed  through  the  fore  and 
hind  ends  of  the  rib,  so  that  their  outer  surfaces  are  turned 
directly  outwards.  In  this  way  a  certain  enlargement  of  the 
thoracic  cavity  is  produced.  As  the  thorax  is  raised  there  is 
alwavs  some  stretching  of  the  rib  cartilages. 


EESPIRATION 


381 


111  expiration  the  processes  are  reversed,  and  the  cavity  of 
the  thorax  is  diminished  in  all  three  dimensions. 

The  movements  of  the  thorax  are  effected  by  means  of 
muscles.     Inspiration  is  performed  by  the  following  muscles  : 

The  diaphragm,  which  is  the  most  important,  and  almost 
suffices  alone  to  carry  out  quiet  respiration. 

The  external  intercostal  muscles,  which  shorten  and  so 
raise  the  ribs. 

The  levatores  costarum  and  serratus  posticus  superior. 

These  muscles  are  the  only  ones  normally  engaged  in 
carrying  out  inspiration.  When,  in  consequence  of  muscular 
exertions   or   from   any  other  cause,  the  inspiratory  efforts 

Fig.  188. 


Four  dorsal  vertebras  and  their  ribs  to  show  attachments  of  respiratory 
muscles.  (A.  Thomson.)  1  1,  levatores  costarum ;  2,  external 
intercostal ;  3,  internal  intercostal. 


become  more  forcible,  a  large  number  of  accessory  muscles 
are  brought  into  play.     These  are  — 

The  scaleni, 

Sterno-mastoid, 

Trapezius, 

Pectoral  muscles. 

Rhomboids,  and 

The  serratus  anticus. 

Normal  expiration  is  chiefly  effected  passively.  When  the 
inspiratory  muscles  cease  to  contract,  the  lungs,  which  were 
stretched  })y  the  previous  inspiration,  contract  by  virtue  of 
the  elastic  tissue  they  contain,  and  the  thorax  itself  sinks  by 
its  own  weight,  and  by  the  elastic  reaction  of  the  stretched 


382 


PHYSIOLOGY 


costal  cartilages.  Probably  under  normal  circumstances  the 
internal  intercostal  muscles  also  contract  with  each  expira- 
tion. 

Although  the  action  of  the  intercostal  muscles  has  been  a  subject  of  debate, 
physiological  experiments  serve  on  the  whole  to  confirm  the  view  first  put 
forward  by  Bamberger  and  based  on  a  consideration  of  the  direction  of  the 
fibres.  The  external  intercostals  pass  from  one  rib  to  the  next  below  down- 
wards and  forwards.     Hence  if  a  pair  of  ribs  be  isolated  from  the  rest  of  the 

Fig.  189. 


chest-wall,  leaving  the  vertebral  and  costal  attachments  intact,  contraction  of 
these  muscles  will  cause  a  rise  of  both  ribs.  This  result  will  be  evident  from 
a  consideration  of  Fig.  189,  where  a  i  is  a  fibre  of  the  external  intercostal 
muscles,  passing  from  the  rib  u  s  to  be  attached  to  the  rib  v'  s'  at  b.  When 
a  b  contracts,  the  tension  it  exerts  on  its  two  attachments  can  be  resolved  into 
two  components  a  c  acting  downwards  and  b  d  acting  upwards,  b  d  however 
acts  at  the  end  of  the  long  lever  b  v',  whereas  a  c  acts  at  the  end  of  a  short 

Fig.  190. 


lever  a  v.  Hence  the  raising  effect  will  overcome  the  depressing  effect,  and 
both  ribs  will  rise. 

The  fibres  of  the  internal  intercostals  run  in  the  opposite  direction  to  the 
external  nuiscles,  and  from  a  consideration  of  Fig.  190  it  is  evident  that  their 
effect  will  be  to  depress  any  pair  of  ribs,  thus  acting  as  expiratory  muscles. 

Owing  to  the  fact  that  the  costal  cartilages  make  an  angle  with  the  bony 
ribs,  the  fibres  of  prolongation  of  the  internal  intercostals,  mtiscitli  intcrcarti- 
laginei,  have  the  same  relation  to  their  attachments  that  the  external  inter- 
costals have  to  the  bony  ribs.     Their  action  therefore  must  be  to  raise  the 


KESPIRATION  383 

cartilages  and  flatten  out  the  angle  between  the  cartilaginous  and  bony  ribs,  so 
that  they  must  act  with  the  external  intercostals  as  inspiratory  muscles. 

In  forced  expiration  a  large  number  of  muscles  may  take 
part — such  as  the  serratus  posticus  inferior,  and  the  muscles 
forming  the  wall  of  the  abdomen,  i.e.  the  rectus,  obliquus,  and 
transversus  abdominis  muscles. 

As  the  lungs  expand  with  each  inspiration,  their  position 
changes  somewhat  in  relation  to  the  thoracic  wall.  The 
roots,  the  hinder  borders,  and  the  apices  of  the  lungs  remain 
nearly  stationary.  The  front  parts  move  downwards  and 
inwards,  so  that  their  inner  borders  in  front  approach  one 
another.  By  percussing  the  chest,  it  may  be  easily  made 
out  that  the  resonant  area,  corresponding  to  the  parts  where 
the  lungs  are  in  contact  with  the  thoracic  walls,  increases 
with  each  inspiration,  and  diminishes  with  each  expiration. 

Even  at  the  end  of  expiration  the  lungs  are  in  a  stretched 
condition.     This  is  shown  by  the  fact  that  if  in  an  animal 
or  in  the  corpse  an  opening  be  made  into  the  pleural  cavity, 
air  rushes  into  the  opening  and  the  lungs  collapse,  driving  a 
certain  amount  of  air  out  through  the  trachea.     Smce  then 
the  lungs  are  always  tending  to  collapse,  it  is  evident  that 
they  must  exert  a  pull  on  the  thoracic  wall.     This  pull  of 
the  lungs  gives  rise  to  a  negative  pressure  in  the  pleural 
cavity.      If   we    connect   a   mercurial   manometer   with    the 
pleural  cavity,  we  find  this  pull  of  the  lungs  amounts  in  the 
corpse  to  6  mm.  of  mercury.    If  the  lungs  are  fully  distended, 
as  after  full  inspiration,  the  elastic  forces  are  more  brought 
into   play,    and  the  negative   pressure   in   the   pleura   may 
amount  to  30  mm.     Since  the  lungs  are  always  tending  to 
collapse,  respiration  becomes  impossible  directly  free  open- 
ings are  made  into  the  pleural  cavities  on  both  sides.     With 
each   inspiratory   movement   air    rushes    in    through    these 
openings,  so  that  the  thoracic  movements  can  no  longer  exert 
any  influence  on  the  volume  of   the  lungs.     The  negative 
pressure  in    the  thorax    is    diminished    by  any   factor    de- 
creasing  the   elasticity  of  the  lung-tissue.     Thus  in  an  old 
man,  where  the  elastic  tissue  is  degenerated  and  the  alveoli 
are  enlarged,  giving  rise  to  the  condition  known  as  emphy- 
sema, the  lungs  may  collapse  only  slightly  or  not  at  all  on 
opening  the  chest.     The  lungs  do  not  collapse  on  making  an 
opening  in  the  chest  of   a  new-born  mammal ;    but  this  is 


384  PHYSIOLOGY 

owing  to  the  fact  that  the  hings  completely  fill  the  thorax 
in  the  expiratory  position,  and  it  is  only  later  that  with  the 
growth  of  the  ribs  the  thorax  gets,  so  to  speak,  too  large  for 
the  lungs,  which  are  therefore  stretched  to  fill  it. 

The  force  exerted  by  the  inspiratory  muscles  is  nearly 
all  spent  in  overcoming  the  elastic  resistance  of  the  lungs 
and  costal  cartilages.  A  free  access  of  air  is  provided  for 
by  contractions  of  certain  accessory  muscles  of  respiration. 
With  each  inspiration  the  glottis  is  widened  by  abduction'  of 
the  vocal  cords.  When  the  glottis  is  observed  by  means  of 
the  laryngoscope,  a  rhythmical  separation  and  approximation 
of  the  vocal  cords  are  observed,  synchronous  respectively 
with  inspiration  and  expiration.  When  inspiration  is  laboured, 
the  alse  nasi  are  dilated  by  the  action  of  the  dilatator  nasi. 
This  movement  of  the  nostril,  which  is  constant  in  many 
animals,  becomes  very  prominent  in  children  suffering  from 
any  respiratory  trouble. 

If  a  manometer  be  connected  with  one  of  the  nostrils,  so 
as  to  register  the  pressure  in  the  air-cavities,  it  is  found  that 
there  is  a  negative  pressure  of  —  1  mm.  Hg  with  inspira- 
tion, and  a  positive  pressure  of  2  or  3  mm.  with  expiration. 
With  forced  inspiration  the  negative  pressure  may  amount 
to  —57  mm.  Hg,  and  with  forced  expiration  there  may  be  a 
positive  pressure  of  +  87  mm. 

Under  no  circumstances  can  we  by  forced  expiration 
empty  the  lungs  of  air.  At  the  end  of  the  most  forcible 
expiration,  if  the  pleura  were  perforated,  the  lungs  would 
collapse  and  drive  more  air  through  the  trachea.  When 
breathing  quietly  a  man  takes  in  and  gives  out  at  each  breath 
about  500  c.c.  of  air,  measured  dry  and  at  0°  C.  If  measured 
moist  and  at  the  temperature  of  the  body,  viz.  37°  C,  the 
volume  would  be  604  c.c.  This  amount  is  known  as  the 
tidal  air.  By  means  of  a  forcible  inspiratory  effort  it  is 
possible  to  take  in  about  1,500  c.c.  more  {complemcntal  air). 
At  the  end  of  a  normal  expiration  a  forcible  contraction  of 
the  expiratory  muscles  will  drive  out  about  1,500  c.c.  more 
{supplemental  air).  These  three  amounts  together  constitute 
the  '  vital  capacity  '  of  an  individual.  This  total  may  be 
determined  by  means  of  the  instrument  known  as  the  spiro- 
meter, which  is  merely  a  small  gas-meter  with  a  gauge  by 
which  the  amount  of  air  in  it  can  be  at  once  read  off.     The 


EESPIKATION  385 

person  to  be  tested  tills  his  lungs  as  full  as  possible,  and  then 
expires  to  the  utmost  into  the  spirometer.  The  air  left  in 
the  Imigs  after  the  most  vigorous  expiration  {residual  air) 
amounts  to  about  2,000  c.c. 

The  residual  air  may  be  determined  by  letting  a  person  expire  to  the 
utmost  extent  and  then  connecting  with  his  mouth  or  nose  a  bag  of  known 
capacity  filled  with  hydrogen.  The  subject  of  the  experiment  then  inspires 
and  expires  into  the  bag  two  or  three  times,  ending  in  the  same  state  of  forced 
expiration  as  he  began.  Any  diminution  of  the  total  volume  of  gas  in  the  bag 
will  represent  the  gas  lost  during  the  experiment  by  diffusion  into  the  blood- 
vessels. By  analysis  of  the  gaseous  mixture  in  the  bag,  it  is  possible  to 
determine  the  amount  of  air  in  the  lungs  at  the  beginning  of  the  experiment. 
Supposing  for  example  the  bag  held  4,000  c.c.  hydrogen,  after  two  respirations 
the  total  volume  is  unaltered,  but  the  gas  is  found  to  consist  of  3,000  c.c. 
hydrogen  and  1,000  c.c.  oxygen,  nitrogen,  and  CO.^,  i.e.  pulmonary  gases.  Since 
the  gas  in  the  lungs  must  have  the  same  composition  and  1,000  c.c.  hydrogen 
have  disappeared  from  the  bag,  it  is  evident  tbat  the  lungs  will  contain  1,000  c.c. 
hydrogen  and  i-— ,  i.e.  330  c.c.  pulmonary  gases.  Thus  the  total  volume  of 
gas  left  in  the  lungs  at  the  end  of  the  forced  expiration  was  1,330  c.c,  which  is 
the  residual  volume  for  the  individual. 

Of  the  500  c.c.  of  tidal  air  taken  in  at  each  inspiration, 
only  a  certain  part  reaches  the  alveoli,  part  being  required 
to  fill  the  air-tubes,  trachea,  bronchi,  and  bronchioles  which 
lead  to  the  air-cells.  The  volume  of  the  air-tubes  has  been 
reckoned  to  amount  to  140  c.c,  so  that  of  the  500  c.c.  about 
360  c.c.  reach  the  alveoli.  For  the  same  reason  the  expired 
air  represents  the  air  from  the  alveoli  (360  c.c.)  diluted  with 
140  c.c.  of  air  which  has  remained  in  the  air-tubes  and 
undergone  very  little  change,  other  than  the  elevation  of 
temperature  and  saturation  with  aqueous  vapour.  We  have 
therefore  to  allow  for  this  air  contained  in  the  so-called 
'  dead  space '  of  the  lungs  when  we  seek  to  arrive  at  the 
composition  of  alveolar  air  from  an  analysis  of  expired  air. 
We  must  remember  however  that  the  air  in  this  dead  space 
is  not  absolutely  wasted,  since  it  is  constantly  midergoing 
interchange  by  diffusion  with  the  alveolar  air,  and  thus 
serves  to  carry  some  of  the  oxygen  to  and  some  of  the  CO^ 
away  from  the  pulmonary  alveoli,  although  never  in  contact 
with  the  latter. 


25 


386  ■    PHYSIOLOGY 

Section  2 
CHEMISTEY   OP   EESPIEATION 

The  respiratory  movements  are  but  means  to  an  end. 
They  enable  the  blood  to  take  up  oxygen  and  give  off  carbon 
dioxide  on  its  way  through  the  lungs,  so  that  the  blood  reaches 
the  tissue-elements  prepared  to  supply  them  with  oxygen  and 
to  take  up  carbon  dioxide,  the  product  of  their  destructive 
metabolism.  We  have  now  to  study  the  conditions  that 
regulate  gaseous  interchange  in  the  lungs  and  in  the  tissues. 

As  we  should  expect,  analysis  shows  marked  differences 
in  the  constitution  of  inspired  and  expired  air.  Inspired 
air — that  is  to  say,  ordinary  atmospheric  air — consists  of 
a  mixture  of  oxygen  and  nitrogen,  with  a  very  small  trace  of 
carbon  dioxide  gas.     Its  composition  is  — 

Oxygen  .....     20-96  vols,  per  cent. 

Nitrogen  ....     79 

Carbon  dioxide        .         .         .       0-04  vol.         ,, 

It  also  contains  a  variable  amount  of  watery  vapour,  but  is 
very  rarely  saturated  with  it.  Its  temperature  of  course 
varies  with  the  season  of  the  year. 

The  chief  change  that  occurs  in  respired  air  is  a  decrease 
of  the  oxygen,  and  a  corresponding  increase  of  carbon  dioxide. 
Its  average  composition  in  man  is — 


Oxygen      . 

.     Ki-O  vols,  per  cent. 

Nitrogen  . 

.     79-6     „ 

Carbon  dioxide 

■       4-4     „ 

It  is  moreover  nearly  saturated  with  watery  vapour,  which  on 
a  cold  day  condenses  in  a  cloud  of  steam  with  every  expira- 
tion. Its  temperature,  which  is  very  slightly  affected  by  that 
of  the  external  air,  is  a  little  below  the  normal  body  tempera- 
ture (about  36°  C).  If  the  inspired  air  is  above  the  body 
temperature,  the  expired  air  is  found  to  be  cooled  down  to 
the  temperature  of  the  body.  If  the  inspired  and  expired  air 
be  carefully  measured  in  a  dry  condition  at  the  same  tempera- 
ture, it  will  be  found  that  the  volume  of  expired  air  is  about 
5V  less  than  that  of  the  inspired.  The  conversion  of  oxygen 
into  carbon  dioxide  would  not  of  course  cause  any  change 
in  the  volume  of  the  gas  ;  for  one  molecule  of  oxygen  (O.J 


RESPIRATION  387 

would,  on  combining  with  carbon,  give  rise  to  one  molecule 

of  carbon  dioxide  (CO.,),  which  at  the  same  temperature  and 

pressure   would   occupy  exactly   the   same   volume.     But   it 

must  be  remembered   that   carbon  is  not  the   only  element 

which  leaves  the  body  in  an  oxidised  condition.     Fats,  for 

example,  contain  a  number  of  unoxidised  atoms  of  hydrogen, 

which  in  the  metabolic  processes  of  the  body  are  fully  oxidised 

to  be  excreted  as  water.     A  certain  amount  of  oxygen  too  is 

used  up  in  the  oxidation  of  the  nitrogenous  elements  of  food, 

which  are  excreted  chiefly  as  urea.     Thus  a  certain  amount 

of  oxygen  is  taken  in  which  does  not  reappear  as   CO.^  in 

expired  air.     Hence,  although  the  total  volume  of  nitrogen 

expired  is  the  same  as  that  inspired,  its  percentage  amount 

is  rather  greater  in  expired  than  in  inspired  air.     We  should 

expect  to  find  this  apparent  loss  of  oxygen  greater  in  carnivora, 

v/hose   food   consists   mainly  of   proteins    and   fats,  than  in 

herbivora,  which    feed   principally  on    carbohydrates.     This 

indeed  is  found  to  be  the  case. 

^,  .  t!0,  expired    .     . 

The  quotient     q    •       • .   i    is  known  as  the  respiratory 

quotient.  From  the  precedmg  remarks,  it  is  evident  that  it 
can  never  be  greater  than  1,  if  the  observation  be  extended 
over  a  fairly  long  period,  and  that  it  is  less  in  carnivora 
than  in  herbivora. 

If  an  animal  could  live  on  a  purely  carbohydrate  diet,  its  respiratory 
quotient  would  be  exactly  1,  since  the  hydrogen  and  oxygen  in  the  carbohydrate 
molecule  are  in  the  exact  proportion  to  form  water  and  can  therefore  be  dis- 
regarded.    Thus  : 

C«H,,0,  +  6  0,  :=  6  CO,  +  6  H,0. 
(Dextrose) 

The  case  is  different  with  fats.     If  we  assume  an  animal  to  be  fed  on  pure 
olein,  the  respiratory  changes  will  be  represented  as  follows  : 
C,H,{C,,H330,)3  +  80  0,,  ^  57  CO,  +  52  H,0. 

57 
Here  the  respiratory  quotient  is  -  -  —  0'71. 

Egg  albumen  may  be  roughly  represented  as  CiooHj^^N^^OjaS.  The  oxidation 
would  take  place  as  follows  : 

C,„,Hi,oN,,033S  +  1071  0,  =  89  CO,  +  5-1  H,0  +  13  CON,H^  +  SO^. 

(Urea) 

89 
Here  the  respiratory  quotient  is  ^^r^^-z  =  0-82. 

In  the  case  of  the  isolated  frog's  muscle,  or  of  the  whole 
frog  in  an  atmosphere  of  nitrogen,  the  respiratory  quotient 
may  be  very  large.     In  warm-blooded  animals  however,  the 


388  PHYSIOLOGY 

intake  of  oxygen  must  always  run  parallel  to  the  output 
of  COg,  and  it  is  not  found  that  moderate  muscular  activity 
alters  in  any  way  the  respiratory  quotient.  With  very 
severe  exercise  leading  to  dyspnoea,  there  may  be  a  temporary 
rise  of  the  quotient,  but  such  a  condition  cannot  be  regarded 
as  normal. 

On  the  other  hand  very  considerable  alterations  may  take 
place  in  the  respiratory  quotient,  if  the  organism  is,  so  to 
speak,  altering  its  invested  capital,  and  converting  a  large 
amount  of  carbohydrates  into  fat,  or  fat  into  carbohydrates. 
This  will  be  evident  if  we  compare  the  formula  for  a  fatty 
acid  with  that  of  dextrose.     Thus  : 

Thus  to  convert  fat  into  carbohydrate,  a  large  amount  of 

oxygen  must  be  taken  in,  which  will  not  appear  at  all  in  the 

CO 
excreta.     The  respiratory  quotient  p.  -  will  be  therefore  very 

small,  and  in  hibernating  animals,  where  this  change  occurs 
to  a  large  extent,  it  may  not  exceed  0*2  or  0-3;  and  the  animal 
may  even  gain  weight  by  the  absorption  of  oxygen.  On  the 
other  hand  when  carbohydrates  are  being  largely  converted 
into  and  stored  up  as  fat,  a  large  amount  of  oxygen  will  be 
available  for  the  production  of  CO^  from  the  simple  decom- 

CO 

position   of    the   carbohydrate.     In   such   cases        ?  will  be 

O2 

large,  and  may  amount  to  as  much  as  1-5. 

As  was  shown  on  p.  385,  only  a  certain  percentage  of  the 
500  c.c.  of  tidal  air  reaches  the  alveoli,  100  to  140  c.c.  being 
required  to  fill  the  trachea  and  bronchial  tubes.  Hence  the 
alveolar  air  must  contain  more  carbon  dioxide  and  less  oxygen 
than  the  tracheal  air ;  and  it  is  found  that,  if  we  take  the  air 
from  the  alveoli  instead  of  that  expired  through  the  mouth 
or  nose,  the  differences  between  it  and  the  inspired  air  are 
much  more  pronounced. 

A  sample  of  alveolar  air  may  be  obtained  for  analysis  in  the  following  way 
(Haldane)  :  A  piece  of  indiarubber  tubing  is  taken  of  about  1  inch  diameter 
and  4  feet  long.  Into  one  end  (Fig.  190a)  is  fitted  a  mouthpiece,  the  other 
being  left  open  or  connected  with  a  spirometer.  About  2  inches  from  the 
mouthpiece  is  fixed  a  gas  sampling-bulb,  which  is  provided  with  three-way  taps 
at  the  upper  and  lower  ends.  Before  an  experiment  the  bulb  is  filled  with 
mercury,  if  the  lower  end  is  open,  or  else  it  is  completely  exhausted.  The  sub- 
ject of  the  experiment,  after  breathing  normally  a  few  times  through  the  tube, 


RESPIRATION  889 

at  the  end  of  a  normal  inspiration,  expires  quickly  and  deeply  and  closes  the 
mouthpiece  with  his  tongue.  The  tap  of  the  sampling-bulb  is  then  turned, 
and  the  air  last  expelled  from  the  lungs  (which  is  therefore  pure  alveolar  air) 
rushes  into  the  bulb.     The  top  of  the  bulb  is  then  turned  ofT,  and  the  gas  may 

Fig.  190a. 

Aifo(/r//-/=/£C£. 


S/IMPL/N6    TUBS. 


be  removed  for  analysis.  A  similar  sample  is  then  taken,  in  which  the  subject 
expires  deeply  at  the  end  of  a  normal  expiration.  This  sample  will,  of  course, 
contain  more  CO,^  and  less  O.,  than  that  obtained  at  the  end  of  inspiration.  The 
mean  of  the  two  samples  is  taken  as  the  average  composition  of  alveolar  air. 

The  difference  between  the  composition  of  expired  air 
and  alveolar  air  is  determined  l)y  the  dilution  of  the  alveolar 
air  with  that  contained  in  the  dead  space.  Hence  with 
shallow  breathing  there  will  be  a  large  difference,  but  this 
will  decrease  with  increased  depth  of  respiration.  Thus  if 
the  alveolar  air  contained  6  per  cent.  CO^  and  the  dead  space 
amounted  to  150  c.c,  the  expired  air  would  only  contain 
3  per  cent.  CO^  when  the  person  was  taking  in  only  300  c.c. 
at  each  respiration.  If  however  he  was  breathing  slowly 
and  deeply  so  as  to  raise  the  tidal  air  to  1,500  c.c  ,  only  one- 
tenth  of  this  would  be  represented  by  the  dead  space,  and  the 
expired  air  would  contam  nine-tenths  as  much  CO^,  as  the 
alveolar  air,  i.e,  5'4  per  cent. 

Bespiratory  Changes  in  the  Blood 

From  100  volumes  of  either  venous  or  arterial  blood  we 
can,  by  means  of  the  mercurial  pump,  remove  about  sixty 
volumes  of  gas. 

A  great  variety  of  pumps  has  been  devised  for  this  purpose.  One  of  the 
simplest  of  these  (Hill's  blood-pump)  is  shown  in  Fig.  191,  and  will  serve  to 
illustrate  the  principle  on  which  all  the  other  pumps  are  constructed.  'In 
extracting  the  gases  of  the  blood  by  means  of  this  pump,  a  blood  receiver  is 
affixed  to  the  end  of  the  tube  e,  and  the  receiver  is  elevated  into  the  position 
indicated  by  the  dotted  outline.  The  receiver  (b)  is  then  put  in  connection  with 
the  tube  (e)  by  means  of  the  three-way  tap  (d),  the  reservoir  (a)  is  raised  above 
the  pump,  and  the  whole  system  is  filled  with  mercury  to  the  top  of  the  blood 
receiver  (f).  The  screw  clip  on  the  rubber  tube,  at  the  upper  end  of  f,  is  then 
closed,  and  the  reservoir  (a)  lowered  until  the  blood  receiver  is  exhausted,  except 
for  2  or  3  c.c.  of  mercury,  which  are  purposely  left  within.     The  screw  clip  on 


890 


PHYSIOLOGY 


the  lower  end  of  f  is  next  closed,  and  the  blood  receiver  now  clipped  at  either 
end,  exhausted,  detached  from  tube  e,  and  weighed.  A  sample  of  blood  is  then 
collected  in  the  following  way: — The  arterial  or  venous  cannula  is  filled  with 
blood,  and  immediately  afterwards  pushed  into  the  rubber  tube  at  the  end  of 

Fig.  191. 


Blood  gas-pump  of  L.  Hill.  This  consists  of  a  mercury  reservoir  (a) 
about  300  c.c.  in  capacity,  which  is  connected  with  a  second  reser- 
voir (b)  by  means  of  120  cm.  of  pressure  tubing.  The  upper  end  of 
the  reservoir  (b)  is  closed  by  a  three-way  tap  (n).  By  means  of  this 
tap  the  reservoir  (b)  can  be  put  in  connection  with  either  the  tube  (e) 
leading  to  the  blood  receiver  (f),  or  with  the  tube  (c)  leading  to  the 
eudiometer  (h).  The  tubes  e  and  c  are  made  of  manometer  tubing. 
The  blood  receiver  (f),  which  also  acts  as  the  froth  chamber,  consists 
of  three  glass  bulbs  connected  by  short  and  wide  junctions.  To 
either  end  of  the  blood  receiver  is  fixed  a  piece  of  pressure  tubing, 
provided  with  a  screw  clip. 


the  blood  receiver,  as  far  as  the  closed  screw  clip.  Before  the  insertion  of  the 
cannula,  the  end  of  the  rubber  tube  is  compressed  with  the  fingers  to  exclude 
the  air  within  it.  A  sufficient  quantity  of  blood  is  now  withdrawn  by  opening 
at  the  same  time  the  screw  clip,  and  the  clip  placed  on  the  vessel  of  the  animal. 


RESPIRATION  391 

The  blood  is  defibrinated  by  shaking  it  with  the  mercury  left  within  the  blood 
receiver  for  that  purpose,  and  the  latter  is  then  again  weighed.  The  weight  of 
the  sample  of  blood  is  then  obtained.  The  blood  receiver  is  once  more  aflSxed 
to  the  tube  (k),  in  the  dependent  position  shown  in  the  figure,  and  the  tube  (e) 
is  exhausted.  Finally,  the  screw  clip  between  e  and  the  blood  receiver  is 
opened,  and  the  gases  are  withdrawn  and  collected  in  the  eudiometer.  Since 
the  blood  receiver  hangs  freely  from  the  tube  (e)  by  means  of  a  piece  of  rubber 
tubing,  it  can  be  both  immersed  in  warm  water,  and  shaken  to  facilitate  tl:  e 
complete  escape  of  the  gases.  The  bulbous  form  of  the  blood  receiver  prevents 
the  blood  from  frothing  over  into  the  pump ;  and  if  the  action  becomes  too 
violent,  it  can  be  immediately  allayed  by  pouring  a  few  drops  of  warm  water  on 
to  the  tube  (e).  The  bubbles  are  thereby  driven  back  into  the  receiver,  and  the 
pump  is  never  fouled.  The  tap  (d)  is  so  manipulated  that  the  gases  only,  and 
not  the  water  which  condenses  in  the  reservoir  (b),  are  driven  over  into  the 
eudiometer.  The  water  is  turned  back  into  the  blood  receiver.  Three  to  four 
exhaustions  are  sufficient  to  extract  all  the  gases  from  about  ten  grammes  of 
blood.     The  gases  are  estimated  by  the  potash  and  pyrogallic  acid  method.' 

The  composition  of  this  gas  varies  considerably  in  venous, 
but  not  so  much  in  arterial  blood. 

The  average  composition  of  the  gases  of  dog's  blood  is 
given  in  the  following  table : 

From  100  vols.  May  be  obtained 

Of  oxygen         0£  carbon  dioxide  Of  nitrogen 

Of  arterial  blood         .         .     20  vols.         .     40  vols.        .     1  to  2  vols. 
Of  venous  blood         .  8  to  12  vols.      .     46     „  .         „        „ 

Measured  at  760  mm.  and  0°  C. 

Thus  the  analyses  of  expired  air  and  of  the  gases  of  the 
blood  show  clearly  that  the  latter,  in  its  passage  through  the 
lungs,  takes  up  oxygen  and  gives  off  carbon  dioxide.  In 
the  tissues  the  reverse  process  occurs,  so  that  the  venous 
blood  returns  to  the  lungs  deprived  of  a  portion  of  its 
oxygen,  and  loaded  with  CO..  In  studying  the  mechanism 
of  this  gaseous  interchange,  it  will  be  convenient  to  treat 
the  two  gases  separately,  since  their  behaviour  in  the  blood 
and  tissues  seems  to  be  largely  independent  of  each  other. 

The  oxygen  in  the  blood  is  nearly  entirely  carried  by 
the  haemoglobin  of  the  red  blood-corpuscles.  The  serum  or 
plasma  of  the  blood  cannot  take  up  more  oxygen  than  the 
same  bulk  of  water — less  than  1  per  cent,  at  the  ordinary 
atmospheric  pressure  and  temperature.  On  the  other  hand, 
if  from  a  given  specimen  of  blood  we  extract  the  haemo- 
globin and  dissolve  this  in  water,  we  find  that  the  pure 
haemoglobin  is  able  to  absorb  as  much  oxygen  as  the  original 
blood  on  being  exposed  to  or  shaken  with  pure  oxygen  or  air. 


392  PHYSIOLOGY 

What  is  the  condition  of  the  ox3^gen  in  the  blood  ?  Is  it 
simply  dissolved,  or  does  it  enter  into  chemical  combination 
with  the  haemoglobin  ? 

It  is  well  known  that,  when  a  gas  is  dissolved  by  a  liquid, 
the  amount  of  gas  taken  up  by  the  liquid  varies  directly  as 
the  pressure  of  the  gas.  Thus  if  one  hundred  volumes  of 
water  at  0°  C.  would  dissolve  four  volumes  of  oxygen  at  a 
pressure  of  one  atmosphere,  it  would  dissolve  eight  volumes 
at  a  pressure  of  two  atmospheres.  At  a  pressure  of  three 
atmospheres  the  amount  dissolved  would  be  twelve  volumes. 
If  the  liquid  be  removed  from  an  atmosphere  of  oxygen  at 
a  pressure  of  two  atmospheres  to  an  atmosphere  at  a  pressure 
of  one,  oxygen  will  be  given  off  by  the  water  until  equilibrium 
is  established  between  it  and  the  surrounding  medium  ;  the 
water  will  then  contain  only  four  volumes  per  cent.  At  a 
pressure  of  half  an  atmosphere  the  amount  dissolved  will  be 
two  volumes.  In  this  case  it  makes  no  difference  to  the 
amount  of  oxygen  dissolved,  whether  the  oxygen  is  alone  or 
whether  it  be  mixed  with  some  other  gas.  Thus  the  amount 
dissolved  will  be  the  same,  whether  the  water  be  exposed  to 
pure  oxygen  at  a  pressure  of  380  mm.  Hg,  or  to  a  mixture 
of  equal  volumes  of  oxygen  and  nitrogen  at  a  pressure  of 
760  mm.  Hg.  In  each  case  the  jjartial  pressure  or  tension 
of  the  oxygen  is  the  same,  and  therefore  the  same  amount  is 
dissolved. 

When  equilibrium  is  established  between  a  gas  and  a 
liquid,  so  that  no  gas  is  being  taken  up  or  given  off  by  the 
liquid,  the  tension  of  the  gas  dissolved  in  the  fluid  is  equal 
to  that  in  the  gaseous  medium.  On  this  fact  is  based  the 
meth(<d  of  determining  the  tension  of  a  gas  dissolved  in 
liquid.  The  liquid  is  brought  into  contact  with  gaseous 
mixtures  containing  various  proportions  of  the  gas  in  question. 
It  is  found  that  the  liquid  gives  off  gas  to  some  of  these 
mixtures,  and  from  others  takes  up  gas.  By  making  various 
experiments  a  gaseous  mixture  will  be  found  with  which  the 
liquid  is  in  equilibrium.  If  we  know  beforehand  the  amount 
of  gas  in  this  gaseous  mixture,  we  know  its  tension  and 
therefore  the  tension  of  the  gas  in  the  liquid. 

Pfliiger's  aerotonometer  (Fig.  192)  consists  of  two  glass  tubes,  r  and  e, 
contained  in  a  vessel  filled  with  water  at  the  temperature  of  the  body.  The 
upper  ends  of  the  tubes  are  connected  by  the  tube  a  with  the  artery  or  vein 


RESPIRATION 


393 


in  which  it  is  desired  to  estimate  the  tension  of  the  blood  gases.  If  for  instance 
we  wish  to  determine  the  tension  of  CO2  in  venous  blood,  where  we  expect  the 
tension  to  amount  to  about  4  per  cent,  of  an  atmosphere,  one  tube  k  is  filled 
with  a  gaseous  mixture  containing  3  per  cent.  C0„,  and  the  other  tube  n  with  a 
mixture  containing  5  per  cent.  C0„.  a  is  now  connected  with  the  distal  end  of 
the  jugular  vein,  or  with  the  central  end  of  the  carotid  artery,  and  blood  is 

Fig.  192. 


Pfliigev's  aerotonometer. 

allowed  to  ilow  and  pass  in  a  thin  stream  down  the  walls  of  the  tubes  r  and  i;, 
thus  presenting  a  large  surface  to  the  contained  gases.  The  blood  collects  in 
the  lower  narrower  portions  of  the  tubes,  and  runs  out  into  the  vessels  b,  b, 
whence  after  defibrination  it  is  returned  at  intervals  into  the  veins  of  the 
animal.  Details  of  an  experiment  of  this  description  carried  out  by  Fredericq, 
using  however  a  single  instead  of  a  double  tube,  will  serve  to  illustrate  the 
method  employed. 


Determination  of  Tension  of  Gases  in  Arterial  Blood 
Dog,  12  kilos.     Received  injection  of  '  peptone '  at  2.15  p.m. 


2.34—3.34 


3.36—4.36 


Percentfige  composition  of  gas  in 
aerotonometer 


At  beginning 

C0.,=   5-07 
O.  =  10-8 

co;=riv53~ 

0„  =  15-175 


At  end 

C0o=   2-106 

0^^13-01 

"C'07^~2^2^ 

0.,=  14-83 


Probable  value  of  0„  and  CO2 

tensions  (in  percentage  of 

an  atmosphere) 

O,  =  13  to  14-8  per  cent. 
CO.,  =  2-41  per  cent. 


In  this  experiment  the  COo  tension  of  the  blood  was  lowered  in  consequence 
of  the  injection  of  albumoses. 


394  PHYSIOLOGY 

One  grm.  of  crystallised  haemoglobin  can  absorb  about 
1'4  c.c.  of  oxygen.  If  a  solution  of  this  oxyhsemoglobin  be 
subjected  in  an  air-pump  to  gradually  diminishing  pressure 
at  the  temperature  of  the  body,  it  will  be  found  that  very 
little  oxygen  is  given  off  until  the  partial  pressure  of  the 
oxygen  is  diminished  to  about  30  mm,  Hg.  At  this  point 
a  large  evolution  of  gas  begins,  and  continues  at  falling 
pressure  until  at  0  mm.  pressure  all  the  oxyhaemoglobin  is 
dissociated  and  converted  into  haemoglobin.  The  same  obser- 
vation may  be  made  in  a  reverse  direction.  If  a  solution 
of  reduced  hemoglobin  be  exposed  to  gradually  increasing 
pressures  of  oxygen,  it  will  be  found  that  the  greatest  absorp- 
tion takes  place  between  0  and  30  mm.  Hg.  After  this  point 
the  oxygen  is  very  slowly  absorbed,  and  the  further  absorp- 
tion goes  on  in  proportion  to  the  partial  pressure  of  oxygen. 

The  following  table  gives  the  relative  proportions  of  haemoglobin  and 
oxyha3moglobin  in  a  blood  containing  13  per  cent,  hasmoglobin  and  exposed  to 
air  at  various  pressures  at  a  temperature  of  37'4°  C.  (Hiifner). 


Atmospheric  pressure 
in  mm.  Hg 

760 

Partial  pressm 
Oxygen  in  mm. 

160 

•e  of 
Hg 

Hremoglobiu 
percentage 

5-4 

Oxyha3moglobiu 
percentage 

94-6 

524-8 

110 

7-6 

92-4 

357-8 

75 

10-8 

89-2 

238-5 

50 

15-4 

84-6 

119-3 

25 

26-7 

73-3 

47-7 

10 

47-6 

52-4 

23-8 

5 

63-9 

36-1 

0-0  ...  0-0        ...         10000         ...  0-00 

In  the  curve  in  Fig.  193  the  divisions  along  the  base  line  correspond  to  the 
partial  pressure  of  the  oxygen  in  mm.  Hg,  and  those  along  the  upright  lines 
(the  ordinates)  to  percentage  of  saturation  of  the  haemoglobin  with  oxygen. 

In  the  case  of  simple  solution,  the  relation  between  the  pressure  of  a  gas 
and  the  amount  absorbed  would  be  represented  by  a  straight  line. 

Since  there  is  no  direct  proportion  between  the  partial 
pressure  of  the  oxygen  and  the  amount  absorbed,  it  is 
evident  that  the  oxygen  combines  with  haemoglobin  to  form 
an  unstable  chemical  compound,  and  that  this  is  not  a  mere 
question  of  solution.  This  is  further  proved  by  the  fact 
that  we  can  displace  the  oxygen  (O2)  from  the  oxyhaemoglobin 
by  equivalent  amounts  {i.e.  by  equal  volumes)  of  CO  or  NO. 

A  knowledge  of  these  facts  makes  it  easy  to  understand 
how  the  oxygen  is  taken  up  by  the  blood  as  it  circulates 
round  the  pulmonary  alveoli.     Arterial  blood,  such  as  that 


RESPIRATION 


B95 


which  hlls  the  puhnonary  vems  and  the  systemic  arteries,  is 
very  nearly  saturated  with  oxygen,  and  will  only  take  up 
about  1  per  cent,  more  on  shaking  it  with  air  at  the  body 
temperature.  Venous  blood  requires  8  to  10  volumes  per 
cent,  of  oxygen  to  saturate  it ;  Init  we  have  already  men- 
tioned that,  at  a  tension  of  30  mm.  oxygen,  the  blood  becomes 
more  than  three-quarters  saturated.  The  tension  of  oxygen  in 
the  alveoli  is  considerably  above  this.  In  the  trachea  the  ten- 
sion of  oxygen  is  about  ^  of  an  atmosphere  (since  the  air  here 
contains  16  volumes  per  cent.),  and  the  tension  in  the  alveoli 

Fir,.  193. 


30 
8o 

70 
6o 
50 
40 
30 
20 


_____ 

[■ 

^ 

' 

,/- 

^' 

/ 

/ 

/ 

/ 

' 

)       I 

0     a 

0       3 

0  -1 

0    J 

0      6 

0      7 

0      8 

0      9 

0       K 

50       I 

10       1 

20      I, 

}0       t 

to     I 

5O     Hi 

0 

Diagram  from  Hiifner,  to  show  the  percentage  saturation  of  hannoglobin 
with  oxygen  at  different  pressures  of  the  gas. 


will  be  only  a  little  lower  than  this.  If  we  take  the  oxygen 
tension  in  the  alveoli  at  I  of  an  atmosphere,^  it  will  still  be 
something  over  100  mm.  Hence  the  venous  blood  brought 
to  the  alveoli  by  the  pulmonary  artery  will,  on  there  coming 
into  intimate  contact  with  the  atmosphere,  take  up  oxygen 
from  it  to  saturation,  or  to  a  point  not  far  removed  from  it. 

The  blood,  thus  laden  with  oxygen,  travels  to  the  left  side 
of  the  heart,  and  from  there  is  sent  through  the  arteries  to 
all  parts  of  the  body.     It  must  be  remembered  that  neither 


'  The  oxygen  tension  in  the  alveoli  has  been  reckoned  at  about  12-G  per 
cent,  to  13-5  per  cent,  of  an  atmosphere. 


896  PHYSIOLOGY 

in  the  lungs  nor  in  the  tissues  does  the  haemoglobin  come  m 
actual  contact  with  the  source  of  the  oxygen,  nor  with  the 
cells  which  it  is  to  supply.  In  both  cases  the  interchange  is 
effected  through  the  intermediation  of  the  plasma  and,  in 
the  tissues,  of  the  lymph  as  well.  Since  the  tissue-elements 
are  constantly  using  up  oxygen,  which  they  build  up  into 
their  living  protoplasm,  they  absorb  any  oxygen  that  is 
present  in  the  surrounding  lymph.  There  is,  in  consequence, 
a  descending  scale  of  oxygen  tensions  from  red  blood-cor- 
puscle through  plasma,  vessel-wall,  lymph,  and  tissue-element. 
The  cell  draws  from  the  lymph,  the  lymph  from  the  plasma, 
so  that  the  oxygen  tension  in  the  plasma  sinks.  This  has  the 
same  effect  as  if  we  put  the  red  corpuscles  in  a  mercurial 
pump  and  lowered  the  pressure  of  gas.  The  immediate  result 
is  an  evolution  of  oxygen,  which  is  taken  up  by  the  plasma,  to 
be  in  turn  passed  on  to  the  lymph  and  the  tissue-cell. 

Under  normal  circumstances  a  blood-corpuscle  never 
stays  long  enough  in  the  proximity  of  the  tissues  to  lose  its 
whole  store  of  oxygen.  If  however  the  further  supply  of 
oxygen  to  the  blood  be  prevented,  as  in  asphyxia,  the  last 
traces  of  oxygen  disappear  from  the  blood.  The  enormous 
avidity  of  the  tissues  for  oxygen  is  shown  by  the  following 
experiment  (Ehrlich).  If  a  saturated  solution  of  methylene 
blue  be  injected  into  the  circulation  of  a  living  animal  and 
the  animal  be  killed  ten  minutes  later,  it  is  found  on  first 
opening  the  body  that  most  of  the  organs  present  their 
natural  colour,  although  the  blood  is  a  dark-blue  colour.  On 
exposure  to  the  atmospheiie  all  the  organs  acquire  a  vivid 
blue  colour.  The  avidity  of  the  tissues  for  oxygen  has  been 
so  great  that  they  have  been  able  to  decompose  the 
methylene  blue  molecule,  with  the  formation  of  a  colourless 
reduction-product,  which  on  exposure  to  the  air  undergoes 
oxydation  again  and  reforms  methylene  blue.  If  then  the 
tissues  are  able  to  effect  the  reduction  of  a  comparatively 
stable  body  like  methylene  blue,  it  is  easy  to  understand  their 
power  of  reducing  oxyhemoglobin,  which  is  so  unstable  that 
it  is  decomposed  by  simple  physical  means  such  as  exposure 
to  a  vacuum. 

It  was  long  debated  whether  the  chief  processes  of  oxida- 
tion took  place  in  the  blood  or  in  the  tissues.  Our  experi- 
ences with  muscle  would  alone  serve  to  convince  us  that  in 


RESPIRATION  397 

some  tissues,  at  any  rate,  processes  of  oxidation  take  place, 
and  the  methylene  blue  experiment  shows  that  these  pro- 
cesses of  oxidation  are  intense  in  all  the  chief  organs  of  the 
body.  It  has  been  found  moreover  that  it  is  possible  to  keep 
a  frog  alive  after  substituting  normal  saline  solution  for  his 
blood,  if  he  be  placed  in  absolutely  pure  oxygen,  and  that  in 
this  case  indeed  the  metabolism  of  the  animal  goes  on  as 
actively  as  before.  As  the  frog  has  no  blood,  it  is  evident 
that  its  metabolic  processes,  consisting  of  the  taking  up  of 
oxygen  and  the  giving  out  of  carbon  dioxide,  must  have  their 
seat  in  the  tissues. 

The  relations  of  carbon  dioxide  in  the  blood  and  the 
manner  of  its  excretion  through  the  lungs  are  rather  more 
complicated  and  obscure  than  in  the  case  of  oxygen.  If  a 
given  volume  of  blood  be  divided  into  plasma  or  serum  and 
corpuscles,  it  will  be  found  that  the  larger  proportion  of  the 
carbon  dioxide  in  the  whole  blood  is  contained  in  the  serum, 
although  a  certain  amount  is  also  present  in  the  corpuscles. 
When  extracting  gases  from  serum  by  means  of  the  mercurial 
pump,  it  is  found  that  about  5  per  cent,  of  the  carbon  dioxide 
present  is  fixed  ^ — that  is  to  say,  is  only  liberated  after  the 
addition  of  some  weak  acid,  such  as  phosphoric  or  tartaric 
acid.  If  however  we  use  whole  blood  for  the  experiment,  it  is 
found  that  the  entire  amount  of  CO.,  is  given  off.  This  is 
shown  by  the  fact  that,  after  extracting  with  the  pump  as 
much  CO.^  as  possible,  no  further  amount  can  be  obtained  on 
addition  of  phosphoric  acid.  Hence  the  red  corpuscles  act  the 
part  of  a  weak  acid,  and  we  can,  in  fact,  in  the  first  experiment 
use  fresh  red  corpuscles  instead  of  phosphoric  acid  to  drive 
off  the  last  trace  of  CO,. 

From  100  volumes  of  venous  blood  we  can  extract  about 
50  volumes  of  CO..  The  question  now  arises  :  Is  this  gas  in 
a  condition  of  solution  or  in  chemical  combination  with  the 
plasma  ?  The  answer  is  easy  to  give.  At  the  temperature 
of  the  body  100  volumes  of  plasma  would  take  up  50  volumes 
of  CO2  at  760  mm.  Hg  pressure,  i.e.  if  we  may  consider  the 

•  It  must  not  be  thought  that  these  5  volumes  per  cent,  represent  the  whole 
of  the  CO.  that  is  chemically  combined.  The  fact  that  a  part  of  the  gas  is 
given  off  on  exposure  to  a  vacuum,  and  a  part  left  in  solution,  shows  merely 
either  that  one  part  is  in  a  state  of  looser  chemical  combination  than  the  other, 
or  that  the  phosphates  in  the  serum  only  suffice  to  take  up  part  of  the  soda 
liberated  by  the  decomposition  of  the  sodium  carbonate. 


398  PHYSIOLOGY 

solubility  of  the  gas  in  plasma  equal  to  its  solubility  in  water. 
If  therefore  CO.^  exists  in  a  state  of  solution  in  the  plasma, 
its  tension  will  be  760  mm.  Hg,  that  is  to  say,  one  atmosphere. 
Now  the  CO.,  tension  in  venous  blood  may  be  determined  by 
the  method  indicated  later  (p.  417),  and  is  found  to  be  equal 
to  only  5  per  cent,  of  an  atmosphere.  This  merely  means 
that  when  the  venous  blood  is  brought  into  an  atmosphere 
containing  5  per  cent.  CO^,  the  relative  proportions  of  CO,^ 
in  the  liquid  and  the  gas  remain  the  same.  We  see  then  that 
only  ^  part  of  50  volumes  can  be  absorbed,  since  the  CO2 
tension  is  only  -^  of  an  atmosphere  ;  and  must  conclude  that 
of  the  50  volumes  |-|}  =  2^  volumes  are  in  simple  solution, 
and  the  remaining  47^  volumes  in  chemical  combination. 

On  analysing  the  ash  of  the  serum,  we  find  that  it  con- 
tains sufficient  sodium  present  to  combine  with  all  the  CO.^, 
besides  that  which  is  necessary  to  satisfy  the  fixed  acids, 
hydrochloric  and  phosphoric.  The  presence  of  phosphates 
as  well  as  proteins  (which  may  act  as  weak  acids)  in  the 
serum  is  probably  of  great  importance  for  the  regulation 
of  the  tension  of  the  00.^.  If  the  two  acids,  carbonic  and 
phosphoric,  are  present  together  in  a  solution  containing  soda, 
the  salts  formed  depend  on  the  relative  masses  of  the  two 
acids.  If  the  carbonic  acid  is  in  excess,  sodium  carbonate  is 
formed,  together  with  monosodium  phosphate  (NaH.^PO,).  If 
however  the  carbonic  acid  be  removed,  or  its  '  mass  influence  ' 
diminished  by  allowing  it  to  escape  freely  into  a  vacuum,  the 
phosphoric  acid  gains  the  upper  hand  and  takes  the  lion's 
share  of  the  sodium,  disodium  phosphate  (Na^HPO^)  being 
formed.  In  this  way,  as  soon  as  the  amount  of  free  carbon 
dioxide  decreases,  however  little,  the  amount  of  the  CO.^  in 
combination  also  diminishes,  and  moreover  to  a  very  con- 
siderable extent.  Thus  in  the  serum,  where  these  two  salts 
are  present,  an  alteration  of  eight  volumes  per  cent,  in  its  CO2 
gives  rise  to  a  change  of  tension  of  only  2*6  per  cent,  of  an 
atmosphere. 

In  other  words  a  distinct  advantage  is  gained  if  the  CO2  in  the  plasma  is,  like 
the  oxygen  in  the  corpuscles,  in  a  condition  of  unstable  chemical  combination. 
A  rise  of  oxygen  tension  from  0  to  25  mm.  gives  a  rise  of  oxygen  carried  from 
0  vols,  per  cent,  to  15  vols,  per  cent,  (taking  20  vols,  per  cent,  as  maximum  satura- 
tion of  the  hemoglobin),  while  a  rise  of  COo  tension  from  0  to  38  mm.  gives  an 
increase  in  CO^  carried  in  the  plasma  from  0  vols,  to  50  vols,  per  cent.  If  the 
CO.,  were  in  simple  solution,  this  change  of  tension  would  raise  the  CO.^  content 


EESPIRATION  399 

only  to  2^  vols,  per  cent.  The  co-existence  of  other  weak  salts  of  sodium,  such 
as  phosphate  or  albuminate,  with  the  carbonate  has  the  eil'ect  of  rendering  the 
latter  more  unstable  and  more  susceptible  to  changes  in  mass  action  of  CO^, 
i.e.  to  changes  in  the  CO.j  tension. 

This  struggle  between  the  COo  and  the  weak  acids  of  the 
serum  for  the  possession  of  the  sodium  is  constantly  going  on. 
In  the  tissues,  where  the  CO^  tension  is  high,  the  mass  influ- 
ence of  this  acid  predominates,  and  a  large  amount  of  it  is 
taken  up  into  the  blood,  where  it  forms  sodium  bicarbonate. 
It  is  difficult  to  be  certain  of  the  tension  of  the  carbon  dioxide 
in  the  cells  themselves.  In  urine  it  is  9  per  cent.,  and  in  bile 
7  per  cent.  It  may  be  estimated  in  the  tissues  of  the  intestinal 
wall  of  an  animal  such  as  the  rabbit,  with  a  thin -walled  gut, 
by  injecting  air  into  a  ligatured  loop  of  intestine,  and 
analysing  the  air  after  two  or  three  hours.  The  air  is  then 
found  to  contain  7  to  8  per  cent.  COo.  Thus  the  tension  is 
much  higher  in  the  tissues  than  even  in  the  venous  blood,  and 
there  must  be  a  continual  flow  of  CO^  from  tissues  to  lymph, 
and  from  lymph  to  blood-plasma. 

We  have  seen  above  that  the  taking  up  of  oxygen  by 
the  blood  in  the  lungs  can  be  explamed  on  purely  physical 
grounds,  since  the  tension  of  oxygen  in  the  alveoli  is  suffi- 
cient to  cause  almost  complete  saturation  of  the  haemoglobin. 
Our  experimental  data  however  do  not  yet  suffice  to  show 
that  the  physical  conditions  at  work  account  for  the  giving 
off  of  CO^,  to  the  air  in  the  alveoli.  In  order  that  this  might 
happen  by  a  mere  process  of  gaseous  diflusion,  the  following 
conditions  must  be  present. 

The  tension  of  the  carbon  dioxide  in  the  pulmonary 
alveoli  must  always  be  less  than  that  in  the  blood.  If  this 
be  so,  the  blood  in  its  passage  through  the  pulmonary  capil- 
laries will  give  oft"  COj  to  the  alveolar  air,  and  the  COo  tension 
in  the  blood  will  be  diminished.  But  this  discharge  of  COo 
can  never  go  on  to  such  an  extent  that  the  COj  tension  in  the 
blood  should  fall  below  that  in  the  alveolar  air ;  for  if  this 
were  the  case  there  would  be  at  once  a  retrograde  movement 
of  the  CO.^,  which  would  then  pass  from  the  alveolar  air 
back  to  the  blood.  Thus  the  COo  tension  in  the  blood  of 
the  pulmonary  vein  can  never  be  less  than  that  of  the 
alveolar  air. 

The   experimental   investigation  of   this  question  is  not 


400  PHYSIOLOGY 

entirely  satisfactory.  According  to  Pfliiger  and  Fredericq, 
the  output  of  carbon  dioxide  as  well  as  the  intake  of  oxygen 
in  the  lungs  can  be  accounted  for  by  simple  processes  of 
diffusion.  The  basis  for  this  statement  lies  in  the  determi- 
nation by  aerotonometric  methods  of  the  tensions  of  oxygen 
and  CO2  in  venous  blood,  in  arterial  blood,  and  in  alveolar 
air.  The  average  results  obtained  by  these  methods  are  as 
follows  : 

Arterial  blood        .         .         .     2-8  per  cent.  ] 

Venous  blood         .         .         .     3-5 — 5'4  per  cent.  V  from  dog.' 

Alveolar  air  ....     2-8  per  cent.  j 

It  would  seem  from  these  figures  that  the  interchange  of 
gases  in  the  lungs  was  practically  complete,  and  such  a  result 
is  not  surprising  when  we  consider  that  the  surface  area  of 
blood  exposed  in  the  alveoli  of  the  lungs  amounts  to  2,000 
square  feet,  every  blood-corpuscle  being  practically  exposed 
on  two  sides  to  the  alveolar  air. 

Objections  have  however  been  raised  to  these  results.  It  has  been  pointed 
out  that  the  tension  of  oxygen  in  the  blood,  even  in  the  arteries,  must  be  con- 
stantly and  rapidly  diminishing  as  the  blood  leaves  the  lungs,  while  the  tension 
of  CO^  must  increase  in  a  like  manner,  although  to  a  less  degree,  since  it 
is  found  that  any  delay  in  the  pumping  of  the  gases  out  of  the  blood  leads 
to  a  diminution  of  total  oxygen  and  to  an  increase  of  CO.^.  This  slight  loss 
of  oxygen,  in  consequence  of  processes  of  oxidation  going  on  in  the  blood 
itself,  would  cause  a  considerable  change  in  the  oxygen  tension,  and  must 
therefore  diminish  the  value  of  the  results  obtained  by  the  aerotonometer.  In 
order  to  avoid  this  source  of  fallacy,  Haldane  has  devised  a  method  of  deter- 
mining the  oxygen  tension  of  the  blood  in  the  lungs,  founded  on  the  use  of 
carbon  monoxide. 

It  has  already  been  mentioned  that  carbon  monoxide  has  the  power  of 
displacing  oxygen  from  oxyha-moglobin  to  form  a  much  more  stable  com- 
pound, carboxyhromoglobin.  If  blood  be  shaken  up  with  a  mixture  of  oxygen 
and  carbon  monoxide,  the  hiemoglobin  distributes  itself  between  the  two 
gases.  In  order  however  to  get  an  equal  distribution,  it  is  necessary  10  take 
a  very  small  percentage  of  carbon  monoxide,  owing  to  its  greater  avidity  for 
ha;moglobin.  Thus  if  hsimoglobin  solution  be  shaken  up  with  air  containing 
0-07  per  cent,  of  CO,  the  result  is  a  mixture  of  equal  parts  of  oxy-  and  carb- 
oxyhsemoglobin.     The   affinity   of   CO  for   haemoglobin  would  thus  appear  to 

21 

be  about =  300  times  the  affinity  of  oxygen  for  haemoglobin. 

Carbon  monoxide  however  is  not  destroyed  in  the  body,  so  that  if  a  mixture 
containing  a  small  proportion  of  CO  be  breathed,  this  gas  will  be  taken  up  until 
a  certain  percentage  of  hemoglobin  is  converted  into  CO  hsemoglobin  and  the 

'  The  dog  in  these  experiments  was  curarised,  and  artificial  respiration  was 
employed.  On  this  account  the  amounts  of  CO^  in  the  expired  air  and  in  the 
blood  were  reduced  below  normal. 


RESPIEATION  401 

tension  of  CO  in  the  tissues  and  fluids  of  the  body  is  equal  to  that  of  the 
inspired  air.  The  amount  of  haemoglobin  therefore,  which  is  converted  into 
carboxyhsemoglobin,  will  serve  as  a  measure  of  the  relative  tensions  of  CO 
and  oxygen  in  the  lungs.  If  the  oxygen  tension  of  arterial  blood  wei'e  the  same 
as  that  of  air,  we  should  expect  that,  with  a  given  percentage  of  CO  in  the  air 
breathed,  the  final  saturation  with  CO  of  the  blood  within  the  body  would  be  the 
same  as  the  saturation  of  blood  when  shaken  outside  the  body  with  air  con- 
taining the  same  percentage  of  CO  as  in  the  air  breathed.  It  was  found  by 
Haldane  however  that  in  all  cases  the  percentage  of  CO  hemoglobin  formed  was 
much  less  in  the  body  than  outside  the  body.  Thus  in  blood  shaken  up  with 
air  containing  20"9  per  cent,  oxygen  and  0'04.5  per  cent.  CO,  the  amount  of 
carbon  monoxide  formed  was  31  per  cent,  of  the  whole  haemoglobin.  When 
the  same  mixture  was  inhaled  for  three  or  four  hours  by  a  man,  the  percentage 
of  CO  haemoglobin  in  his  blood  rose  only  to  26  per  cent.,  at  which  figure  it 
remained  stationary.  This  would  correspond  to  an  oxygen  tension  of  about 
25  per  cent,  of  an  atmosphere,  whereas  we  have  already  seen  that  the  oxygen 
tension  in  the  alveoli  cannot  be  greater  than  15  per  cent.  Hence  it  would  appear 
that  the  epithelial  cells  of  the  alveoli  play  an  active  part  in  the  respiratory  inter- 
change, taking  up  the  oxygen  on  one  side  at  a  tension  of  15  per  cent,  and  piling  it 
up  on  the  other  until  the  pressure  in  the  blood  is  double  that  in  the  alveolar  air. 
Theoretically  there  is  no  reason  to  deny  the  possibility  of  such  powers  to  the 
pulmonary  epithelium.  We  know  that  the  secreting  cells  of  the  kidney  take  up 
urea  from  the  blood  which  contains  only  about  0"05  per  cent,  of  this  substance, 
and  excrete  it  into  the  renal  tubule,  into  a  medium  containing  about  2  per  cent. ; 
and  if  the  data  given  by  Haldane  are  correct,  we  must  ascribe  an  analogous 
function  to  the  pulmonary  epithelium.  These  data  however  were  obtained  by 
a  colorimetric  method  working  with  very  minute  quantities  of  blood,  and  in 
the  absence  of  further  control  experiments  the  question  must  be  regarded  as 
undecided. 

I  have  already  mentioned  that  part  of  the  carbon  dioxide 
in  the  blood  is  in  combination  with  the  red  blood-corpuscles. 
A  solution  of  hsemoglobin  is  also  found  to  have  a  power  of 
combining  with  CO.^  to  form  a  loose  chemical  compound.  It 
is  thought  by  some  that  this  carbon  dioxide  haemoglobin  acts 
as  a  carrier  of  CO.,  between  the  plasma  and  the  alveolar  air, 
and  that,  under  the  influence  of  the  oxygen  of  the  alveolar 
air,  the  CO.^  is  expelled  from  the  corpuscles,  causing  a  sudden 
rise  of  CO^,  tension  in  the  plasma,  and  therefore  a  discharge 
of  this  gas  into  the  alveoli. 


26 


402  PHYSIOLOGY 


Section  3 
THE   EEGULATION   OF   EESPIRATION 

For  the  normal  carrying  out  of  respiration  a  complicated 
series  of  co-ordinated  movements  is  necessary.  And  this 
is  not  all.  The  respiratory  movements  must  be  adjusted  in 
rhythm  and  strength  to  the  varying  needs  of  the  organism. 
When  the  animal  is  performing  active  work,  when  the  muscles 
of  the  body  are  contracting  vigorously  and  producing  large 
quantities  of  carbon  dioxide,  the  respiratory  movements  must 
be  also  quickened  and  deepened  in  order  to  provide  for  the 
due  aeration  of  the  blood,  and  the  discharge  of  the  excess  of 
carbon  dioxide  produced. 

This  co-ordination  of  the  activities  of  the  respiratory 
muscles  and  their  adaptation  to  the  varying  needs  of  the 
organism  are  brought  about  through  the  agency  of  the 
nervous  system,  and  in  particular  by  a  circumscribed  portion 
situated  in  the  medulla  oblongata. 

If  a  section  be  made  just  above  the  pons  dividing  the  brain 
from  the  lower  parts  of  the  central  nervous  system,  it  will 
be  observed  that  the  respiratory  movements  go  on  normally. 

If  another  section  be  made  at  the  lower  border  of  the 
medulla,  the  diaphragm  and  ribs  will  be  motionless,  but 
respiratory  movements  may  be  still  observed  to  take  place  in 
the  facial  muscles  and  larynx.  These  experiments  show  that 
the  part  of  the  nervous  system  presiding  over  the  movements 
of  respiration  is  situated  somewhere  between  the  two  sections. 
This  *  centre '  can  be  localised  still  more  exactly.  Injury  to  a 
small  portion  of  the  medulla  in  the  immediate  neighbourhood 
of  the  nuclei  of  the  vagus  nerves,  and  just  below  the  vaso- 
motor centre,  causes  total  cessation  of  resj^iratory  movements 
and  death  of  the  animal.  Hence  this  part  of  the  nervous 
system  was  called  by  Flourens,  its  discoverer,  the  noeud 
vital. 

In  new-born  animals  a  few  abortive  attempts  at  respira- 
tion are  sometimes  observed  even  after  destruction  of  this 
centre  ;  and  it  is  therefore  supposed  that  there  are  subsidiary 


RESPIRATION  403 

centres  in  the  cord  controlled  and  regulated  by  the  centre  in 
the  medulla.  This  is  true,  since  the  anterior  cornual  cells, 
whence  the  motor  fibres  to  the  respiratory  muscles  arise,  are 
situated  all  the  way  down  the  cord.  But  these  are  not 
res])hratonj  centres,  since  respiration  involves  a  co-ordination 
of  the  various  motor  centres.  The  spasmodic  movements 
that  may  occur  under  the  influence  of  strychnine  or  asphyxia 
after  destruction  of  the  medulla  are  not  co-ordinated,  inspira- 
tory and  expiratory  muscles  contracting  at  irregular  intervals 
and  in  many  cases  simultaneously. 

The  question  now  arises  whether  the  activity  of  this 
centre  in  the  medulla  may  be  regarded  as  automatic  or 
reflex :  that  is  to  say,  do  the  rhythmic  discharges  proceed- 
ing from  it  depend  merely  on  local  changes  taking  place  in 
the  centre,  induced  perhaps  by  changes  in  the  surrounding 
lymph  or  blood,  or  on  a  rhythmic  or  continuous  excitation  of 
the  centre  by  stimulation  of  some  afferent  nerve?  If  the 
vagus  nerves  be  cut  and  the  spinal  cord  divided  just  below 
the  medulla,  respiratory  movements  of  the  alae  nasi  are  seen 
to  continue  and  to  grow  more  pronounced  as  the  blood 
becomes  venous  in  consequence  of  the  cutting  off  of  the  chief 
respiratory  muscles  from  the  medullary  centre. 

A  still  more  thorough  isolation  of  the  centre  has  been 
accomplished  by  dividing  the  vagi,  cutting  through  the  upper 
part  of  the  medulla  and  through  the  cord  at  the  first  dorsal 
nerve,  and  then  dividing  all  the  cervical  posterior  roots.  In 
this  case  movements  of  the  diaphragm  still  continued,  but 
consisted  simply  in  a  series  of  prolonged  spasms,  at  first 
about  two  to  four  per  minute  and  gradually  getting  less  fre- 
quent till  the  animal  died.  We  might  argue  from  this  that 
the  centre  was  capable  of  a  very  imperfect  degree  of  auto- 
matic action,  but  needed  the  stimulus  of  afferent  impulses  from 
the  vagi  or  from  the  higher  parts  of  the  brain  to  render  these 
actions  adequate  for  the  respiratory  needs  of  the  organism. 

In  the  above  experiment  the  centre  cannot  be  regarded  as  free  from  all 
afferent  stimuli,  since  the  mere  closure  of  the  demarcation  current  in  the  cut 
ends  of  the  nerves  would  cause  a  certain  amount  of  excitation,  and  the  animal 
does  not  survive  sufficiently  long  to  allow  this  condition  to  pass  off.  Hering 
has  shown  that  in  the  '  spinal  cord  frog '  {i.e.  one  in  which  the  brain  has  been 
destroyed)  section  of  all  the  posterior  roots  absolutely  destroys  all  mobility,  the 
injection  of  strychnine  being  without  effect.  A  typical  spasm  however  can  be 
at  once  produced  by  exposing  and  stimulating  the  stump  of  one  of  the  cut 
posterior  roots.      We   might  suppose   that   the   respiratory   centre   would   be 


404  PHYSIOLOGY 

siniihirly  devoid  of  automatism  if  absolutely  free  from  afferent  stimuli."  It  can 
be  shown  however  that  the  centre  tends  to  respond  to  all  stimuli,  continuous 
or  rhythmic,  by  means  of  rhythmic  discharges. 

Whether  or  no  we  accept  as  proved  the  possible  auto- 
matism of  the  centre,  there  is  no  doubt  that  the  activity  of 
the  centre  is  intimately  dependent  on  its  local  chemical  state 
or  environment. 

If,  the  nervous  centres  being  intact,  the  proper  aeration 
of  the  respiratory  centre  be  interfered  with  in  any  way,  as  by 
obstruction  of  the  respiratory  passages,  or  by  perforation  of 
both  pleura,  or  by  ligature  of  all  the  arteries  to  the  brain,  or 
by  extensive  loss  of  blood,  the  respiratory  movements  increase 
in  strength  and  frequency,  and  if  the  disturbing  factor  be  not 
removed  the  animal  dies,  presenting  a  train  of  phenomena 
which  are  classified  together  under  the  term  '  asphyxia.' 

The  phenomena  of  asphyxia  may  be  divided  into  three 
stages  : 

(1)  In  the  first  stage,  that  of  hyperpnoea,  the  respiratory 
movements  are  increased  in  amplitude  and  in  rhythm.  This 
increase  affects  at  first  both  inspiratory  and  expiratory 
muscles.  Gradually  the  force  of  the  expiratory  movements 
becomes  increased  out  of  all  proportion  to  the  inspiratory, 
and  the  first  stage  merges  into  : 

(2)  The  second,  which  consists  of  expiratory  convulsions, 
in  which  almost  every  muscle  of  the  body  may  be  involved. 
Just  at  the  end  of  the  first  stage  consciousness  is  lost,  and 
almost  immediately  after  the  loss  of  consciousness  we  may 
observe  a  number  of  phenomena  extending  to  almost  all  the 
functions  of  the  body,  some  of  which  have  been  already 
studied.  Thus  at  this  time  the  vasomotor  centre  is  excited, 
causing  universal  vascular  constriction.  There  is  often  also 
secretion  of  saliva,  inhibition  or  increase  of  intestinal  move- 
ments, constriction  of  the  pupil,  and  so  on. 

(3)  At  the  end  of  the  second  minute  after  the  stoppage 
of  the  aeration  of  the  blood,  the  expiratory  convulsions  cease 
almost  suddenly,  and  give  way  to  slow  deep  inspirations. 
With  each  inspiratory  spasm  the  animal  stretches  itself  out. 

'  According  to  Sherrington,  it  is  possible  to  excite  strychnine  or  asphyxia 
!ipasms  in  a  dog  or  cat  with  isolated  spinal  cord,  in  which  all  the  afferent  roots 
below  the  transection  have  been  divided  six  or  seven  hours  previously.  These 
experiments  appear  to  indicate  that  in  the  mammal  the  motor  nervous 
mechanisms  can  be  set  into  activity  apart  from  the  incidence  of  afferent 
impressions. 


RESPIRATION  405 

and  opens  its  mouth  widely  as  if  gasping  for  breath.  The 
whole  stage  is  one  of  exhaustion  ;  the  pupils  dilate  widely, 
and  all  reflexes  are  abolished.  The  pauses  between  the 
inspirations  become  longer  and  longer,  until  at  the  end  of  four 
or  five  minutes  the  animal  takes  its  last  breath. 

Somewhat  similar  movements  of  respiration,  but  on  a 
smaller  scale,  may  be  produced  b}^  increasing  the  activity  of 
the  centre,  and  therefore  its  gaseous  interchanges,  by  warm- 
ing the  blood  m  the  carotid  arteries  on  its  way  to  the  brain. 
Under  these  circumstances  there  may  be  a  considerable 
quickening  of  respiration  unaccompanied  by  any  deepening, 
which  is  often  spoken  of  as  tacliypnoea.  On  the  other  hand, 
we  may  slow  the  respiratory  movements  by  placing  a  small 
piece  of  ice  on  the  floor  of  the  fourth  ventricle. 

In  the  production  of  the  phenomena  of  asphyxia  two 
factors  must  be  at  work.  In  the  first  place  there  is  an 
accumulation  of  carbon  dioxide  in  the  blood  bathing  the 
centre  or  an  increased  tension  of  this  gas  in  the  centres 
themselves,  either  as  a  result  of  deficient  excretion  or 
increased  production.  On  the  other  hand,  the  centre  is 
deprived  of  oxygen,  either  by  failure  of  renewal  of  the  oxygen 
supply,  or  by  increased  using  up  of  this  gas  in  the  metabol- 
ism of  the  centre.  The  question  arises.  Which  of  these  two 
changes  is  responsible  for  the  different  phj'siological  events 
which  characterise  asphyxia  ?  At  various  times  these 
phenomena  have  been  ascribed  either  to  the  increased 
tension  of  CO^  or  to  the  diminished  tension  of  oxygen  in  the 
centre.  As  a  matter  of  fact,  both  factors  are  at  work,  and  we 
must  now  proceed  to  discuss  the  exact  part  played  by  each. 
In  order  to  investigate  this  point  we  may  try  the  effect  on  the 
respiratory  movements  of  altering  the  tension  of  these  two 
gases  in  the  air  breathed.  The  results  of  such  experiments 
are  very  striking.  It  is  found  that  even  a  slight  increase  in 
the  percentage  of  CO.,  in  the  air  causes  an  increase  first  in 
the  depth  and  later  on  in  the  rhythm  of  respiration.  This  is 
well  shown  in  the  following  Table  by  Haldane,  which  repre- 
sents the  average  depth  and  frequency  of  the  respirations 
when  the  subject  was  breathing  normal  air  and  air  charged 
with  varying  percentages  of  CO..  It  is  seen  that  a  rise  of 
COo  in  the  atmosphere  to  2  per  cent,  increases  the  depth  of 
respirations  by  30  per  cent.,  and  the  total  alveolar  ventilation 


40G 


PHYSIOLOGY 


by  50  per  cent.  A  rise  of  CO.^  to  3  per  cent,  increases  the 
total  ventilation  of  the  alveoli  by  126  per  cent.  An  amount 
of  CO.,  equivalent  to  6  per  cent,  increases  the  depth  of  each 
respiration  by  27'2  per  cent.,  and  the  total  alveolar  ventilation 
by  757  per  cent. 


Percentage  CO., 

Average  depth 

Average 
frequency  of 

Ventilation  of  Alveoli 

with  inspired  Air, 

(Normal  =100) 

100 

COj  Percentage 

in  inspired  Air 
0-04 

of  Respirations 

Jiesjiirations 
per  Minute 

14 

in  Alveolar  Air 

673 

5-6 

((■(•(JO  litres  per  rain.) 

0-79 

789 

14 

lie 

5-5 

202 

864 

15 

153 

5-6 

307 

1,216 

15 

220 

5-5 

5-14 

1,771 

19 

498 

G-2 

6-02 

2,104 

27 

857 

6-6 

If  we  examine  the  last  column  of  figures  in  this  Table, 
representing  the  percentage  of  CO.;  in  the  alveolar  air,  it  will 
be  seen  that,  in  spite  of  the  very  large  variations  in  the  air 
breathed,  the  alveolar  content  in  CO^  remained  practically 
constant  until  the  CO^  in  the  atmosphere  was  increased  to 
such  an  extent  that  the  processes  of  compensation  were  no 
longer  efficient.  We  must  conclude  therefore  that  the 
respiratory  centre  is  so  arranged  as  to  react  to  the  slightest 
increase  of  CO2  tension  in  the  blood,  any  increase  in  this 
gas  giving  at  once  a  compensatory  increase  in  depth  and 
frequency  of  respiration,  so  that  the  alveolar  CO.,  content  may 
be  maintained  almost  constant. 

That  it  is  the  tension  of  CO.^  in  the  alveolar  air,  and 
therefore  in  the  blood  bathing  the  centres,  and  not  the  per- 
centage amount  of  this  gas,  which  is  the  determining  factor, 
is  shown  by  a  comparison  of  the  composition  of  the  alveolar 
air  under  different  atmospheric  pressures.  Thus,  when  the 
subject  of  the  experiments,  from  which  the  above  Table  was 
derived,  was  placed  in  an  air-chamber  compressed  to  a 
pressure  of  1,261  mm.,  the  mean  percentage  of  CO^  in  the 
alveolar  air  was  3*42,  corresponding,  however,  to  a  tension  of 
5-6  per  cent,  of  an  atmosphere,  a  figure  almost  identical  with 
those  given  in  the  last  column  of  the  Table  above.  At  the  top 
of  Ben  Nevis,  where  the  barometric  pressure  was  646  mm.  the 
percentage  of  CO.^  in  the  alveolar  air  was  6*6,  corresponding 
to  a  tension  of  5-2  per  cent,  of  an  atmosphere,  i.e.  of  760  mm. 


RESPIKATIOM  407 

Thus  the  pressure  of  CO.,  in  alveolar  air  remains  practically 
constant  with  widely  varying  limits  of  atmospheric  pressure 
and  with  very  different  percentages  of  CO,  in  the  inspired 
air,  showing  that  the  reactions  of  the  organism  are  directed 
so  as  to  maintain,  by  alterations  in  the  respiratory  depth  and 
rhythm,  a  constant  tension  of  this  gas  in  the  alveoli. 

Very  different  are  the  phenomena  observed  on  alteration 
of  the  partial  pressure  of  oxygen.  Here,  within  wide  limits, 
the  partial  pressure  of  oxygen  in  the  alveolar  air  is  deter- 
mined by  its  pressure  in  the  inspired  air.  Thus,  if  we  take 
the  same  series  of  observations,  with  a  pressure  of  646  mm., 
the  percentage  of  oxygen  in  the  alveolar  air  was  13*  19, 
corresponding  to  a  tension  of  10"4  per  cent.  At  an  atmo- 
spheric pressure  of  755  mm.  the  percentage  of  oxygen  in  the 
alveolar  air  was  13*97,  corresponding  to  a  tension  of  13'06 
per  cent.  In  air  compressed  to  a  pressure  of  1,260  mm.  the 
percentage  of  oxygen  was  16*79,  corresponding  to  a  tension  of 
26*8  per  cent,  of  an  atmosphere  of  760  mm.  The  same  sort 
of  results  are  obtained  by  altering  the  percentage  of  oxygen 
in  the  air  breathed.  It  is  found  that  the  oxygen  tension  or 
percentage  in  the  inspired  air  can  be  lowered  from  its  normal 
of  20*93  to  12  or  13  per  cent,  without  altering  in  any  way 
the  depth  or  rhythm  of  respiration,  and  in  fact  without  any 
change  being  noticed  by  the  individual  who  is  the  subject 
of  the  experiment.  A  percentage  of  13  per  cent,  of  oxygen 
corresponds  to  an  alveolar  content  in  oxygen  of  8  per  cent., 
and  with  a  further  reduction  of  the  oxygen  content  there 
is  increased  pulmonary  ventilation,  but  the  diminution  in 
oxygen  may  be  pushed  to  such  an  extent  that  the  patient 
becomes  blue  from  the  deficient  aeration  of  his  haemoglobin, 
without  any  considerable  distress  being  caused.  In  fact,  in 
many  cases,  the  subject  of  such  an  experiment  may  lose  con- 
sciousness suddenly,  before  he  has  been  aware  of  any  serious 
deficiency  in  his  aeration.  We  must  conclude,  therefore,  that 
the  respiratory  centre  possesses  a  specific  excitability  for 
carbon  dioxide,  and  that  it  is  by  this  specific  excitability  that 
the  normal  depth  and  rhythm  of  the  respiratory  movements 
are  determined.  The  uses  of  such  an  excitability  to  the 
organism  are  obvious. 

We  may  take  as  an  example  the  changes  in  respiration 
which  occur  in  an  animal  as  the  result  of  muscular  exercise. 


408  PHYSIOLOGY 

Under  normal  circumstances  any  changes  in  the  amount  of 
oxygen  in  the  air  breathed  will  be  of  the  rarest  possible 
occurrence.  As  the  result  of  muscular  exercise  a  large 
amount  of  carbon  dioxide  is  produced  in  the  muscles.  The 
venous  blood  passing  from  the  muscles  to  the  lungs  will 
therefore  be  more  highly  charged  with  this  gas,  and  a  rapid 
rise  in  the  alveolar  CO.,  pressure,  and  therefore  in  the  arterial 
CO,,  pressure,  will  be  occasioned.  The  respiratory  centre  is 
thus  stimulated  to  increased  activity,  with  consequent  lower- 
ing of  the  alveolar  CO.,  pressure,  until  a  point  is  reached  at 
which  an  equilibrium  is  maintained  between  the  effect  of  the 
increased  production  of  CO.2  in  raising  the  arterial  CO.^  pressure 
and  that  of  the  increased  respiratory  activity  in  lowering  it. 

In  certain  experiments  by  Zuntz  and  Geppert  on  the  causation  of  the 
increased  respiratory  movements  during  muscular  exercise,  these  observers 
found  that  the  movements  were  increased  to  such  an  extent  as  to  bring  the 
amount  of  CO.^  in  the  arterial  blood  below  the  normal.  In  these  experiments, 
however,  the  muscular  contractions  were  not  produced  by  normal  exercise,  but 
were  obtained  by  tetanising  through  the  spinal  cord  the  lower  limbs  of  an 
animal.  Under  these  circumstances,  the  activity  of  the  muscle  would  be 
associated  with  a  diminished  blood-flow,  so  that  the  contractions  would  be  carried 
out  in  the  absence  of  a  sufficient  amount  of  oxygen.  As  we  have  seen  earlier 
in  dealing  with  the  metabolism  of  muscle,  in  the  absence  of  sufiicient  oxygen, 
muscular  contractions  result  in  the  production,  not  of  CO.j,  but  of  lactic  acid, 
and  it  is  highly  probable  that,  in  the  experiments  in  question,  there  was  a 
discharge  of  acid  substances  into  the  blood,  diminishing  the  alkalinity  of  this 
fluid  and  therefore  lowering  its  carrying  power  tor  CO.^.  As  a  matter  of  fact, 
one  can  produce  dyspnoea  by  diminishing  the  alkalinity  of  the  blood  by  the 
injection  of  acids,  and  attacks  of  dyspnoea  are  observed  in  the  later  stages  of 
diabetes,  when  the  alkalinity  of  the  blood  is  decreased  in  consequence  of  the 
enormous  production  of  such  bodies  as  oxybutyric  acid.  A  diminished  carry- 
ing power  of  the  blood  for  CO.,  will  necessarily  raise  the  tension  of  this  gas  in 
the  tissues  where  it  is  formed,  and  a  diminished  alkalinity  of  the  blood  will 
therefore  tend  to  cause  a  higher  tension  of  CO2  around  the  respiratory  centre. 
It  is  probable  that  when  muscular  exercise  is  carried  to  exhaustion,  this 
diminishing  of  alkalinity  of  the  blood  also  plays  its  part  in  the  hyperpnoea  and 
dyspnoea,  but  in  every  case  the  determining  factor  seems  to  be  the  increased 
tension  of  the  CO.^  in  the  respiratory  centre  itself.  We  must  therefore  con- 
clude that  the  depth  and  rhythm  of  respiration — that  is,  the  regulation  of  the 
rate  of  alveolar  ventilation  and  of  respiration —  depend  under  normal  conditions 
on  the  CO.2  pressure  in  the  respiratory  centre.  This  part  of  the  central  nervous 
system  has  developed  a  specific  sensibility  for  COj,  a  rise  of  0'2  per  cent,  of 
an  atmosphere  in  the  tension  of  this  gas  in  the  alveoli,  and  therefore  in  the 
arterial  blood,  being  sufficient  to  double  the  amount  of  alveolar  ventilation 
during  rest. 

It  is  only  the  first  phase  in  the  phenomena  of  as^jhyxia  which  is  thus 
conditioned  by  the  changes  in  the  CO.^  tension.  The  respiratory  centre  shares 
with  the  rest  of  the  central  nervous  system  a  sensitiveness  to  the  absence  of 


RESPIRATION 


409 


.  /»3  n,  deprivation  of  oxygen  having  tirst  an  excitatory  and  later  a  paralytic 
ect.  In  asphyxia  the  first  centres  to  feel  this  effect  are  those  of  the  cortex, 
d  during  the  first  stage  there  is  mental  excitation  terminating  rapidly  in 
abolition  of  consciousness  at  the  end  of  this  stage.  During  the  second  stage 
here  is  a  discharge  of  energy,  which  spreads  throughout  the  whole  nervous 
system,  beginning  in  the  bulbar  centres  and  causing  a  great  rise  of  blood 
pressure  with  slowing  of  the  heart,  and  extending  thence  to  all  the  spinal 
centres  with  the  production  of  muscular  spasms.  At  this  stage,  too,  there  is  a 
discharge  of  impulses  giving  contraction  of  the  pupil,  and  a  little  later  dis- 
charge along  the  whole  sympathetic  system  producing  the  various  phenomena 
of  vaso-constriction,  erection  of  hairs,  sweating,  salivation,  which  are  generally 
brought  about  by  stimulation  of  different  parts  of  this  system.  The  phenomena 
of  the  third  stage  are  due  to  a  gradual  exhaustion  of  the  nerve-centres,  accom- 
panied or  preceded  by  an  exhaustion  and  dilatation  of  the  hea»t,  the  circulation 
failing  before  the  excitation  of  the  lower  centres  has  entirely  come  to  an  end. 
In  this  third  stage,  however,  it  is  impossible  by  the  strongest  stimuli  to  evoke 
any  reflex,  and  the  general  phenomena  are  those  of  exhaustion. 

Although  we  must  regard  the  specific  sensibihty  of  the 
respiratory  centre  to  COo  as  the  most  important  factor  in 
determining  the  depth  and  rhythm  of  the  respiratory  move- 
ments, these  movements  and  the  condition  of  the  respiratory 
centre  itself  are  modified  in  a  large  degree  by  impulses  arriv- 
ing at  the  centre  along  both  vagi.  Through  other  sensory 
nerves  of  the  body  the  respiratory  movements  can  be  altered 
reflexly,  but  it  is  only  through  the  vagi  that  a  continuous 
stream  of  impulses  passes  to  the  centre  under  normal  circum- 
stances, so  that  exevy  respiratory  movement  is  modified  by 
these  impulses. 

iuG.  194. 


/] 


Normal  tracing  of  diaphragm  slip  (Head's  method). 

In  studying  the  nervous  mechanism  of  respiration,  it  is  necessary  to  have 
some  accurate  method  of  recording  the  resisiratory  movements.  They  may  be 
registered  by  means  of  a  tambour  applied  to  the  chest,  communicating  with 
another  tambour  provided  with  a  lever,  which  is  arranged  to  write  on  a 
blackened  surface ;  or  a  side  tube  to  a  cannula  in  the  trachea  may  be  con- 
nected with  the  registering  tambour.  In  the  first  case,  movements  of  the 
thoi'ax  are  registered;    in  the  second,  changes  of  intra-pulraonary  pressure. 


410  PHYSIOLOGY 

These  methods  are  obviously  useless  when  it  is  wished  to  study  the  effects  of 
artificial  distension  or  collapse  of  the  lungs.  In  this  instance  we  may  use  the 
ingenious  method  described  by  Head.  In  the  rabbit  a  slip  of  the  diaphragm 
on  either  side  of  the  ensiform  cartilage  is  so  disposed  that  the  end  of  it  may  be 
freed  and  attached  by  a  thread  to  a  lever  without  injury  to  its  blood-  or  nerve- 
supply.  It  is  found  that  this  slip  contracts  synchronously  with  the  rest  of  the 
diaphragm,  so  that  it  serves  as  a  sample  of  the  diaphragm,  the  coniractions-of 
which  may  be  recorded  uninfluenced  by  passive  movements  of  the  chest-wall  or 
artificial  increase  of  intra-pulmonf^ry  pressure. 

If,  while  the  respiratory  movements  are  being  recorded 
in  one  of  the  aforementioned  ways,  both  vagi  be  divided,'  a 
marked  change  in  the  respiratory  rhythm  is  at   once    seen 

Fig.  195. 


Tracings   of   respiratory  movements. — A.    Normal.      B.    After 
•     division  of  one  vagus.     C.  After  section  of  both  vagi.     D. 

Both  vagi  cut.     The  central  end  of  one  vagus  stimulated 

with  weak  induced  current  at  Exc. 
Lower     line    =   time-marking,    indicating     seconds.       (From 

Waller.) 

(Fig.  195).     The  first  effect  is  an  increased  inspiratory  tonus, 
but  this  rapidly  disappears,  and  the  respiratory  movements 

'  The  division  of  the  vagi  is  best  effected  by  putting  them  on  a  hooked 
copper  wire,  of  which  the  upper  end  is  inserted  in  a  freezing  mixture.  In 
this  way  complete  functional  division  of  the  nerves  is  obtained  without  any 
excitation.  If  the  nerves  be  cut,  a  certain  amount  of  stimulation  takes  place 
in  consequence  of  the  closure  of  the  demarcation  current  produced  by  the 
cross-section. 


RESPIEATION  411 

become  less  frequent  and  are  increased  in  amplitude.  If  now 
the  central  end  of  one  of  the  vagi  be  stimulated  with  an 
interrupted  current,  the  respiration  may  be  quickened  (as  in 
the  experiment  represented  in  Fig.  195),  or,  as  is  more 
commonly  the  case,  the  inspiratory  movements  may  be 
increased  at  the  expense  of  the  expiratory,  so  that  finally 
a  condition  of  inspiratory  standstill  is  produced,  and  the  slip 
of  the  diaphragm  enters  into  prolonged  contraction. 

With  a  very  weak  stimulus  it  is  sometimes  possible  to 
produce  augmentation  of  the  expiratory  movements  or  rather 
inhibition  of  the  inspiratory,  and  this  is  the  invariable  result 
of  passage  of  a  constant  current  through  the  vagus  in  an 
ascending  direction.  This  effect  however  may  be  more 
strikingly  brought  about  by  stimulation  of  the  central  end 
of  the  superior  laryngeal  nerve.  Excitation  of  this  nerve  pro- 
duces first  an  inhil)ition  of  inspiration,  so  that  the  respiratory 
muscles  come  to  a  standstill  in  the  position  of  expiration,  and 
then  a  forcible  contraction  of  the  expiratory  muscles  takes 
place.  This  illustration  of  the  presence  of  expiratory  fibres 
in  the  superior  laryngeal  nerve  is  not  confined  to  laboratory 
experience,  but  is  constantly  occurring  in  everyday  life. 
The  superior  laryngeal  nerve  supplies  sensory  fibres  to  the 
mucous  membrane  of  the  glottis,  and  we  know  that  the 
slightest  irritation  of  these  fibres — the  presence  of  a  crumb 
or  a  particle  of  mucus — causes  forcible  expiratory  spasms, 
with  spasmodic  closure  of  the  glottis,  which  we  term  a 
cough.' 

So  we  see  that  the  vagus  nerve  contains  two  kinds  of 
afferent  fibres,  or  at  any  rate  afferent  fibres  with  two  distinct 
functions.  Stimulation  of  the  one  kind  stops  inspiration  and 
produces  expiration  ;  stimulation  of  the  other  stops  expiration 
and  produces  inspiration. 

Since  section  of  both  vagi  causes  slowing  of  respiration,  it 
is  evident  that,  under  normal  circumstances,  impulses  which 
exert  some  influence  on  the  respiratory  centre  and  quicken 
respiration,  must  travel  up  the  vagi  from  the  lungs.     The 

'  It  must  not  be  imagined  that  the  fibres  of  the  superior  laryngeal  nerves 
are  concerned  in  the  reflex  maintenance  of  the  normal  respiratory  rhythm. 
They  are  cited  here  merely  because  the  result  of  their  stimulation  resembles 
that  which  would  be  caused  by  stimulation  of  the  analogous  expiratory  fibres 
which  run  in  the  trunk  of  the  vagus  from  the  lungs  to  the  respiratory  centre. 


412  PHYSIOLOGY 

respiratory  movements  cause  an  alternate  distension  and 
contraction  of  the  Imigs,  and  it  has  long  been  thought  that 
it  is  these  changes  in  the  volume  of  the  lungs  which  start  the 
accelerating  impulses  that  travel  up  the  vagus  nerves.  To 
test  the  truth  of  this  hypothesis  it  is  necessary  to  study  the 
two  phases  of  respiration  separately ;  that  is,  to  see  first  the 
result  on  the  respiratory  impulses  of  repeated  distension  of 
the  lungs,  and  secondly  the  result  of  a  sudden  collapse  or  a 
contraction  caused  by  sucking  air  out  of  the  lungs.  The  first 
mode  of  experiment,  when  air  is  driven  repeatedly  into  the 
lungs,  is  spoken  of  as  positive  ventilation  ;  and  the  second, 
when  air  is  sucked  repeatedly  out  of  the  lungs,  as  negative 
ventilation.  In  these  experiments  it  is  advantageous  to 
employ  Head's  method  of  registering  the  diaphragmatic 
movements.      If,    in    a    rabbit    breathing    quietly,    air    be 

Fig.  19G. 


Pop.  ventilation 


Positive  ventilation  (Head).  Under  the  influence  of  positive 
ventilation,  the  inspiratory  contractions  of  the  diaphragm 
become  less  and  less  till  they  disappear  completely. 

repeatedly  blown  into  the  lungs,  the  inspiratory  movements, 
as  evidenced  by  the  contraction  of  the  diaphragm,  are 
gradually  knocked  down,  till  finally  the  animal  is  in  a  con- 
dition in  which  no  inspiratory  movements  whatever  are  made 
(Fig.  196)  ;  or  the  diaphragmatic  standstill  may  be  followed 
by  a  strong  contraction  of  the  expiratory  muscles.  Thus 
distension  of  the  lungs  has  the  same  effect  as  stimulation  of 
the  superior  laryngeal  nerve  in  stopping  inspiration  and  pro- 
ducing expiration. 

If,  on  the  other  hand,  air  be  sucked  out  of  the  lungs  at 
regular  intervals  (negative  ventilation),  the  movements  of  the 
diaphragm  are  amplified,  and  it  does  not  relax  completely 
after  each  individual  respiration.  The  relaxation  becomes 
more  and  more  incomplete,  until  finally  the  diaphragm  enters 


RESPIEATION  413 

into  a  condition  of  continued  contraction,  which  may  last  for 
several  seconds  (Fig.  197).  Thus  collapse  of  the  lung  inhibits 
expiration  and  augments  inspiration. 

The  effects  of  distension  or  collapse  of  the  lung  may  be 
still  more  readily  shown  by  simply  closing  the  trachea  at  the 
end  of  inspiration  or  of  expiration.  The  results  of  such  an 
experiment  are  shown  in  Fig.  198. 

These  experiments  throw  complete  light  on  the  quickening 
action  of  the  vagus  on  respiration.  Normal  respiration  is  a 
series  of  reflex  acts.  Each  inspiratory  movement  causes  an 
expansion  of  the  lung,  which  in  its  turn  stimulates  the  vagus 
nerve-endings,  inhibiting  the  movemeiit  which  has  given  rise 

Fig.  197. 


Negative  ventilation  (Head).  At  a  negative  ventilation  was 
commenced.  The  expiratory  relaxation  of  the  diaphragm  is 
seen  to  become  more  and  more  incomplete,  until  it  finally 
enters  into  continued  contraction. 

to  the  stimulus,  and  causing  the  ensuing  expiratory  movement. 
The  collapse  of  the  lung  attending  expiration  acts  like  the 
negative  ventilation  in  the  experiment  above  mentioned  in 
stimulating  the  inspiratory  nerve-endings  of  the  vagus  ;  and 
the  impulse  thus  produced  acting  on  the  medullary  centre 
checks  the  expiratory  and  hurries  on  the  inspiratory  move- 
ment. In  this  way,  under  normal  circumstances,  the  rhythm 
of  the  respiratory  centre  is  determined  reflexly  through  the 
agency  of  the  vagi,  while  the  chief  factor  in  determining  the 
total  pulmonary  ventilation,  i.e.  the  depth  of  the  movements, 
is  the  CO.^  tension  of  the  blood. 

In  the  foregoing  account  we  have  spoken  of  the  expiratory  and  inspiratory 
effects  of  the  vagus  as  if  they  were  of  equal  importance.  It  seems  probable 
however  that  the  inhibitory  or  expiratory  impulses  started  by  the  inspiratory 
movement,  the  only  active  part  of  normal  respiration,  play  a  more  prominent 


414  PHYSIOLOGY 

part  in  the  regulation  of  respiration  than  do  the  inspiratory  impulses;  and 
one  observer  (Gad)  goes  so  far  as  to  deny  altogether  the  existence  of  two  kinds 
of  respiratory  fibres  in  the  vagus.  According  to  Gad,  the  vagus,  as  regards  the 
respiratory  centre,  is  a  purely  inhibitory  nerve.  Hence  the  primary  effect  of 
dividing  both  vagi  is  an  increased  inspiratory  tone.  This  view  at  first  seems 
paradoxical,  in  that  it  explains  the  final  slowing  of  respiration  after  section  of 
the  vagi  as  due  to  the  cutting  off  of  previous  inhibitory  impulses.  But  inhibi- 
tion in  all  tissues  has  a  twofold  effect.  Although  the  immediate  effect  is 
diminution  of  activity,  yet  the  diminished  disintegration  necessarily  associated 
with  diminished  activity  means  an  increase  of  the  anabolic  at  the  expense  of 
the  katabolic  processes  of  the  tissues.  In  this  way  we  explained  the  diminished 
excitability  occurring  in  a  nerve  at  the  anode  of  a  constant  current,  and  it  will 

Fig.  198. 


Effects  of  distension— collapse  of  lung.  Both  curves  are  described 
by  a  lever  attached  to  a  slip  of  the  diaphragm  of  a  rabbit.  A 
contraction  of  the  diaphragm  (inspiration)  raises  the  lever ;  during 
relaxation  of  the  diaphragm  the  lever  falls. 

In  A,  the  trachea  is  closed  at  x,  the  height  of  inspiration ;  a  pause 
follows,  during  which  the  lever  gradually  sinks  until  an  inspira- 
tion (a  very  powerful  one)  sets  in. 

In  B,  the  trachea  is  closed  at  the  end  of  expiration,  x  ;  there  follow 
powerful  inspirations.     (From  Foster.) 

be  remembered  that  the  secondary  result  of  anelectrotohus  was  increased 
irritability  and  consequent  excitation  at  break  of  the  constant  current.  The 
same  sort  of  process  must  occur  in  the  respiratory  centre.  A  continued 
restraint  of  its  rhythmic  activity  must  lead  to  a  heaping  up  of  its  irritable 
material,  so  that  the  final  result  is  a  state  of  hyperexcitability  in  which  the 
centre,  so  to  speak,  boils  over  on  the  slightest  provocation. 

In  this  condition  a  cutting  off  of  the  inhibitory  impulses  must  at  first 
increase  the  activity  of  the  centre  leading  to  the  increased  inspiratory  tonus 
already  described.  But  unchecked  by  any  reining  impulses,  the  centre  enters 
upon  a  career  of  spendthrift  activity.  Each  inspiratory  contraction  is  maximal, 
but  the  centre,  exhausted  by  the  effort,  has  to  wait  a  considerable  time  before  it 
can  accumulate  sufficient  energy  for  the  next ;  hence  the  final  result  of  section 


RESPIRATION 


415 


of  both  vagi  is  deepening  and  slowing  of  respiration.  In  the  normal  state  we 
must  imagine  that  the  function  of  the  vagus  is  to  increase  the  excitability  of 
the  respiratory  centre,  and  so  make  it  more  susceptible  to  slight  changes  in 
the  COo  tension  of  the  blood. 

Although  Gad  has  rendered  great  service  in  emphasising  the  importance  of 
the  inhibitory  or  expiratory  impulses  which  ascend  the  vagi,  there  is  no  doubt 
that  he  went  too  far  in  denying  the  existence  of  inspiratory  fibres  in  the  vagus. 
This  is  shown  by  the  following  experiment  of  Head.  According  to  Gad's  view, 
collapse  of  both  lungs  implies  simply  a  removal  of  the  normal  inhibitory 
impulses  ascending  the  vagi,  and  is  therefore  equivalent  to  division  of  these 
two  nerves.  If  in  the  rabbit  the  left  vagus  be  divided,  a  tube  can  be  introduced 
into  the  left  bronchus  and  artificial  respiration  can  be  performed  by  alter- 
nate inflation  and  collapse  of  the  left  lung,  without  in  any  way  affecting  the 
respiratory  centre,  all  connections  with  the  latter  being  destroyed  {v.  Fig.  199). 

Fig.  199. 


R'  Lung. 


artif  resp  app. 


L^  Lun 


Diagram  to  illustrate  Head's  experiment  on  the  effect  of  collapse 
of  the  lung.  E.G.,  respiratory  centre  ;  E.V.,  L.V.,  right  and  left 
vagi. 


Meanwhile  the  animal  carries  out  normal  respiratory  movements,  which  can  be 
recorded  by  the  diaphragm  slip  method.  While  the  slip  is  contracting  regularly, 
the  right  pleura  is  opened  and  the  right  lung  allowed  to  collapse.  The  effect  of 
this  collapse  carried  up  by  the  right  vagus  to  the  centre  is  an  extreme  contrac- 
tion of  the  diaphragm,  and  since  the  onset  of  asphyxia  is  prevented  by  the 
artificial  respiration  carried  out  on  the  left  lung,  the  tonic  standstill  of  the 
diaphragm  may  last  over  a  minute.  In  this  case  therefore,  the  effect  of  collapse 
of  one  lung  is  enormously  greater  than  that  produced  by  section  of  both  vagi, 
showing  that  the  effect  is  due,  not  to  abolition  of  the  ordinary  tonic  inhibitory 
stimuli,  but  to  excitation  of  special  inspiratory  fibres  in  the  vagus  by  the  collapse 
of  the  lunK. 


416 


PHYSIOLOGY 


The  important  part  played  by  the  vagi  in  the  regulation 
of  normal  respiration  is  shown  still  more  strikingly  if  the 
respiratory  centre  in  the  medulla  be  separated  from  the  higher 
parts  of  the  brain  before  the  section  of  the  vagi  is  carried  out. 
Separation  of  the  medulla  from  the  higher  parts  of  the  brain, 
as  by  a  section  just  behind  the  corpora  quadrigemina,  has 
practically  no  influence  on  the  respiratory  rhythm.  If  now 
both  vagi  be  divided,  the  normal  respiratory  movements  cease 

Fio.  200. 


Different  forms  of  apntta  produced  by  ventilation  (Head).  A.  Normal 
tracing  of  diaj)liragmatic  slip  ;  B.  Effect  of  positive  ventilation. 
Standstill  of  diaphragm  in  complete  relaxation ;  C.  Effect  of 
negative  ventilation.  Standstill  of  diaphragm  in  a  state  of  tonic 
contraction ;  D.  Effect  of  combined  positive  and  negative  ventila- 
tion. Cessation  of  movements,  the  diaphragm  being  in  a  state  of 
moderate  tone. 

entirely,  being  replaced  by  a  series  of  inspiratory  spasms, 
each  of  which  lasts  several  seconds  and  is  followed  by  a  pause 
of  half  to  one  minute  duration.  These  spasms  are  inadequate 
for  the  proper  oxygenation  of  the  blood.  They  become  gradu- 
ally less  and  less  frequent,  and  the  animal  dies  in  about  half 
an  hour  of  asphyxia.  We  must  conclude  therefore  that  the 
respiratory  centre  with  the  help  of  the  vagi  is  able  to  carry 
out  normal  respiratory  movements.  If  both  vagi  are  cut, 
impulses  arrive  at  the  centre  from  the  higher  parts  of  the 
brain  regulating  its  activity,  and  enabling  it  to  carry  out 
modified    but   sufficient   respiratory   movements.      Kemoved 


EESPIEATION  417 

from  both  these  sources  of  afferent  impulses,  the  centre  dis- 
charges only  a  series  of  spasms  which  are  totally  inadequate 
for  the  renewal  of  the  blood  gases,  so  that  the  animal  dies. 

We  may  summarise  these  results  as  follows  : 

Respiratory  centre  with  vagi^normal  respiration. 

Respirator}^  centre  with  brain — modified  respiration. 

Respiratory  centre  alone — inadequate  spasmodic  contrac- 
tions of  respiratory  muscles,  and  death  of  animal. 

Apnoea.—li  positive  and  negative  ventilation  be  used 
together,  so  that  air  is  blown  into  and  sucked  out  of  the 
lungs  at  a  rate  quicker  than  the  animal's  respiratory  rhythm, 
both  inspiratory  and  expiratory  processes  are  inhibited,  and 
a  standstill  of  respiration  ensues.  This  condition  is  called 
apnoea.  It  has  been  thought  to  be  chiefly  reflex  in  origin,  in 
consequence  of  the  fact  that  an  apnceic  pause  may  be  observed 
after  artificial  respiration  for  a  short  time  with  inert  gases, 
such  as  hydrogen  or  nitrogen.  The  pause  however  does  not 
last  so  long  as  when  air  or  oxygen  is  employed,  since  in  the 
former  case  the  blood  becomes  so  venous  that  the  stimulation 
of  the  centre  thereby  produced  overcomes  the  effects  of  the 
reflex  inhibition,  and  a  violent  inspiratory  movement  ensues. 

One  factor  that  has  not  been  sufficiently  allowed  for  in 
these  ventilation  experiments  is  the  effect  that  rapid  ventila- 
tion with  any  gas  will  have  in  reducing  the  CO.,  tension  in  the 
blood  circulating  round  the  pulmonary  alveoli,  and  therefore 
round  the  respiratory  centre.  The  respiratory  centre  has  a 
specific  sensibility  to  CO.^,  while  it  shares  with  the  rest  of  the 
medulla  and  cord  the  susceptibility  to  absence  of  oxygen. 
Hence  any  change  in  the  amount  of  CO,  in  the  air  breathed 
(irrespective  of  changes  in  the  oxygen)  produces  correspond- 
ing changes  in  the  respiratory  rhythm.  If  for  instance  a  man 
be  made  to  breathe  a  mixture  of  oxygen,  nitrogen,  and  CO.^, 
containing  the  ordinary  percentage  of  oxygen  (21  per  cent.), 
but  2  or  3  per  cent,  of  CO^,  he  experiences  a  feeling  of  breath- 
lessness,  and  objectively  the  respiratory  movements  are  in- 
creased in  rhythm  and  extent  (hyperpnoea)  so  as  to  keep  the 
COo  tension  in  the  alveolar  air,  and  therefore  in  the  blood,  at 
its  normal  point.  If  the  CO^  content  be  increased  to  about 
4-5  per  cent.,  it  is  impossible  to  produce  an  apnoeic  pause, 
however  rapidly  the  respiratory  movements  be  carried  out. 
It  would  seem,  therefore,  that  ordinary  apncea  is  entirely  due 

27 


418  PHYSIOLOGY 

to  deficiency  of  CO^  tension  in  the  respiratory  centre,  and 
that  I  although  the  vagus  nerve  is  inhibitory  of  respiration,  it 
is  impossible  to  summate  a  series  of  vagus  inhibitions  by 
artificial  respiration  so  as  to  produce  a  lasting  cessation  of 
respiratory  movements.  The  chief  use  of  the  vagi,  in  fact,  in 
respiration  seems  to  be  for  maintaining  by  frequent  inhibitions 
the  excitability  of  the  respiratory  centre  at  a  maximum. 

"We  have  seen  that  if  a  man  breathe  a  mixture  of  nitrogen 
and  oxygen  free  from  CO.,,  and  the  oxygen  be  gradually 
diminished,  no  feeling  of  '  want  of  breath  '  may  l)e  experienced. 
Eespiration  is  often  practically  unaltered,  although  the 
deficient  oxygenation  of  the  blood  may  be  shown  by  the 
blueness  of  the  lips  and  face.  In  certain  cases  great  oxygen 
deficiency  excites  the  respiratory  centre,  l)ut  in  many  cases 
no  ill  effects  may  be  felt  by  the  subject  before  he  suddenly 
becomes  unconscious  from  lack  of  oxygen.  Immediately 
following  this  loss  of  consciousness  may  come  the  convulsive 
phenomenon  of  asphyxia. 

From  the  lack  of  any  mechanism  in  the  respiratory 
centre  to  respond  to  minute  changes  in  the  oxygen  tension  of 
the  surrounding  atmosphere,  it  follows  that  any  diminution 
in  oxygen  tension  (as  by  change  of  altitude)  must  cause  a 
corresponding  diminution  in  the  degree  of  saturation  of  the 
haemoglobin  of  the  blood  with  oxygen.  This  change  in 
oxygen  saturation  is  at  once  felt  by  the  blood-forming  organs  ; 
so  that  a  change  of  habitation  from  a  low-lying  to  a  mountain 
district  is  followed  by  an  increased  production  of  blood  cor- 
puscles, until  the  oxygen  carrying  capacity  of  the  blood  is  the 
same  at  the  lower  oxygen  tension  as  it  was  previously  at  the 
higher  oxygen  tension  of  the  plains. 

Broncho-motor  functions  of  the  vagus.— -The  unstriated  muscular  fibres, 
which  form  a  prominent  element  in  the  walls  of  the  bronchioles,  receive  fibres 
from  the  vagi  which  are  efferent  in  function.  If  positive  ventilation  under  a 
constant  pressure  be  maintained  in  an  animal  and  the  excursions  of  the  chest- 
wall  recorded,  a  perfectly  regular  series  of  movements  are  obtained,  which  serve 
as  a  measure  of  the  expansion  of  the  lungs  under  each  inspiratory  blast  of  air. 
If  now  the  peripheral  ends  of  both  vagi  be  stimulated,  the  excursions  of  the 
lever  at  once  are  diminished  in  extent,  showing  that  the  air  does  not  enter  and 
expand  the  lungs  with  its  former  freedom.  The  vagi  in  fact  are  the  motor 
nerves  to  the  bronchial  muscle.  Stimulation  of  these  nerves  therefore  causes 
occlusion  of  the  smaller  air-tubes  and  diminished  entry  of  air  into  the  lungs. 
The  attacks  of  dyspnoea  which  characterise  asthma  are  due  to  spasmodic  con- 
traction of  the  bronchial  muscle,  probably  in  consequence  of  abnormal  stimula- 
tion of  the  central  origin  of  the  vagi. 


EESPIRATION  419 

Cha7iges  in  Composition  of  Air  breathed 

The  oxygen  tension  of  the  air  can  be  considerably  reduced 
without  causing  inconvenience.  If  however  a  mammal  be 
exposed  to  air  at  a  pressure  of  300  mm.  (corresponding  to  a 
partial  oxygen  pressure  of  GO  mm.  Hg),  it  becomes  dyspna-ic 
and  dies  of  asphyxia.  The  experience  obtained  in  balloon 
and  mountain  ascents  is  in  complete  harmony  with  the  result 
of  this  experiment.  Dyspncea  does  not  begin  till  a  height  of 
5,000  metres  is  reached,  which  corresponds  to  a  mercurial 
pressure  of  400  mm.  Hg. 

It  is  interestmg  however  to  note  that  the  extreme  dyspnoea 
with  which  mountaineers  are  attacked  at  a  height  of  about 
5,000  metres  passes  off  after  some  time,  and  then  they  may 
pursue  their  way  another  1,000  metres  higher  without  any 
discomfort  (Whymper).  In  view  of  the  results  given  on 
p.  394,  it  seems  at  first  diiKcult  to  explain  the  deleterious 
effects  of  high  altitudes  or  reduced  oxygen  pressures.  It  was 
there  mentioned  that  even  at  a  partial  pressure  of  oxygen 
of  25  mm.  Hg  (corresponding  to  an  atmospheric  pressure  of 
119*3  mm.  Hg,  or  to  a  height  above  the  sea  of  about  14,000 
metres),  as  much  as  73*3  per  cent,  of  the  haemoglobin  was  con- 
verted into  oxyhaemoglobin,  and  yet  dyspncea  is  produced  at  a 
height  of  6,000  metres,  corresponding  to  a  pressure  of  358  mm. 
(75  mm.  oxygen).  We  must  remember  however  that  an 
important  factor  in  the  respiratory  exchanges  is  the  rapiditij 
of  passage  of  oxygen  from  air  to  blood.  The  blood  is  con- 
stantly circulating  through  the  lungs,  and  during  the  short 
time  of  its  passage  becomes  nearly  saturated  with  oxygen. 
The  velocity  of  passage  of  oxygen  across  the  epithelial 
membrane  will  vary  directly  as  the  difference  of  pressure  on 
the  two  sides  of  the  membrane.  As  this  difference  diminishes, 
the  organism  seeks  to  increase  the  velocity  of  passage  by 
increasing  the  area  of  lung  surface,  i.e.  by  deepening  of  the 
respiratory  movements.  At  the  same  time  there  is  quicken- 
ing of  the  heart's  action  and  increased  velocity  of  blood-flow 
so  as  to  increase  the  amount  of  oxygen  carried  by  the  blood 
in  a  given  time.  These  processes  of  compensation  however 
cannot  go  on  indefinitely,  and  distress,  which  is  chiefly  cir- 
culatory in  character,  begins  to  be  experienced  when  the  limit 
of  compensation  is  arrived  at.    This  limit  occurs  in  trained  men 


420  PHYSIOLOGY 

at  an  atmospheric  pressure  of  about  400  mm.  Hg ;  and  it  is 
of  course  reached  sooner  if  the  respiratory  needs  of  the 
organism  are  increased  by  severe  muscular  exercise  (as  in 
mountain  climbing).  The  gradual  accustoming  to  heights, 
which  is  a  common  experience,  is  a  complex  phenomenon, 
due  partly  to  training  of  respiratory  and  heart  muscles,  and 
partly  to  an  increase  of  red  corpuscles  and  haemoglobin  con- 
tents of  the  blood.  It  is  probable  that  the  dwellers  in  mountain 
regions,  such  as  the  lofty  South  American  plateaus  of  Potosi, 
Quito,  etc.,  would  be  found  to  have  a  greater  lung  area  per 
unit  body  weight  than  the  dwellers  on  the  plains. 

If  the  oxygen  in  the  air  supplied  to  an  animal  be  reduced 
to  3  per  cent.,  it  rapidly  dies  of  asphyxia  with  convulsions. 
Excess  of  carbon  dioxide  also  proves  fatal,  but  in  a  different 
manner.  If  an  animal  be  placed  in  an  atmosphere  contain- 
ing 6  per  cent,  of  CO^,  after  a  stage  of  hyperpncea  it  gradu- 
ally becomes  narcotised  and  dies  without  convulsions.  CO.,  is 
therefore  looked  upon  as  a  narcotic  poison. 

Gases  such  as  nitrogen,  hydrogen,  and  methane  (CH^) 
are  termed  indifferent  gases.  They  may  be  respired  if  mixed 
with  20  per  cent,  of  oxygen,  and  either  of  the  other  gases 
may  be  used  instead  of  nitrogen  to  dilute  the  oxygen  that  we 
breathe,  without  harm  or  inconvenience. 

Carbon  monoxide  is  rapidly  poisonous  by  its  action  on  the 
red  corpuscles.  It  combines  with  haemoglobin,  forming  CO 
haemoglobin,  a  compound  which  is  much  more  stable  than 
oxyhaemoglobin.  The  blood  is  therefore  deprived  of  its 
oxygen  carrier,  and  the  animal  dies  of  asphyxia.  We  have 
seen  however  that  the  displacement  of  oxygen  by  CO  is  not 
absolute,  but  only  relative.  Hence,  although  the  avidity  of 
CO  for  haemoglobin  is  140  times  that  of  oxygen,  we  can 
convert  the  CO  back  into  oxyhaemoglobin  by  increasing  the 
mass  influence  of  the  oxygen.  This  may  be  done  by  giving 
the  poisoned  animal  pure  oxygen  to  breathe,  or  even  oxygen 
under  pressure.  In  pure  oxygen  at  a  pressure  of  two  atmo- 
spheres an  animal  can  breathe  and  live,  even  though  the 
whole  of  its  haemoglobin  is  converted  into  CO  haemoglobin, 
the  amount  of  oxygen  which  is  simply  dissolved  by  the  blood - 
plasma  being  sufficient  at  this  pressure  for  the  respiratory 
needs  of  the  animal. 

Other  gases  which  have  special  poisonous  properties  are 


RESPIRATION  421 

hydrocyanic     acid,    sulphuretted    hydrogen,     phosphuretted 
hydrogen  (PH3),  arseniuretted  hydrogen,  etc. 

Irrespirahle  gases  are  those  which  are  so  irritating  that 
they  produce  spasm  of  the  glottis.  Such  are  ammonia, 
chlorine,  sulphur  dioxide,  nitric  oxide,  and  many  others. 

Ventilation 

A  point  of  practical  importance  is  the  securing  to  each 
individual  of  sufticient  fresh  air,  so  that  he  may  always  have 
a  plentiful  supply  of  oxygen,  and  may  be  relieved  of  his 
waste  products.  It  is  found  that  a  dwelling-room  becomes 
unpleasant  and  stuffy  when  the  percentage  amount  of  CO^ 
has  reached  0-1  per  cent.  This  stuffiness  is  supposed  to  be 
due  to  organic  exhalations  from  the  skin,  lungs,  and  alimen- 
tary canal,  some  of  which  have  a  poisonous  effect,  giving  rise 
to  headache  and  sleepiness.  Since  these  cannot  be  measured, 
it  is  taken  as  a  cardinal  rule  in  ventilation  that  the  amount 
of  CO,,  should  never  rise  above  0*1  per  cent.  An  adult  man 
gives  oft'  about  0*6  cubic  foot  of  CO2  every  hour.  Hence  in 
that  time  he  raises  the  amount  of  CO.,  in  1,000  cubic  feet  of 
air  from  0*04  per  cent,  (the  normal  amount  in  the  atmosphere) 
to  0-1  per  cent.  He  must  therefore  be  supplied  with  2,000 
cubic  feet  of  air  per  hour  in  order  to  keep  the  amount  of  CO., 
down  to  0-07  per  cent. 

(Ordinary  air  contains  0'04  per  cent.  CO.,,  therefore  2,000 
cubic  feet  would  contain  0*8  cubic  foot  CO.^,  which  with  the 
0*6  cubic  foot  given  off  by  the  man  would  be  1-4,  which  is 
0-07  per  cent.) 

In  order  that  the  air  may  be  easily  renewed  without 
giving  rise  to  excessive  draught,  a  certain  amount  of  cubic 
space  must  be  allotted  to  each  man.  Each  adult  should  have 
in  a  room  1,000  cubic  feet  of  space,  and  be  supplied  every 
hour  with  2,000  to  3,000  cubic  feet  of  air. 


422  PHYSIOLOGY 


Section  4 

EFFECTS   OF   THE   EESPIEATORY   MOVEMENTS   ON 
THE   CIECULATION 

The  Pulmonary  Circulation 

Although  the  maintenance  of  the  ch'culation  through  the 
lungs  is  the  sole  function  of  the  right  side  of  the  heart,  and 
so  is  of  equal  importance  with  the  whole  systemic  circulation, 
yet  owing  to  the  simple  features  of  this  circulation  it  can  be 
dealt  with  in  a  very  short  space.  In  the  lungs  we  have  an 
extensive  system  of  wide  capillaries  presenting  very  little 
resistance  to  the  flow  of  blood.  The  arterioles  are  also  wide 
and  have  only  a  slight  amount  of  muscular  fibre  in  their  walls, 
and  we  find  therefore  that  the  pressure  necessary  to  drive  the 
blood  from  right  to  left  heart  is  also  small.  The  determina- 
tion of  the  normal  average  pressure  in  the  pulmonary  artery 
presents  considerable  difficulties,  but  it  probably  does  not 
exceed  25  mm.  Hg,  i.e.  about  one-sixth  of  the  mean  aortic 
pressure. 

The  capillaries  of  the  lungs  may  vary  passively  consider- 
ably in  size  according  to  the  conditions  under  which  they  may 
be  placed.  Thus  whereas  at  ihe  height  of  inspiration  the 
blood  contained  in  the  lungs  is  about  y^  o^  the  whole  blood 
in  the  body,  this  amount  is  diminished  during  expiration  to 
between  -^K^  and  y^-,  and  by  forcible  artificial  inflation  of  the 
lungs  may  be  lessened  to  ^.  These  changes,  as  we  shall  see 
later,  exercise  a  considerable  effect  on  the  systemic  blood- 
pressure  and  are  largely  responsible  for  the  respiratory  varia- 
tions observed  in  the  systemic  blood-pressure. 

On  the  other  hand  this  distensibility  of  the  lung  capillaries 
may  play  an  important  part  in  enabling  the  lungs  to  act  so  to 
speak  as  a  reservoir  for  the  left  side  of  the  heart.  Any  tem- 
porary excess  of  output  on  the  right  side  that,  in  consequence 
of  raised  arterial  pressure  or  other  factor,  cannot  be  dealt 
with  at  once  by  the  left  heart,  is  taken  up  for  a  time  in  the 
lung  capillaries. 

Vaso-motor  fibres  to  the  lung  vessels  have  been  described 


EESPIRATION 


423 


as  running  in  the  anterior  roots  of  the  3rd,  4th,  and  5th  dorsal 
nerves.     Their  action  is  however  of  little  importance. 

The  Bespiratory  Undulations  in  the  Systemic  Blood 
Br  ess  lire 

If  we  examine  a  tracing  of  the  arterial  blood-pressure, 
we  notice  that  it  presents  certain  periodic  oscillations  which 
accompany  the  movements  of  respiration.  With  each  inspira- 
tion the  blood-pressure  rises  ;  with  each  expiration  it  falls. 
The  synchronism  of  the  rise  and  fall  with  the  respiratory 
movements  is  not  exact,  since  the  rise  continues  for  a  short 
time  after  the  beginning  of  expiration,  before  it  begins  to  fall, 
and  the  fall  continues  right  into  the  beginning  of  the  next 
inspiration,  so  that  the  highest  pomt  of  the  curve  occurs  at  the 
beginning  of  expiration  and  the  lov>'est  point  at  the  beginning 
of  inspiration.  During  the  fall  which  accompanies  expira- 
tion, the  heart-beats,  as  shown  in  the  diagram  (Fig.    201), 


Fig.  201. 


Bespiratoiy  (racuiff. 

Diagram  of  blood-pressure  curve,  showing  effects  of  the  respiratory 
movements  on  blood-pressure  and  pulse-rate. 


become  less  frequent,  and  an  obvious  explanation  of  the  fall 
of  pressure  would  be  to  ascribe  it  to  a  reflex  inhibition  of  the 
heart.  On  dividing  both  vagi,  this  difference  in  the  pulse- 
rate  during  inspiration  and  expiration  disappears,  but  the 
main  features  of  the  blood-pressure  curve  remam  the  same  ; 
so  that  we  must  look  for  some  mechanical  explanation  of  the 
respiratory  undulations. 

We  have  already  seen  that  under  normal  conditions  the 
lungs  are  in  a  state  of  over-distension,  and  that  in  consequence 
of  this  condition  they  are  constantly  tending  to  collapse,  and 
are  therefore  exerting  a  pull  on  the  chest-wall.     As  soon  as 


424  PHYSIOLOGY 

we  admit  air  into  the  pleural  cavity  by  perforating  the  chest- 
wall,  the  lungs  collapse.  The  force  with  which  the  lungs  are 
normally  trying  to  collapse  amounts  to  6  mm.  Hg,  so  we 
say  that  in  the  pleural  cavity  there  is  normally  a  negative 
pressure  of  —  6  mm.  Hg. 

As  the  chest  expands  in  inspiration,  it  drags  the  lungs  still 
more  open.  As  these  become  more  distended,  their  tendency 
to  collapse  becomes  greater,  and  hence  the  negative  pressure 
in  the  pleura  may  be  increased  during  forcible  inspiration  to 
—  30  mm.  Hg. 

It  must  be  remembered  that  the  heart  and  great  veins  and 
arteries  are  in  the  thorax  only  separated  from  the  pleural 
cavity  by  a  thin,  yielding  membrane,  so  that  they  are  practi- 
cally exposed  to  any  pressure,  positive  or  negative,  which  may 
exist  in  the  pleural  cavity. 

Hence  even  at  the  end  of  expiration  the  heart  and  large 
vessels  are  subjected  to  a  negative  pressure  of  ~6  mm.  Hg. 
Outside  the  thorax  all  the  vessels  are  exposed  to  a  positive 
pressure,  conditioned  in  the  neck  by  the  elasticity  of  the 
tissues,  and  in  the  abdomen  by  the  contractions  of  the  dia- 
phragm and  abdominal  muscles. 

Now  blood,  like  any  other  fluid,  will  always  flow  from  a 
point  of  higher  to  a  point  of  lower  pressure.  There  must 
thus  be  a  constant  aspiration  of  blood  from  peripheral  parts 
into  the  thorax.  This  aspiratory  force  will  however  not 
influence  arteries  and  veins  alike.  The  arteries,  having  thick 
comparatively  non-distensible  walls,  will  be  very  little  affected 
by  the  negative  pressure  obtaining  in  the  thoracic  cavity, 
whereas  the  thin-walled  distensible  veins  will  be  very  largely 
influenced  by  the  same  factor.  The  total  result  then  of  the 
negative  pressure  in  the  pleural  cavities  is  to  increase  the 
flow  of  blood  from  the  veins  into  the  heart,  without  afl"ecting 
to  any  appreciable  degree  the  outflow  of  blood  from  the  heart 
into  the  arteries.  The  more  pronounced  the  negative  pres- 
sure in  the  thorax,  the  greater  will  be  the  amount  of  blood 
sucked  into  the  heart  from  the  veins.  During  inspiration 
therefore,  the  heart  will  be  better  supplied  with  blood  than 
during  expiration,  and  this  factor  in  itself  will  tend  to  raise 
the  arterial  blood- pressure. 

The  mspiratory  descent  of  the  diaphragm  will  moreover 
tend  to  increase  the  inflow   into   the   heart    by  raising  the 


KESPIRATION  425 

positive  pressure  in  the  abdomen,  so  that  blood  is  jiressed  out 
of  the  abdominal  veins  and  sucked  into  the  heart  and  thoracic 
veins. 

Still  more  important  is  the  influence  of  the  respiratory 
movements  on  the  circulation  through  the  lungs.  In  trying 
to  understand  this  influence,  it  must  be  remembered  that  the 
pulmonary  capillaries  lie  in  a  certain  amount  of  elastic  and 
connective  tissue,  and  are  separated  on  the  one  side  by  the 
alveolar  epithelium  from  air  at  the  ordinary  atmospheric 
pressure,  and  on  the  other  by  the  pleural  endothelium  from 
the  pleural  cavity,  where  the  pressure  varies  from  6  to 
30  mm.  Hg  below  the  atmospheric  pressure.  We  may  there- 
fore consider  the  pulmonary  capillaries  as  lying  between, 
and  attached  to,  two  concentric  elastic  bags,  as  represented 
in  Fig.  202.  Under  normal  conditions,  since  these  bags  are 
always  tendmg  to  collapse,  the  inner  one  must  be  pulling 
away  from  the  outer  one,  and  the  outer  one  from  the  chest- 
wall.  Hence  there  must  be  a  negative  pressure  in  the 
tissues  between  these  two  bags — a  negative  pressure  which  in 
the  expiratory  condition  will  be  something  between    0  and 

—  6  mm.  Hg,  and  in  the  inspiratory  condition  between  0  and 

—  30  mm.  Hg.  If  we  regard  the  average  pressure  within 
the  pulmonary  capillaries  as  constant,  these  capillaries  must 
be  more  dilated  in  the  inspiratory  than  in  the  expiratory 
condition,  as  shown  in  the  diagram  (Fig.  203).  Now  this 
dilatation  of  the  pulmonary  capillaries  will  have  two  effects. 
Their  capacity  will  be  increased  and  the  resistance  they 
present  to  the  flow  of  blood  will  be  diminished. 

Let  us  now  consider  what  effect  these  changes  will  have 
on  the  general  arterial  blood-pressure.  We  will  assume  that 
during  expiration  (Fig.  202)  the  pulmonary  vessels  have  a 
capacity  of  25  c.c,  and  that  the  beat  of  the  right  heart  is 
forcing  through  them  10  c.c.  of  blood  per  second.  So  long  as 
the  chest  remains  in  the  expiratory  condition,  10  c.c.  of  blood 
will  be  flowing  into  the  left  heart  and  into  the  aorta,  so  that 
the  sj^stemic  blood-pressure  will  remain  constant.  Now  let 
us  suppose  that  an  inspiratory  enlargement  of  the  thorax 
takes  place  (Fig.  203).  The  negative  pressure  in  the  pleura 
is  increased,  the  two  walls  of  the  lung  are  pulled  farther  away 
from  one  another,  and  there  is  a  general  enlargement  of  the 
pulmonary  capillaries.     We   will   assume  that  this  enlarge- 


426 


PHYSIOLOGY 


ment  increases  the  capacity  of  the  pulmonary  capillaries  from 
25  to  30  c.c.  Owing  to  this  increased  capacity,  the  first  5  c.c. 
of  blood,  which  flows  into  the  kings  after  the  beginning  of 
inspiration,  will  not  flow  out  through  the  pulmonary  vein,  but 


Fig.  202. 


Mariometes 


Diagram  to  show  condition  of  pulmonary  vessels  in  expiration. 

will  simply  serve  to  bring  the  capillaries  into  the  same  state 
of  distension  as  before.  Hence  at  the  beginning  of  inspiration 
the  flow  through  the  pulmonary  vein  will  be  diminished  ; 
there  will  be  less  blood  discharged  into  the  left  heart,  and 


Fig.  203. 


^&^\ 


II 


Manomefer 
connected 

with 
I'leuw.lCaviti/ 


■  Pulm.  Veiiv. 
Pulmonary 
Artertf. 

Diagram  to  show  condition  of  pulmonary  vessels  in  inspiration. 


therefore  a  fall  in  systemic  pressure.  As  soon  however  as  the 
increased  capacity  of  the  pulmonary  vessels  is  made  up,  the 
dilating  effect  of  the  inspiratory  movement  of  these  vessels 
will  aid  the  flow  through  the  lungs,  in  consequence  of  the 
diminution  of  resistance,  so  that  the  same  force  of  the  right 


RESPIRATION  427 

heart,  which  drove  10  c.c.  of  blood  per  second  through  the 
former  resistance  during  expiration,  will  now  drive  more,  say 
12  c.c.  of  blood.  There  is  thus  more  blood  entering  the  left 
heart,  and  therefore  a  rise  of  systemic  pressure  during  the 
last  three  quarters  of  the  inspiratory  movement. 

Expiration  will  have  exactly  the  reverse  effect.  At  the 
beginning  of  expiration  there  is  a  diminution  of  capacity  in 
the  pulmonary  vessels  from  30  to  25  c.c.  Hence  during  the 
first  second  of  expiration  the  outflow  of  blood  from  the 
pulmonary  vein  into  the  left  heart  will  be  17  c.c.  (12  c.c.  + 
5  c.c).  After  this,  the  increased  resistance  in  the  pulmonary 
capillaries  in  consequence  of  their  constriction  will  come 
into  play,  and  the  flow  of  blood  through  them  will  fall  once 
more  from  12  c.c.  to  10  c.c.  Hence  at  the  beginning  of  expira- 
tion the  inflow  of  blood  from  the  pulmonary  vein  into  the  left 
heart  is  greater  than  at  any  period.  The  arterial  pressure 
will  therefore  rise  to  its  greatest  height  at  the  beginning 
of  expiration,  and  will  fall  during  the  last  three  quarters  of 
expiration,  but  will  attain  its  minimum  only  at  the  beginning 
of  the  next  inspiration. 

In  this  way  the  effect  of  the  respiratory  movements  on 
the  systemic  blood -pressure  can  be  entirely  explained  by  the 
influence  they  exert  on  the  lung  vessels  or  lesser  circulation. 


428 


PHYSIOLOGY 


Section  5 
VOICE   AND   SPEECH 

Voice.  —The  voice  is  produced  by  an  expiratory  blast  of 
air  being  forced  through  the  narrow  interval  between  the 
true  vocal  cords.  These,  which  are  thin,  membranous,  and 
elastic,  are  set  into  vibration  by  the  current  of  air,  and  the 

Fk;.  204. 


Anterior  half  of  the  larynx,  seen  from  behind.  The  section  on  the 
right  side  is  somewhat  in  front  of  the  left  side,  e,  epiglottis ; 
c',  cushion  of  epiglottis ;  t,  thyroid  cartilage ;  s,  s',  ventricle 
of  larynx ;  h,  great  cornu  of  hyoid  bone ;  t  a,  thyro-arytaenoid 
muscle ;  v  I,  vocal  cords.  Above  the  ventricles  are  the  false  vocal 
cords,     r,  first  ring  of  trachea.     (A.  Thomson.) 

vibrations  are  communicated  in  turn  to  the  air  in  the  upper 
air-passages.  The  pitch  of  the  sound  produced  depends  on 
the  rapidity  of  vibration  of  the  vocal  cords. 

The  vocal  cords  are  composed  of  elastic  and  muscular  fibres,  covered  over 
by  mucous  membrane,  and  running  from  the  anterior  processes  of  the  arytse- 
noid   cartilages   behind   to  the  posterior   surface  of   the  angle  of  the  thyroid 


RESPIEATION 


429 


cartilage  in  front.  The  arytsenoicl  cartilages  are  small  pyramidal  masses  of 
fibro-eartilage  resting  on  the  back  part  of  a  ring-shaped  cartilage,  the  cricoid 
cartilage.  This  is  thin  in  front,  but  thick  behind,  being  produced  upwards  at 
its  back  part  for  the  attachment  of  the  arytaenoids. 

The  thyroid  cartilage  consists  of  two  wings  or  alae,  articulated  to  the  sides 
of  the  cricoid,  so  that  the  back  part  of  the  cricoid  can  move  backwards  and 
forwards. 

These  four  cartilages  form  the  skeleton  of  the  larynx.  By  their  relative 
positions  the  tension  of  the  vocal  cords  and  the  shape  of  the  rima  glottidis 
are  determined.     These  movements  are  carried  out  by  the  following  muscles  : 

The  crico-thyroid  muscle  passes  downwards  and  forwards  from  the  lower 
part  of  the  thyroid  cartilage  to  the  front  part  of  the  cricoid. 

The  loosteriar  crico-arytcEnoid  muscle  arises  from  the  posterior  surface  of 
the  cricoid  cartilage  and  is  inserted  into  the  outer  angle  of  the  arytsenoid 
cartilage. 

Fig.  205.  Fig.  20G. 


^— -  c.a.p 


Fig.  206.— Side  view  of  cartilages  of  larynx,     th,  thyroid  cartilage ; 

ar,   arytsenoid  cartilage ;  cr,   cricoid   cartilage  ;   ctm,   crico-thyroid 

muscle  ;  cli,  vocal  cords. 
Fig.  207.— Back   view  of   larynx,     ar.m.  arytsenoid  muscle  with  the 

oblique  fibres  which  pass  round  to  join  the  arytseno-epiglottidean 

muscles  ;  ca-p.  posterior  crico-arytsenoid  muscles. 


The  lateral  crico-arytcenoid  runs  upwards  and  backwards  from  the  middle 
third  of  the  upper  border  of  the  cricoid  cartilage  to  the  anterior  margin  of  the 
base  of  the  arytsenoid  cartilage. 

The  arytcenoid  is  a  single  flat  muscle  running  horizontally  between  the 
posterior  borders  of  the  two  arytsenoid  cartilages. 

The  thyro-arytanoid  muscles  run  from  the  internal  surface  of  the  thyroid 
cartilage,  close  to  the  angle,  backwards,  to  be  inserted  into  the  lateral 
border  of  the  arytsenoid  cartilage,  the  most  internal  fibres  being  contained 
in  the  vocal  cords,  and  inserted  into  the  processus  vccalis  of  the  arytsenoid. 

The  false  vocal  cords  are  two  ridges  of  mucous  membrane,  lying  over  and 
parallel  to  the  true  vocal  cords,  and  separated  from  them  by  a  lateral  recess 
known  as  the  ventricle  of  the  larynx. 

During  ordinary  respiration  the  glottis  or  opening  between 
the  vocal  cords  remains  about  half  open,  being  rhythmically 
widened  with  every  inspiration.  For  the  production  of  voice, 
the  free  borders  of  the  vocal  cords  must  be  brought  so  near 


430  PHYSIOLOGY 

to  one  another  that  they  almost  touch,  and  a  narrow  chhik 
with  parallel  sides  is  formed.  At  the  same  time  they  must 
be  more  or  less  tense,  according  to  the  pitch  of  the  note  to 
be  produced.  Both  these  changes  are  effected  by  the  agency 
of  muscles,  the  narrowing  of  the  glottis  by  the  contraction 
of  the  lateral  crico-arytsenoid  muscles  acting  in  conjunction 
with  the  posterior  arytenoid  and  the  external  thyro-arytse- 
noid  muscles.  The  tightening  is  brought  about  by  the  con- 
traction of  the  crico-thyroid.  This  muscle  raises  the  anterior 
part  of  the  cricoid  cartilage  so  that  its  upper  part,  to  which 
the  arytaenoids  are  attached,  is  carried  backwards,  and  thus 
the  vocal  cords  are  put  on  the  stretch.  At  the  same  time 
the  tension  may  be  regulated  by  the  contraction  of  the 
internal  thyro-aryt?enoid  muscles. 

The  pitch  of  the  tone  depends  on — ■ 

1.  The  length  of  the  vocal  cords.  It  is  well  known  that 
the  pitch  of  a  stretched  string  varies  inversely  as  its  length. 
A  piece  of  catgut  one  foot  long  will,  on  being  struck,  give  a 
note  an  octave  higher  than  a  similar  piece  two  feet  long,  if 
both  are  under  the  same  tension.  Comparative  measurements 
show  that  the  vocal  cords  of  men  are  one  and  a  half  times 
the  length  of  those  of  women,  and  this  explains  the  difference 
in  the  pitch  of  their  voices.  Among  men  those  with  a  tenor 
voice  have  shorter  vocal  cords  than  those  with  a  bass  one. 

2.  Tension  of  the  vocal  cords,  which  is  modulated  by  the 
degree  of  contraction  of  the  crico-thyroid  muscle. 

The  intensity  or  loudness  of  the  voice  depends  on  the 
strength  of  the  expiratory  blast  of  air,  since  the  more 
powerful  this  the  greater  the  amplitude  of  the  vibrations  of 
the  vocal  cords  produced. 

The  changes  in  the  glottis  accompanying  phonation  are  best  studied  with 
the  aid  of  the  laryngoscope.  This  consists  essentially  of  two  mirrors ;  the 
larger,  which  has  a  small  hole  in  the  middle,  is  fastened  on  to  a  spectacle 
frame,  which  the  observer  wears.  This  mirror  is  used  to  reflect  a  powerful 
Ijo-ht  into  the  back  of  the  pharynx  of  the  person  to  be  observed.  A  small 
round  mirror  about  half  an  inch  in  diameter,  mounted  on  a  handle,  is  then 
introduced  into  the  pharynx  until  it  is  directly  over  the  opening  of  the  larynx. 
The  observer  then  sees  in  the  small  mirror  a  reflected  image  of  the  epiglottis 
and  arytseno-epiglottidean  folds,  with  the  true  and  false  vocal  cords  lying 
between  them. 

The  human  voice  extends  to  about  3i  octaves,  although 
it  is  rare  to  find  any  individual  compass  extending  over  two 


RESPIKATION  431 

octaves.  In  men  two  kinds  of  voice  can  be  distinguished — 
the  chest  register  and  the  head  register  or  falsetto.  In  the 
latter  form  of  voice  it  is  said  that  the  vocal  cords  are  wider 
apart,  and  that  only  their  innermost  margins  are  set  into 
vibration  by  the  current  of  air. 

Speech. — Articulate  speech  is  produced,  not  in  the  larynx, 
but  in  the  mouth  and  pharynx.  If  these  parts  alone  are 
called  into  play,  the  expiratory  blast  of  air  is  so  modified  as 
to  give  rise  to  whispering  speech  ;  if  at  the  same  time  there 
is  a  production  of  voice  in  the  larynx,  ordinary  speech  is 
the  result. 

The  sounds  that  take  their  origin  in  this  way  are  divided 
into  vowel  sounds  and  consonants. 

The  voivels  a,  e,  i,  o,  u  (pronounced  ah,  eh,  ee,  o,  oo)  are 
tones,  i.e.  are  produced  by  a  regular  series  of  vibrations. 

The  special  characters  of  each  vowel  sound  were  shown 
by  Helmholtz  to  be  due  to  the  combination  of  overtones, 
which  is  different  for  each  vowel.  This  was  determined  in 
the  following  way  :  —A  person  was  made  to  tone  the  vowel 
sounds  on  one  particular  note,  and  by  means  of  resonators 
the  component  vibrations  of  each  vowel  sound  were  analysed. 
These  being  fomid,  the  experiment  was  completed  by  reform- 
ing the  vowel  soimds  synthetically  by  means  of  tunmg-forks 
arranged  to  vibrate  at  the  same  notes  as  the  notes  of  the 
resonators  that  picked  out  the  sounds  in  the  first  experi- 
ment. 

Thus  if  h  be  taken  for  the  fundamental  tone, 

b  +  b,  =  u(ooj. 

b  +  b,  (loud)  +  f,  (soft)  =  0. 

b  +  b,  and  f^  (moderately  loud),  f,  (louder),  l^,  Rj  and  bj  (loud)  =  e  (eh). 

i  =  ee  could  not  be  reproduced,  since  its  higher  overtones 
could  not  be  artificially  represented  by  means  of  tuning- 
forks. 

The  difference  in  the  overtones  accompanying  the  fun- 
damental tone,  and  therefore  in  the  vowel  soimds,  is  brought 
about  by  changes  in  the  shape  of  the  cavity  of  the  mouth 
and  pharynx  (Fig.  207).  When  o  and  ou  are  sounded,  the 
mouth-cavity  has  the  shape  of  a  flask  without  a  neck,  the 
opening  being  situated  at  the  mouth.  The  opening  is  narrow 
when  00  is  sounded,  wider  with  o. 


432 


PHYSIOLOGY 


When  a  {ah)  is  sounded,  the  mouth-cavity  assumes  a  wide 
conical  form,  the  widest  part  of  the  cone  being  at  the  mouth. 
With  e  {eh)  and  i  {ee)  the  cavity  assumes  the  form  of  a  flask 
with  a  long  narrow  neck  which  is  formed  by  the  raising  of 
the  tongue,  leaving  a  narrow  canal  between  this  organ  and 
the  hard  palate. 

These  changes  can  be  observed  roughly  by  any  one  on 
himself  if  he  intones  oo,  and  then  gradually  changes  the 
sound  to  0,  ah,  e,  i,  directing  close  attention  to  the  changes 
that  he  is  making  in  his  mouth. 

The  vowel  sounds,  we  may  conclude,  are  brought  about  by 
variations  in  the  shape  of  the  cavity  of  the  mouth  and  pharynx, 
which  alter  the  quality  of  the  tone  produced  in  the  larynx 


Fi«.  207. 


A  (ah)  U  (oo)  I  (ce) 

Shape  of  the  oral  cavity  in  the  production  of  the  vowel  sounds,  A,  U,  1 
(Griitzner). 

by  intensifying  some  and  suppressing  other  harmonics  or 
overtones. 

Diphthongs  are  produced  by  changing  the  form  of  the 
mouth-cavity  from  that  of  one  vowel-sound  to  the  other, 
so  that  one  sound  follows  directly  after  the  other  ;  thus 
ai  =  ah-ee  run  together  and  abbreviated.  Consonants  are 
sounds  produced  by  a  sudden  check  being  placed  in  the 
course  of  the  expiratory  blast  of  air  by  closure  of  some  part 
of  the  pharynx  or  mouth.  They  are  classified  into  labials, 
dentals,  or  gutturals,  according  as  the  check  takes  place  at 
the  lips,  between  teeth  and  tongue,  or  between  back  of  tongue 
and  soft  palate. 

In  the  production  of  nasal  sounds,  such  as  m,  n,  or  ng, 


RESPIKATION  433 

the  mechanism  is  the  same  as  for  the  production  of  h,  d,  and 
g,  except  that  the  posterior  opening  of  the  nares  is  not 
kept  shut  by  the  soft  palate,  so  that  part  of  the  sound  comes 
continually  through  the  nasal  passages,  when  it  acquires 
a  peculiar  resonance.  These  sounds  are  on  this  account 
often  spoken  of  as  resonants.  The  aspirates  are  produced 
by  the  passage  of  a  simple  blast  of  air  through  a  narrow 
opening  which  may  be  at  the  throat  as  in  li,  between 
tongue  and  teeth  as  in  tli,  or  between  lips  and  teeth  as  in 
ph  or  /. 


28 


434  PHYSIOLOGY 


CHAPTER   X 
EXCRETION— FUNCTIONS    OF    THE    KIDNEYS    AND    SKIN 

Section  1 

THE   UEINAEY   CONSTITUENTS    AND    THEIR   OEIGIN 
IN   THE   BODY 

The  consideration  of  the  lungs,  which  are  organs  engaged 
at  the  same  time  in  absorption  and  excretion,  leads  us  on 
naturally  to  those  organs  by  which  the  remaining  waste 
products  of  the  organisms  are  eliminated,  i.e.  the  kidneys 
and  skin. 

The  main  work  of  the  kidneys  is  the  excretion  of  urea, 
the  product  of  the  nitrogenous  waste  of  the  hodj.  This  is 
turned  out  dissolved  in  water,  together  with  certain  other 
nitrogenous  extractives,  salts,  and  water,  which  together  make 
up  the  urine. 

Human  urine  in  a  fresh  condition  is  a  clear  yellow  fluid, 
with  characteristic  odour  and  sour  reaction.  Its  specific 
gravity  is  on  the  average  1016 — 1020.  It  is  free  from 
organised  elements.  An  average  man  of  66  kilos  weight 
passes  in  twenty-four  hours  about  1,500  grms.  of  urine. 
This  contains  about  73  grms.  of  solids,  which  are  made  up 
as  follows : 

Urea 33    grms. 

Uric  acid        .......  0'5  „ 

Hippuric  acid 0*4  ,, 

Creatinine      . 0'9  „ 

Pigment  and  other  substances       .         .         .  10  „ 

Sulphuric  acid 2  ,, 

Phosphoric  acid 3  ,, 

Chlorine 7  „ 

Potassium 25  „ 

Sodium  .......  11  „ 

Ammonia 0*7  „ 

Calcium ) 

Magnesium 1  " 


FUNCTIONS  OF  THE  KIDNEYS  AND   SKiN  435 

It  also  contains  about  15  volumes  per  cent,  of  gas, 
consisting  chiefly  of  carbon  dioxide,  with  a  small  amount  of 
nitrogen. 

The  acidity  of  the  urine  is  due  to  the  presence  of  acid 
sodium  phosphate,  and  is  equivalent  to  about  2  grms.  of 
oxalic  acid  in  twenty-four  hours.  While  active  digestion  is 
going  on,  the  urine  may  be  for  a  while  alkaline,  owing  to  the 
secretion  of  hydrochloric  acid  by  the  stomach.  The  reaction 
varies  with  the  nature  of  the  food.  In  herbivora  the  urine 
is  normally  alkaline,  becoming  acid  only  when  they  have  had 
no  food.  In  this  condition  their  metabolism  is  going  on  at 
the  expense  of  their  own  bodies,  so  that  they  may  be  regarded 
as  carnivorous  for  the  time  being.  The  acidity  of  the  urine 
diminishes  on  standing,  and  if  the  fluid  be  exposed  to  the  air, 
a  development  of  micro-organisms  {Micrococcus  urecB)  takes 
place.  By  their  agency  the  urea  is  combined  with  two  mole- 
cules of  water  to  form  ammonium  carbonate,  and  the  urine 
becomes  strongly  alkaline  and  ammoniacal. 


Urea 

Of  these  constituents,  urea  or  carbamide  is  by  far  the 
most  important,  since  the  greater  bulk  of  the  nitrogen 
produced  by  the  disintegration  of  proteins  leaves  the  body 
in  the  form  of  urea. 

It  may  be  prepared  from  urine  in  the  following  way  : — The  urine  is  evapo- 
rated to  a  small  bulk  (^),  and  strong  pure  nitric  acid  is  added  in  excess, 
keeping  the  mixture  cool.  Crystals  of  urea  nitrate  are  deposited.  These 
are  collected  and  dried  between  filter-paper,  and  made  into  a  paste  with  a 
large  quantity  of  barium  carbonate  and  spirit.  The  paste  is  dried  in  a 
water-bath  and  extracted  with  alcohol.  The  alcoholic  extract  is  filtered  off 
and  allowed  to  evaporate,  when  crystals  of  urea  separate  out.  These  may  be 
redissolved,  decolorised  by  animal  charcoal,  and  allowed  to  crystallise  out  once 
more. 

Urea  crystallises  in  four-sided  prisms.  It  is  odourless 
and  colourless,  readily  soluble  in  alcohol  and  water.  With 
nitric  acid  it  forms  nitrate  of  urea,  which  crystallises  in 
octahedra  (Fig.  208).  It  also  forms  typical  insoluble  crystals 
with  oxalic  acid  (Fig.  209). 

On  heatmg  it  melts  and  decomposes,  giving  oft"  ammonia 
and  forming  biuret. 


436 


PHYSIOLOGY 


The  reaction  may  be  represented  as  follows 


/NH., 

\nh., 


CO 


/ 


NH., 


CO 


\nh 
\nh. 


On  further  heating  cyanuric  acid  C3H3N3O3  is  formed. 

Urea  may  be  formed  from  ammonium  cyanate,  with  which 
it  is  isomeric,  by  simply  heating  with  water. 

On  exposure  to  the  air  urine  becomes  strongly  alkaline, 
and  smells  of  ammonia.  At  the  same  time  the  earthy  phos- 
phates are  precipitated.  This  change  (which  may  be  prevented 
by  boiling  the  urine  in  a  flask,  the  neck  of  which  is  closed 
by  cotton  wool)  is  due  to  infection  of  the  urine  by  a  micro- 
organism,  Micrococcus   urecB.     Under  the  influence  of  this 


Fig.  209. 


Urea  nitrate. 


Urea  oxalate. 


organism  the  urea  undergoes  hydration,  taking  up  two  mole- 
cules of  water  and  being  converted  into  ammonium  carbonate. 

/NH.,  /ONH^ 

CO;      "+2H„o  =  coC 

\NH, 


Urea 


\onh/ 


Ammoniuui  carbonate 


On  treatment  with  an  alkaline  hypobromite  it  is  de- 
composed with  the  formation  of  free  nitrogen  and  carbon 
dioxide : 

CO(NH.,).,-H3NaBrO  =  C0,  +  N,  +  3NaBr  4-2H,0. 

This  reaction  is  taken  advantage  of  in  the  quantitative 
estimation  of  urea.  5  c.c.  of  urine  are  treated  in  a  closed 
vessel  with  about  20  c.c.  of  alkaline  sodium  hypobromite 
solution.  The  CO2  produced  is  dissolved  by  the  excess  of  alkali 
present,  and  the  nitrogen  is  collected  in  a  graduated  cylinder 


FUNCTIONS   OF   THE   KIDNEYS  AND   SKIN  437 

over  water.  From  the  amount  of  nitrogen  given  off,  the 
amomit  of  urea  present  in  the  urine  may  be  calculated. 
35'4  c.c.  of  nitrogen  correspond  to  1  decigram  of  urea. 

Since  urea  is  the  end-product  of  the  metabolism  of  the 
proteins  taken  in  with  the  food,  whether  these  have  been 
built  up  to  form  constituent  parts  of  the  living  cells  of  the 
organism  or  have  been  broken  down  at  once  on  their  entry 
into  the  body,  the  amount  of  it  excreted  in  the  day  is  an 
index  to  the  activity  of  the  protein  metabolism.  Hence  it  is 
increased  by  a  large  protein  diet  ;  as  well  as  under  conditions 
such  as  fevers,  when  a  rapid  disintegration  of  the  tissues 
is  going  on.  It  is  moreover  increased  by  administration  of 
nitrogenous  extractives,  such  as  glycine  or  leucine,  or  of 
combinations  of  ammonia  with  carbon  dioxide  or  vegetable 
acids,  or  of  large  quantities  of  water. 

Origin  of  Urea 

Urea  is  not  formed  in  the  kidneys.  If  the  kidneys  of  an 
animal  be  extirpated,  urea  accumulates  in  all  the  tissues  and 
organs  of  the  body,  in  which  it  is  found  at  death  in  large 
quantities.  Circulating  blood  constantly  contains  a  small 
proportion  of  urea,  and  the  kidney-cells  merely  take  up  this 
urea  and  turn  it  out  into  the  urinary  tubule.  So  we  have  to 
inquire  what  are  the  immediate  precursors  of  urea,  and  in 
what  organ  or  organs  their  transmutation  into  this  body  is 
effected. 

Some  clue  in  our  investigation  of  this  question  is  furnished 
by  the  results  obtained  in  the  experiments  on  the  disintegra- 
tion of  proteins  described  in  Chapter  II.  We  saw  there  that 
a  protein,  on  hydrolytic  dissociation,  broke  up  into  a  large 
number  of  bodies,  belonging  to  the  following  classes : 

1.  Amino-acids,  such  as  leucine,  tyrosine,  aspartic  and 
glutamic  acids. 

2.  Ammonia. 

3.  Diamino-acids  and  bases,  such  as  lysine,  arginine,  and 
histidine. 

We  have  to  inquire  whether  any  of  these  or  allied  sub- 
stances occur  as  stages  in  the  normal  metabolic  processes  of 
the  body  and  by  what  chemical  processes  they  are  converted 
into  the  end-product,  urea.     In   the    case   of   one   of   these 


438  PHYSIOLOGY 

bodies,  arginine,  the  derivation  of  urea  is  extremely  simple, 

since  mere  boiling  with  baryta  water  serves  to  split  off  part 

of  the  molecule  in  the  form  of  urea,  according  to  the  following 

equation  : 

NH.  NH,,. 

V(C5H„N„0.,)  +  H.,0  =  >C0  +  C3H„(NH,),p.. 

NH./  ■    "  '         NH,/ 

Arsriuine  "IJrea  Diaiiuno-vnleriaiiic  acid 

This  mode  of  origin  will  however  account  at  the  most 
only  for  one-ninth  of  the  total  protein.  The  other  eight- 
ninths  of  the  urea  must  be  obtained  from  the  other  decom- 
position products,  especially  from  the  amino-acids.  Amino- 
acids  are  known  to  be  formed  in  the  intestine  in  the 
pancreatic  digestion  of  proteins,  and  are  also  formed  in  the 
ordinary  metabolic  processes  of  the  body.  We  may  instance 
glycine  and  taurine,  which  are  manufactured  by  the  liver 
and  are  combined  with  cholalic  acid  to  form  the  bile  acids. 
It  is  impossible  however  to  conceive  chemically  the  direct 
conversion  of  these  amino-acids  into  urea.  These  bodies  all 
contain  a  much  larger  proportion  of  carbon  to  nitrogen  than 
does  urea,  and  we  must  therefore  assume  that  the  first  stage 
in  the  change  is  one  of  oxidation.  The  carbon-holding  part 
of  the  molecule  is  oxidised  to  CO,,  and  part  of  this  CO^ 
unites  with  the  ammonia  of  the  amino-radical  to  form 
ammonium  carbonate,  from  which,  by  a  simple  process  of 
dehydration,  carbamide  or  urea  may  be  formed. 

CH.,.NH.,  /O.NH, 

(1)  2  I     "        '  +  30.,  =  C0(  +  SCO.,  +  H.O. 

CO.OH  '  \O.NH, 

Glycine  Amuioiiinni  carbonate 

/O.NHj  /NH., 

(2)  C0(  -  2H.,0  =  C0( 

\O.NH^  "  ^NH, 

Carbamide  oi'  area 

If  ammonium  carbonate  be  administered  to  an  animal,  no 
increase  is  found  in  the  ammonia  of  the  urine,  but  a  large 
increase  in  the  urea,  showing  that  the  ammonium  carbonate 
has  been  converted  into  urea. 

Where  does  this  conversion  take  place  ? 

If  defibrinated  blood  mixed  with  ammonium  carbonate  be 
passed  through  the  blood-vessels  of  a  recently  excised-mam- 
pialian  liver,  the  urea  in  the  blood  is  found  to  be  increased 
200  or  300  per  cent.,  and  there  is  a  corresponding  decrease 


FUNCTIONS   OF   THE   KIDNEYS   AND   SKIN  439 

in  the  amount  of  ammonium  carbonate  in  the  blood.  The 
same  result,  viz.  a  formation  of  urea,  is  obtained  if  amino- 
acids,  such  as  leucine  or  glycine,  be  added  to  the  blood  that  is 
perfused  through  the  liver.  We  see  therefore  that  the  liver 
has  the  power  of  carrying  out  the  series  of  oxidations  and 
dehydrations  described  in  the  previous  paragraphs,  which 
result  in  the  formation  of  urea. 

Many  experimental  facts  point  to  the  conclusion  that  this 
power  is  normally  exercised  during  life,  and  is  indeed  one  of 
the  chief  functions  of  the  liver.  Comparative  analyses  of  the 
blood  of  the  portal  vein  and  of  the  carotid  artery  or  hepatic 
vein  show  a  marked  difference  in  the  ammonia  content  of  the 
blood.  Whereas  carotid  blood  contains  only  2-3  mg.  of  NH3 
in  100  c.c,  portal  blood  contains  4-6  or  during  digestion 
even  8  mg.  per  100  c.c.  If  by  any  means  the  proportion  of 
ammonia  in  carotid  blood  is  raised  to  that  in  portal  blood, 
the  animal  shows  symptoms  of  poisoning,  being  affected 
with  convulsions  or  coma,  which  may  end  in  death.  The 
liver  therefore  is  constantly  engaged  in  removing  ammonia 
from  the  blood  and  converting  it  into  an  innocuous  sub- 
stance (urea). 

The  decisive  experiment  of  extirpation  of  the  liver  pre- 
sents in  mammals  considerable  difficulties.  Since  all  the 
blood  from  the  alimentary  canal  passes  through  the  liver, 
extirpation  of  this  organ  involves  enormous  venous  conges- 
tion of  all  the  portal  area.  The  wall  of  the  intestines 
becomes  thickened  and  black  from  effused  blood,  the  blood 
in  the  general  circulation  becomes  concentrated  and  the 
animal  dies  within  a  few  hours.  It  is  evident  that  this 
difficulty  might  be  overcome  if  we  could  in  any  way  make 
an  opening  between  the  portal  vein  and  inferior  vena  cava, 
so  that  the  liver  might  be  excised  without  interfering  with 
the  circulation  through  the  gut.  This  difficult  operation 
has  been  carried  out  by  Pawlow,  though  previously  suggested 
by  Eck,  a  Kussian  surgeon.  In  a  number  of  animals  in 
which  this  operation  had  been  performed,  the  portal  vein 
was  ligatured  so  that  the  liver  was  supplied  with  blood  only 
through  the  hepatic  artery.  In  this  case  the  portal  blood 
had  to  pass  through  the  general  circulation  before  arriving 
at  the  liver,  and  it  was  therefore  found  that  any  increase  of 
nitrogenous  metabolism,  such  as  that  caused  by  a  protein 


440  PHYSIOLOGY 

meal,  brought  about  symptoms  of  ammonia>Ania,  the  convul- 
sions and  coma  mentioned  above.  On  a  farinaceous  diet,  the 
animals  (dogs)  could  be  kept  alive  a  considerable  time,  and  it 
was  observed  that  the  dogs  themselves  soon  recognised  the 
evil  effects  of  a  meat  diet  and  changed  their  tastes  in  conse- 
quence. In  animals  suffering  from  these  symptoms,  the  urea 
in  the  urine  was  somewhat  diminished,  its  place  being  taken 
by  ammonia.  A  large  proportion  of  this  substance  was 
present  in  the  form  of  ammonium  carbamate.  This  fact 
points  to  the  possibility  that  the  combination  of  ammonia 
and  carbon  dioxide  in  the  tissues  results  in  the  formation 
of  this  salt  rather  than  of  the  carbonate,  and  that  the 
ammonium  carbamate  is  therefore  the  immediate  precursor 
of  urea.  In  dogs  with  an  Eck's  fistula  and  the  portal  vein 
ligatured,  obstruction  of  the  hepatic  artery  brings  about 
death  within  twelve  to  twenty-four  hours.  During  this  time 
a  small  amount  of  urine  may  be  secreted,  containing  a  fair 
proportion  of  urea,  a  fact  which  has  been  interpreted  as 
showing  that  other  tissues  besides  the  liver  take  part  in  the 
formation  of  urea. 

The  importance  of  the  liver  for  the  transformation  of  the 
amino-acids  is  illustrated  by  the  fact  that  in  acute  yellow 
atrophy  of  the  liver,  as  well  as  in  the  similar  condition 
brought  about  by  the  administration  of  phosphorus,  the 
urea  may  disappear  from  the  urine,  its  place  being  taken  by 
leucine,  tyrosine,  and  other  amino-acids,  as  well  as  ammonia. 

Confirmatory  evidence  on  this  subject  is  furnished  by 
experiments  on  birds.  In  this  class  there  is  no  need  to  per- 
form Eck's  fistula,  since  there  is  normally  a  communication 
(the  vein  of  Jacobson)  between  the  portal  vein  and  the  renal 
veins,  and  so  with  the  vena  cava  (Fig.  210).  Hence  they 
may  survive  the  operation  of  extirpation  of  the  liver  for 
several  days,  during  which  time  the  kidneys  continue  to 
perform  their  normal  functions.  Unfortunately  for  our 
present  question,  in  birds  the  greater  part  of  the  nitrogen 
is  excreted  as  uric  acid,  and  not  as  urea.  Extirpation  of  the 
liver  causes  an  almost  total  disappearance  of  the  uric  acid 
in  the  urine,  and  a  corresponding  appearance  of  ammonia. 
In  healthy  geese,  for  instance,  the  nitrogen  eliminated  as 
uric  acid  amounts  to  from  60  to  70  per  cent.,  and  as 
ammonia  to  from   9  to   18  per  cent,  of  the  total  nitrogen. 


FUNCTIONS   OF   THE   KIDNEYS   AND   SKIN 


441 


After  removal  of  the  liver  the  uric  acid  nitrogen  represents 
only  3  to  6  per  cent.,  and  the  ammonia  50  to  60  per  cent. 
These  figures  show  clearly  that  in  birds  ammonia  is  a  pre- 
cursor of  uric  acid,  and  that  the  presence  of  the  liver  is 
essential  for  its  conversion  into  this  substance. 

It  is  interesting  to  notice  that  under  such  conditions  a 
large  quantity  of  lactic  acid  occurs  in  the  urine  combined 
with  the  ammonia  as  ammonium  lactate — a  fact  not  without 
significance  in  view  of  the  artificial  synthesis  of  uric  acid 
from  trichlorlactic  acid  and  urea. 


Iliac  vein- 
Kidney 


Fig.  210. 
Inf.  Vena  Cava 


V.  of  Jacobson 
Inf.  mes.  V. 

Caudal  v.  — 


Poctal  V. 


Diagram  to  show  the  arrangement  of  the  veins  in  the  bird,  with 
the  communication  of  the  renal  and  portal  veins.  (After 
Morat.) 

Another  important  precursor  of  urea  is  represented  by 
creatine.  This  nitrogenous  substance  occurs  in  the  organism 
in  far  larger  quantities  than  any  other  extractive.  The  body 
of  a  normal-sized  man  contains  about  90  grms.,  chiefly  in  the 
muscles. 


Creatine  can  be  prepared  by  treating  Liebig's  extract  of  meat  with  baryta  water 
to  precipitate  phosphates,  removing  the  excess  of  baryta  with  COo,  and  then 
concentrating  the  filtrate  on  a  water-bath  to  a  thick  syrup.  In  a  few  days  a 
crystalline  deposit  of  creatine  is  formed.  The  crystals  are  transparent  colourless 
prisms,  soluble  in  water,  almost  insoluble  in  absolute  alcohol.     On  heating  with 


442  PHYSIOLOGY 

dilute  mineral  acids  creatine  loses  a  molecule  of  water  and  is  converted  into 
creatinine.     Thus 

NH^  /CH3  NH5.  /CH3 

NH,/  ^CH.,  NH/  \CH. 

I  -        I     " 

COOH  CO 

On  boiling  creatine  with  baryta  water  it  is  split  up  into 
urea  and  sarcosine  — an  amino-acid,  according  to  the  following 
equation : 


NH^ 


.CH3  NH,.  .CH3 


;C— N<  +H.0=         >CO  +  H— N< 

NH/  \CH,.COOH         "       NH/  \CH,.COOH 

Urea  Sarcosine 

and  it  is  conceivable  that  a  similar  decomposition  takes 
place  in  the  muscles,  the  sarcosine  passing  on  to  the  liver, 
and  there  being  converted  into  ammonium  carbonate  and 
then  into  urea.  The  following  fact  seems  at  first  against 
this  view.  Creatine  taken  with  the  food  or  injected  into  the 
blood  calls  forth  no  increase  of  urea  in  the  urine,  the  whole 
quantity  being  excreted  in  the  urine  as  creatine  or  creatinine. 
We  must  conclude  from  this  experiment  that  the  creatine 
does  not  leave  the  muscle  as  such,  but  that  it  is  broken 
down  in  the  muscle  into  urea  and  an  amino-acid,  or  still 
further  oxidised  into  ammonia  and  carbonic  acid,  which  are 
then  turned  out  into  the  lymph  and  blood -stream. 

Direct  experiments  by  Schondorff  seem  to  point  to  a  formation  of  ammonia 
in  muscles  during  activity,  and  we  know  that  C0._,  is  the  most  prominent 
product  of  muscular  metabolism.  But  in  these  experiments,  which  consisted 
in  the  estimation  of  the  ammonia  in  detibrinated  blood  before  and  after  passage 
through  the  vessels  of  a  hind  limb,  the  muscles  of  which  were  tetanised  through 
the  nerve,  there  was  also  an  increased  formation  of  lactic  acid.  It  is  therefore 
possible  that  the  ammonia  was  the  direct  result  of  the  production  of  acid  and 
therefore  only  indirectly  due  to  the  state  of  activity  of  the  muscle.  We 
do  not  yet  know  whether  lactic  acid  is  produced  in  contraction  of  a  muscle 
under  absolutely  normal  conditions,  and  therefore  cannot  regard  Schondorff's 
experiments  as  absolute  proof  of  the  formation  of  ammonia  under  such 
normal  conditions. 

The  various  stages  in  the  formation  of  urea  from  protein 
may  be  roughly  represented  by  the  following  schema  : 


FUNCTIONS   OF   THE   KIDNEYS   AND   SKIN  443 

Protein 
hy  hydrolysis  breaks  up  into 

Ammonia  Amino-acids  '  Hexone  bases  '  ' 


Urea 


Alternative  process  ? 


Protein 

I    . 
Creatine  {mode  of  conversion 

unknown) 


By  oxidation) 
\ 

1 

Ammonia 
By  dehydration-. 

1 
and  CO, 

s8 


■  CO 
O 

53:) 


555 


Sarcosine        Urea 
Oxidation  | 

NH,  and  CO., 

Dehydration        \ 

Urea 

'  Nothing  is  known  as  to  the  fate  of  these  substances  in  the  body,  though  it 
is  highly  probable  that  they,  as  well  as  the  residue  left  after  the  hydrolysis  of 
arginine,  undergo  changes  similar  to  those  which  have  been  proved  to  take  place 
in  the  case  of  the  simple  amino-acids. 


444  PHYSIOLOGY 


Uric  Acid 

Ukic  acid  (C.:iH^N403),  which  occurs  constantly  in  the 
urine  in  small  quantities,  is  a  weak  dibasic  acid.  It  is 
almost  insoluble  in  cold  water — a  little  more  soluble  in  hot 
water.  It  is  easily  soluble  in  alkaline  solutions  and  in  solu- 
tions of  the  alkaline  phosphates.  In  solutions  of  the  latter 
salts  a  chemical  change  takes  place,  the  uric  acid  with- 
drawing a  part  of  the  alkali  from  the  phosphate  to  form  an 
acid  urate  of  the  alkali,  which  is  more  soluble  in  water  than 
uric  acid.     Thus  : 

Na.HPO,  +  C^H.N.Oa  =  NaH^PO,  +  NaC.HgN.Oa. 

The  acid  reaction  of  the  urine  is  due  to  the  presence  of 
the  acid  sodium  phosphate.   If  the  urine  be  allowed  to  cool,  it 

Fig.  211. 


0^*^  □ 


Various  forms  of  uric  acid  crystals.     (Frey.) 

is  often  found  that  crystals  of  uric  acid  separate  out,  and 
the  urine  becomes  less  acid  or  even  alkaline.  This  is  due  to 
the  fact  that  in  cooling  the  mass-influence  of  the  uric  acid  is 
diminished.  The  phosphoric  acid  of  the  phosphate  combines 
with  the  soda  of  some  of  the  acid  sodium  urate,  forming 
disodium  phosphate  and  setting  free  uric  acid,  which  is  pre- 
cipitated. If  the  urine  be  warmed  again  to  the  temperature 
of  the  body,  the  converse  reaction  takes  place,  acid  sodium 
urate  and  acid  sodium  phosphate  being  formed,  and  the  uric 
acid  is  dissolved. 

It  may  be  prepared  from  urine  by  adding  5  c.c.  of  hydro- 
chloric acid  to  200  c.c.  of  urine,  and  allowing  the  mixture 
to  stand  for  twenty-four  hours.     Crystals  of  uric  acid  then 


FUNCTIONS   OF   THE   KIDNEYS   AND   SKIN  445 

separate  out  (Fig.  211).  These  are  generally  coloured  dark 
red  and  form  rhombic  prisms.  The  red  colour  of  the 
crystals  is  due  to  the  fact  that  they  carry  down  with  them 
part  of  the  pigment  of  the  urme.  If  the  urine  is  concen- 
trated, there  is  very  often  a  brick-coloured  precipitate  pro- 
duced on  cooling,  which  dissolves  up  again  if  the  urine  be 
warmed.  This  is  spoken  of  as  the  '  lateritious  '  deposit,  and 
consists  of  the  mixed  urates  of  potassium,  sodium,  calcium, 
and  ammonium. 

Three  classes  of  urates  are  described.  Taking  H.,U  to  stand  for  uric  acid, 
the  salts  would  be  represented  as  M.,\J  (normal  urates),  HMU  (acid  urates  or 
biurates),  and  HMLMI,_,U,  or  quadriurates.  The  biurates  are  the  only  stable  com- 
pounds. The  normal  urates,  which  are  the  most  soluble,  form  strongly  alkaline 
solutions.  The  quadriurates  are  supposed  to  form  the  greater  part  of  the 
urates  of  urine,  but  split  up  spontaneously  into  biurates  and  uric  acid.  It  is 
doubtful  whether  the  bodies  described  under  this  name  are  anything  more  than 
mixtures  of  biurates  and  uric  acid. 

Tests  for  uric  acid. — A  few  crystals  of  uric  acid  are 
warmed  with  a  little  concentrated  nitric  acid  in  a  porcelain 
capsule  until  the  nitric  acid  is  evaporated.  On  adding  a  drop 
of  ammonia  to  the  yellow  residue,  a  brilliant  purple  colour 
is  produced  (murexide).  If  potassium  or  sodium  hydrate  be 
used  the  colour  is  blue. 

Another  method  is  to  dissolve  the  substance  in  sodium 
carbonate  solution  and  to  place  a  drop  on  filter  paper  pre- 
viously moistened  with  silver  nitrate.  If  uric  acid  is  present 
a  yellow  or  black  coloration  is  produced,  due  to  the  reduction 
of  the  silver  salt.     This  is  known  as  Schiffs  test. 

Quantitative  determination. — Hopkins'  method  for  the 
estimation  of  uric  acid  is  founded  on  the  fact  that  saturation 
of  a  fluid  containing  uric  acid  or  urates  with  ammonium 
chloride  causes  complete  separation  of  all  the  uric  acid 
present  in  the  form  of  ammonium  urate.  In  applying  this 
method  the  urine  is  saturated  with  crystals  of  ammonium 
chloride,  and  a  few  drops  of  strong  ammonia  solution  are 
added.  The  precipitate  of  ammonium  urate  which  forms  is 
collected  on  a  filter,  washed  into  a  beaker,  and  boiled  with 
dilute  hydrochloric  acid.  The  urates  are  broken  up,  and  the 
uric  acid  thus  set  free  is  deposited  in  a  crystalline  form  on 
cooling.  The  precipitate  of  uric  acid  is  collected  on  a  weighed 
filter,  dried,  and  weighed. 


446  PHYSIOLOGY 

Chemical  Relationships  of  Uric  Acid A  knowledge  of  the  chemical  rela- 
tionships of  uric  acid  is  a  necessary  preliminary  to  any  investigation  of  its  mode 
of  origin  in  the  body.  We  have  already  seen  that  it  is  a  member  of  the  group 
of  purine  bodies. 

N-CH 

I         1 

The  formula  of  purine  being  CH  C— NH\ 

II  II  ^CH 

N  C  -N    '^ 

HN     CO 

I  I 

that  of  uric  acid  is  CO       C— NHv 

I         II  CO  (i.e.  trioxypurine). 

HN— C— NH/ 

It  is  apparent  from  this  formula  that  uric  acid  consists  of  a  central  three 
carbon  chain  to  which  are  attached  two  urea  groups,  and  it  is  easy  to  under- 
stand the  synthesis  of  uric  acid  by  fusing  together  trichlorlactic  acid  or 
trichlorlactamide  and  urea. 

Under  the  action  of  oxidising  agents  one  or  both  of  the  urea  groups  is 
split  off. 

Thus  nitric  acid  splits  off  the  right-hand  group,  forming  urea  and  a  body 
known  as  alloxan. 

NH— CO  NH— CO 

I  I  II 

CO       C— NH\  CO      CO       NH,.\ 

I       II         ;co  +  H„o  +  0=1       I    +        ;co. 

NH— C     NH/  NH— CO       NH,/ 

Further  oxidation  converts  the  alloxan  into  parabanic  acid 

NH— CO 


CO 


and  C0.>, 


NH— CO 

and    parabanic    acid    by    hydrolysis   is   finally    converted    into    oxalic    acid 

CO-OH 

I  and  urea. 

CO— OH 

Potassium  permanganate  on  the  other  hand  attacks  the  central  three  carbon 
chain  at  once,  forming  allantoin. 

NH— CO     NH., 


CO 


CO    and  CO... 


NH-CH-NH 

From  the  allantoin  by  processes  of  oxidation  and  hydration  both  urea  groups 
may  be  split  off  as  before. 


FUNCTIONS   OF   THE   KIDNEYS   AND   SKIN  447 

Origin  of  Uric  Acid 

We  have  already  seen  that  in  bh'ds  uric  acid  is  formed 
mainly  in  the  liver  from  ammonia  and  probably  lactic  acid. 
In  this  class  the  administration  of  amino-acids,  ammonia,  or 
urea  gives  rise  in  every  case  to  increased  output  of  uric  acid, 
and  we  have  reason  to  believe  that  the  greater  part  of  the 
transformation  is  effected  by  the  liver.  These  results  how- 
ever are  no  guide  to  the  question  of  the  origin  of  uric  acid 
in  mammals,  where  this  substance  forms  only  a  very  small 
proportion  of  the  total  nitrogen  output. 

The  first  question  we  have  to  decide  is  whether  uric  acid 
is  a  by-product  in  the  ordinary'  processes  of  nitrogenous 
metabolism  or  whether  it  is  formed  along  special  lines  from 
certain  precursors  contained  in  the  food.  If  the  former  were 
the  case,  we  should  expect  to  find  a  constant  ratio  between 
the  uric  acid  and  the  urea  in  the  urine.  Although  on  a  con- 
stant diet  such  a  ratio  does  exist  (average  about  1  to  50),  the 
ratio  is  at  once  upset  by  certain  alterations  in  the  character 
of  the  food.  Thus  the  uric  acid  output  is  very  small  on  a 
farinaceous  diet,  and  is  very  little  affected  by  adding  to  such 
a  diet  a  large  amount  of  protein  in  the  form  of  white  of  egg. 
A  (relatively)  large  increase  is  at  once  observed  if  a  full  meal 
of  meat  be  taken,  and  a  still  greater  rise  is  produced  by 
the  ingestion  of  foodstuffs  rich  in  cells,  such  as  sweetbreads 
(thymus  or  testis)  or  liver.  The  rise  of  uric  acid  output 
following  such  a  meal  is  very  rapid,  and  precedes  the  normal 
post-prandial  increase  of  urea  (Fig.  212,  p.  448). 

This  connection  of  uric  acid  excretion  with  the  ingestion 
of  cellular  organs  at  once  suggests  a  possible  mode  of  origin 
for  the  uric  acid.  We  have  already  seen  that  the  main  con- 
stituents of  undifferentiated  cells,  as  of  their  nuclei,  belong  to 
the  class  of  nucleo-proteins  and  nucleins,  and  that  these,  on 
hydrolytic  dissociation,  give  rise  to  a  series  of  bases,  xanthine, 
h3^poxanthine,  adenine,  belonging  to  the  purine  group,  i.e.  to 
the  same  class  of  bodies  as  uric  acid  itself.  We  have  at  present 
no  adequate  evidence  as  to  the  effect  of  the  administration 
of  these  bases  on  the  output  of  uric  acid  in  man.  It  seems 
probable  however  that  part  of  the  basic  residue  of  nucleo- 
proteins  may  undergo  oxidation  in  the  body  and  appear  in 
the  urine  as  uric  acid,  while  another  part  may  avoid  oxidation 


448 


PHYSIOLOGY 


and  appear  as  other  less  oxidised  members  of  the  group,  the 
so-called  alloxuric  bases  of  the  urine.  The  nucleo-proteins 
which  give  rise  to  the  uric  acid  may  be  contained  in  the  food, 
or  be  produced  in  the  disintegration  of  the  nuclei,  etc.,  of  the 
tissues  of  the  body  itself.  Corresponding  to  these  two  origins 
we  may  distinguish  the  exogenous  uric  acid,  dependent  on  the 
food,  and  an  endogenous  moiety  arising  from  the  metabolism 
of  the  tissues.  As  to  the  organ  in  which  the  formation  of 
uric  acid  is  carried  on,  we  have  very  little  evidence.  It  has 
been  found  however  that,  whereas  extract  of  fresh  spleen 
contains  a  fair  proportion  of  xanthine,  the  passage  of  oxygen 
through  the  splenic  pulp  for  some  hours  causes  a  conversion 
of  this  xanthine  into  uric  acid. 


Fig.  212. 


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Curves  showing  the  hourly  excretion  of  uric  acid  and  urea  after  a 
single  meal  (Hopkins).  The  continuous  line  =  uric  acid  output; 
the  dotted  line  =  urea  output. 

The  only  disease  in  which  there  is  a  large  increase  in  the  output  of  uric  acid 
is  leucliEemia,  which  is  generally  associated  with  enlarged  spleen  and  augmen- 
tation of  the  number  of  leucocytes  in  the  blood.  Since  the  post-prandial  rise  of 
uric  acid  is  also  often  associated  with  a  certain  amount  of  leucocytosis,  it  has  been 
suo^ested  by  Horbaczewsky  that  the  uric  acid  is  a  special  product  of  the 
metabolism  of  the  leucocytes.  Other  observers  have  however  failed  to  detect  a 
parallelism  between  the  number  of  leucocytes  and  the  uric  acid  secretion  under 
all  conditions. 

The  deposition  of  sodium  urate  in  the  joints,  which  occurs  in  gout,  seems  to 
be  due  to  a  deficient  excretion  rather  than  to  an  increased  formation  of  this 
substance.  The  whole  question  is  however  extremely  involved  and  in  need  of 
further  evidence  with  regard  to  the  origin  and  significance  of  uric  acid  in  the 
normal  organism. 


FUNCTIONS   OF   THE    KIDNEYS   AND   SKIN  449 

In  dogs  the  administration  of  uric  acid  or  of  hypoxantliine  causes  an 
increased  excretion  in  the  urine,  not  of  uric  acid  but  of  allantoine.  Admi- 
nistration of  adenine,  another  purine  base,  apparently  causes  no  alteration  at 
first  in  the  nitrogenous  constituents  of  the  urine.  The  dog  however  finally 
dies  with  suppression  of  urine,  and  it  is  found  that  the  epithelial  cells  of  the 
renal  tubules  are  desquamating  and  in  many  places  full  of  uric  acid  infarcts. 
It  seems  evident  from  these  various  facts  that  there  is  a  chapter  of  purine 
metabolism  in  the  body,  of  which  at  present  we  possess  only  a  few  disconnected 
pages. 

The  following  table  will  serve  to  represent  the  relation  of  uric  acid  to  the 
other  xanthine  or  purine  bodies. 

Uric  acide CjH^NPj. 

Xanthine CsH^N^O.,. 

Hypoxantliine  .         .         •         .  C^H^NjO. 

Adenine  ....  .  C5H5N5. 

Guanine.         ...  .  C.H.N.O. 


Other  Nitrogenous  Constituents 

Creatinine. — The  creatinine  in  the  urine  is  nearly  all 
derived  from  the  creatine  contained  in  the  meat  that  is  taken 
as  food.'  It  does  not  however  disappear  in  hmiger,  so  that 
a  certain  amount  must  arise  in  the  metabolic  processes  of  the 
body  itself.  It  may  be  that  this  quantity  merely  represents 
a  small  percentage  of  creatine  which  has  escaped  further 
decomposition  in  the  muscles. 

HiPPURic  Acid. —  This  is  only  present  in  small  quantities 
in  human  urine.  The  large  amount  found  in  the  urine  of 
herbivora  is  due  to  the  fact  that  in  their  food  are  bodies 
belonging  to  the  aromatic  group — the  benzoic  acid  series. 
If  benzoic  acid  be  administered  to  a  man,  it  is  excreted  in 
the  urine  as  hippuric  acid,  which  is  a  combination  of  glycine 
with  benzoic  acid,  with  the  elimination  of  a  molecule  of 
water : 

CH.,.NH..    CH,NH-CO.C,H, 
C,H  — COOH  +   I     "        ■=  I     ■  +  H..0. 

Benzoic  acid  COOH         CO.OH 

Glycine  Hiiipuric  acid 

This  synthesis  is  effected  by  the  living  cells  of  the  kidney. 
If  defibrinated  blood  containing  benzoic  acid  and  glycine  be 

'  According  to  Folin,  however,  all  the  creatinine  of  the  urine  is  derived  from 
the  metabolic  processes  of  the  body,  any  creatine  contained  in  the  diet  being 
oxidised,  or  if  in  excess,  appearing  in  the  urine  unaltered  as  creatine. 

29 


450  PHYSIOLOGY 

passed  through  the  vessels  of  the  kidney  for  some  time,  it 
will  be  found  that  their  place  has  been  taken  by  hippuric 
acid.  In  the  same  way,  a  small  amount  of  hippuric  acid  is 
formed  if  the  kidney  be  chopped  up  and  mixed  with  blood 
containing  benzoic  acid  and  glycine.  If  the  kidney  cells  be 
first  killed  by  exposure  to  a  temperature  of  65°,  or  to  the 
action  of  alcohol,  no  hippuric  acid  is  formed,  showing  that 
this  synthesis  is  not  effected  by  a  mere  ferment  action,  but 
is  intimately  dependent  on  the  life  of  the  cell. 

Ammonia. — Although  this  constituent  occurs  in  normal 
human  urine  only  to  the  extent  of  about  0-7  gram  2^er 
diem,  it  is  of  considerable  importance,  since  it  is  a  measure 
of  the  formation  or  ingestion  of  acids  by  the  body.  If 
ammonium  chloride  be  given  to  a  fed  rabbit,  it  reacts  with 
the  alkaline  carbonates  derived  from  the  vegetable  food  to 
form  ammonium  carbonate,  and  this  is  converted  into  urea, 
and  appears  as  such  in  the  urine.  If,  on  the  other  hand, 
ammonium  chloride  be  given  to  a  carnivorous  animal,  there 
is  no  such  store  of  alkali  to  take  up  the  hydrochloric  acid. 
The  salt  therefore  appears  in  the  urine  unchanged. 

In  the  same  way  if  dilute  mineral  acid  be  injected  into 
the  veins  of  a  dog,  the  reaction  of  the  blood  remains  normal, 
but  the  excretion  of  ammonia  in  the  urine  is  increased  in 
proportion  to  the  acid  injected.  The  tissues  defend  them- 
selves agamst  the  acid  intoxication  by  turning  out  ammonia, 
and  the  ammonia  in  the  urine  is  therefore  increased  at  the 
expense  of  the  urea. 

This  self-protective  ammonia  formation  is  less  marked  in 
herbivorous  animals.  In  these,  when  the  store  of  vegetable 
alkaline  salts  is  used,  the  acid  causes  a  lowering  of  the 
alkalinity  of  the  blood.  The  carrying  power  of  the  blood 
for  carbon  dioxide  is  therefore  diminished  (fixed  acid  having 
taken  the  place  of  CO.,  and  P.^0.,),  and  the  animal  sufi"ers 
from  a  retention  of  CO2  in  all  his  tissues,  giving  rise  to 
hyperpnoea  and  dyspnoea  and  finally  to  a  condition  of  coma 
which  may  end  fatally. 

An  analogous  condition  occurs  in  cases  of  diabetes  in 
man.  This  '  diabetic  coma '  has  been  shown  to  be  associated 
with  the  abnormal  production  of  large  quantities  of  oxy- 
butyric  and  diacetic  acids.  The  alkalinity  of  the  blood  as 
well   as   its   CO.,  contents  is  decreased,  while  the  ammonia 


FUNCTIONS   OF   THE    KIDNEYS   AND   SKIN  451 

of  the  urine  is  largel}'  increased  in  the  vain  attempt  of  the 
organism  to  saturate  the  offending  acids. 


The  Salts  of  Urine 

The  greater  part  of  the  salts  of  the  urine  is  derived 
directly  from  the  salts  taken  in  with  the  food.  The  com- 
binations of  the  vegetable  acids  with  alkalies,  e.g.  citrates 
and  tartrates,  are  oxidised  to  carbonates,  and  are  in  this 
form  eliminated  with  the  urine. 

The  pliofii-)liatef>  originate  partly  from  the  breaking  down 
of  complex  phosphorised  molecules,  such  as  lecithin,  nuclein, 
and  the  nucleo-proteins,  partly  from  the  phosphates  taken 
in  with  the  food.  When  the  urine  becomes  alkaline,  the 
calcium  and  magnesium  phosphates  are  deposited  as  an 
amorphous  precipitate.  If  the  urine  is  ammoniacal,  ammonio- 
magnesium  phosphate  m^y  be  formed  and  precipitated  in 
a  crystalline  form. 

The  sulphur  in  the  urine  arises  partly  from  the  sulphates 
of  the  food,  and  partly  from  protein  metabolism.  It  is  found 
in  the  urine  in  three  forms  : 

1.  Small  traces  of  an  unoxidised  sulphur  compound,  allied 
to  cystine. 

2.  As  simple  sulphates  of  the  alkalies. 

3.  As  conjugated  ethereal  or  aromatic  sulphates. 

These  latter  bodies  are  important  from  the  fact  that  they 
are  dependent  on  putrefactive  changes  occurring  in  the 
intestine,  so  that  their  quantity  in  the  urine  is  an  index  to 
the  extent  of  these  processes.  In  the  bacterial  putrefaction 
of  proteins,  bodies  of  the  aromatic  series,  such  as  skatol, 
indol,  and  phenol,  are  formed.  These,  after  absorption, 
unite  in  the  blood-stream  with  an  alkaline  sulphate  to  form 
conjugated  or  ethereal  sulphates,  and  as  such  are  excreted 
by  the  kidnej^s.  The  poisonous  aromatic  body  is  thus 
rendered  innocuous. 

OH  .O.C.H, 

C,H,OH  +  S0.<  =  SO.,  <  +  H.,0. 

\0K  "    \0K 

Phenol  Acid  potassium  Potassium 

sulphate  phenyl  sulphate 


452  pnvsioLOGY 

Indol  and  skatol  undergo  a  preliminary  oxidation  to 
indoxyl  and  skatoxyl  before  the  conjugation  with  sulphuric 
acid  takes  place.     Thus  : 

NH.  .NH. 

C,H,<         >CH  +  0  -  C,H,<         >COH. 

Indol  Indoxyl 

the  indoxyl  sulphate  of  potash  having  the  formula 

Indoxyl  sulphate  of  potash  is  often  spoken  of  as  indican, 
since  on  oxidation  it  yields  indigo  blue.  If  urine  containing 
this  body  be  treated  with  hydrochloric  acid  and  a  drop  of 
chlorine  water,  a  liright-blue  colour  is  produced  from  the 
formation  of  indigo. 

Urinary  Pigments 

An  exact  knowledge  of  the  pigments  of  the  urine  is 
still  wanting.  The  chief  colouring  matter  is  a  body  known 
as  urochrome.  The  chemical  relationships  of  this  body 
have  yet  to  be  discovered.  It  has  no  distinctive  absorption 
spectrum.  On  treating  however  an  alcoholic  solution  of 
urochrome  with  aldehyde,  a  pigment  is  produced  having  an 
absorption  spectrum  identical  with  that  of  urohilin,  thus 
pointing  to  a  common  origin  of  the  two  bodies.  Urobilin 
itself  does  not  exist  in  normal  urine,  which  presents  no 
absorption  bands.  On  precipitating  normal  urine  with  lead 
acetate,  and  extracting  the  precipitate  with  acid  alcohol,  a 
small  trace  of  urobilin  is  obtained.  Normal  urine  therefore 
contains  a  small  amount  of  a  chromogen,  which  on  appropriate 
treatment  may  give  rise  to  urobilin.  In  certain  diseases, 
especially  cirrhosis  of  the  liver,  the  urine  may  contain  large 
amounts  of  pre-formed  urobilin. 

Urobilin  free  from  other  pigments  may  be  extracted  from 
urine  in  the  following  way :-  The  urine  is  saturated  with 
ammonium  chloride  to  throw  down  urates  with  the  adherent 
pigment  iiroerytlirin,  then  filtered,  acidified,  and  saturated 
with  ammonium  sulphate.     This  throws  down  the  urobilin, 


FUNCTIONS   OF   THE   KIDNEYS   AND   SKIN  453 

but  instead  of  waiting  for  the  precipitate  to  form,  the  urobilin 
may  be  dissolved  out  by  shaking  the  ammonium  sulphate 
solution  with  a  mixture  of  chloroform  (one  part)  and  ether 
(two  parts).  The  solution  thus  obtained  is  yellowish-red, 
with  a  well-marked  absorption  band  in  the  blue  end  of  the 
spectrum  near  the  Fraunhofer  line  F. 

Urobilin  is  certainly  derived  from  bile  pigment,  and  there- 
fore indirectly  from  htiemoglobin.  The  stercobilin  which  is 
formed  from  bilirubin  in  the  intestines,  under  the  reducing 
action  of  putrefaction,  is  identical  with  urobilin.  Urochrome 
is  probably  a  product  of  oxidation  of  urobilin. 

Hydrohiliruhin,  which  is  formed  by  the  action  of  sodium 
amalgam  on  bilirubin,  presents  many  similarities  to  uro- 
bilin, with  which  it  was  formerly  thought  to  be  identical.  It 
contains  however  about  twice  the  proportion  of  nitrogen  that 
is  contained  in  urobilin. 


454 


PHYSIOLOGY 


Section  2 
THE   SECEETION   OP   UEINE 

The  kidney  may  be  considered  as  a  compound  tubular 
gland.  On  cutting  into  a  kidney  it  is  seen  to  consist  of  two 
parts,  a  granular  cortex,  and  a  medullary  portion  presenting 
radial  striation.  The  medulla  in  the  human  kidney  is 
divided  into  a  number  of  pyramidal  segments,  the  Malpighian 

Fig.  213. 


Section  of  human  kidney  (Cacliat).     a,  cortex  ;  h,  medulla 
or  Malpighian  pyramids  ;  c,  papilla  ;  d,  ureter  ;  c  e,  boundary  zone. 

pyramids,  each  consisting  of  a  number  of  tubules  which  open 
at  the  apex  of  the  pyramid,  the  papilla,  into  the  dilated 
extremity  of  the  duct  of  the  kidney — the  ureter.  Between 
the  cortex  and  medulla,  in  the  region  known  as  the  boundary 
zone,  are  seen  a  number  of  large  blood-vessels  cut  across. 
Small  striated  prolongations  of  the  medulla  stretch  up  from 


FUNCTIONS   OF   THE   KIDNEYS  AND   SKIN  455 

the  base  of  the  pyramids  into  the  cortex,  forming  the  medul- 
lary rays,  or  pyramids  of  Ferrein. 

The  urmary  tubule  starts  in  the  cortex  in  a  small  dilata- 
tion— the  Malpighian  capsule,  which  is  Imed  by  a  single 
layer  of  flattened  cells.  Into  this  capsule  projects  the 
glomerulus,  a  little  bunch  of  capillary  blood-vessels,  also 
covered  by  flattened  cells.  The  capsule  leads  into  the  first 
convoluted  tubule,  Imed  with  peculiar  '  rodded  '  epithelium. 
The  tubule  now  becomes  much  narrower,  and  dips  down  into 
the  medullary  pyramids  as  the  descending  loop  of  Henle, 
lined  with  flattened  hyaline  epithelium.  It  then  widens  as  it 
turns  up  again,  and  on  reaching  the  cortex  forms  the  irregular 
tubule  and  the  second  convoluted  tubule.  These  three  last- 
named   parts  are  lined  with  rodded  epithelium.     From  the 

Fig.  214. 


(.i;. 


A  iiortion  of  a  convoluted  tubule  with  '  rodded '  epithelium. 
(Heidenhain.) 


second  convoluted  tubule  a  junctional  tubule  leads  into  the 
collecting  tubule,  which  is  lined  with  hyaline  cylindrical  cells. 
There  are  thus  four  different  varieties  of  epithelial  cells 
in  the  various  parts  of  the  tubule,  i.e.  scaly  cells  in  the  Mal- 
pighian capsule,  peculiar  rodded  epithelium  in  the  convoluted 
and  irregular  tubules,  flattened  cells  in  the  descending  loop 
of  Henle,  and  ordinary  cylindrical  cells  in  the  straight  col- 
lecting tubes. 

There  are  also  certain  peculiarities  connected  with  the 
blood-supply  to  the  kidney.  The  renal  artery  breaks  up  into 
numerous  vessels  at  the  boundary  zone  between  the  pyramids 
and  the  cortex.  From  these  the  straight  interlobular  arteries 
pass  towards  the  surface,  giving  off  lateral  branches  which 
form  the  afferent  arteries  of  the  neighbouring  Malpighian 
capsules  and  break  up  in  the  glomerulus  into  a  cluster  of 
fine  capillaries.   These  unite  again  to  form  the  efferent  vessel, 


456 


PHYSIOLOGY 


which  is  only  two-thirds  the  diameter  of  the  afferent  vessel. 
The  efferent  vessel  leaves  the  glomerulus  and  breaks  up 
again  into  capillaries  which  supply  the  walls  of  the  convoluted 
tubules.  Thus  the  arrangement  of  the  portal  system  of 
vessels  is  repeated  in  the  kidney  on  a  microscopic  scale — the 
vessel  taking  the  blood  from  the  glomerulus  breaks  up  again 
into  a  system  of  capillaries,  just  as  the  portal  vein  does  in 
the  liver.  The  pyramids  are  supplied  by  branches  of  the  vasa 
recta  which  pass  inwards  from  the  arteries  in  the  boundary 
zone. 

Fig.  215. 


Cortex. 


B  mndary  zone. 


Medulla. 


Diagram  showing  course  of  urinary  tubules,  and  the  distribution 
of  the  blood-vessels  (from  Yeo). 


This  arrangement  of  blood-vessels  must  determine  a  high 
pressure  in  the  capillaries  of  the  glomerulus,  and  a  low 
pressure  in  the  vessels  supplying  the  remaining  parts  of  the 
tubule.  Since  these  capillaries  are  covered  only  by  a  thin 
layer  of  scaly  epithelium,  it  has  been  thought  that  filtration 
plays  a  great  part  in  the  secretion  of  urine,  and  that  perhaps 
the  fluid  parts  of  the  blood  are  merely  filtered  off  in  the 
capsule,  and  the  useful  constituents  of  the  filtrate,  together 
with  the   excess  of  water,  reabsorbed  in  the  tubules  where 


FUNCTIONS   OF   THE   KIDNEYS   AND   SKIN  457 

the  pressure  in  the  surroundmg  capillaries  is  low.  At  any 
rate  the  histological  differences  between  the  glomeruli  and 
tubules  indicate  that  in  the  secretion  of  urine  we  have  pro- 
bably two  distinct  mechanisms  at  work,  the  glomerular  and 
tubular.  It  will  be  convenient  to  deal  with  these  two  func- 
tions of  the  kidney  separately. 

According  to  the  theory  just  enunciated,  the  glomerular 
epithelium  acts  simply  as  a  filter  allowing  the  water  and  salts 
of  the  blood-plasma  to  pass  into  Bowman's  capsule.  Such 
a  filtration  presents  no  difficulties  from  a  mechanical  stand- 
point. If  blood-plasma  be  filtered  through  a  clay-cell,  the 
composition  of  the  filtrate  obtained  differs  very  little  from 
that  of  the  original  plasma.  If  however  we  make  the  filter- 
ing medium  denser  by  soaking  it  with  gelatin,  we  find  that 
only  the  water  and  salts  of  the  plasma  will  pass  through,  the 
whole  of  the  protein  remaining  behind.  Under  these  circum- 
stances, a  certain  limiting  pressure  is  found  at  which  no 
filtration  at  all  takes  place.  This  pressure,  which  in  the  case 
of  blood  serum  amounts  to  about  30  mm.  Hg,  represents 
the  force  necessary  to  effect  a  separation  between  the  colloid 
and  fluid  constituents  of  the  blood-plasma.  If  the  filtration 
theory  be  correct,  we  should  expect  to  find  that  a  certain 
minimal  blood -pressure  is  necessary  for  any  flow  of  urine 
to  take  place  at  all,  and  that  when  the  pressure  exceeds  this 
minimum,  the  rate  of  secretion  will  be  proportional  to  the 
blood-pressure  in  the  glomerular  capillaries,  or  at  any  rate 
to  the  difference  of  pressure  obtainmg  between  the  glomerular 
capillaries  and  the  urinary  tubule. 

The  experimental  investigation  of  this  question  shows 
that  in  many  cases  at  least  the  urinary  secretion  follows  the 
mechanical  conditions  just  laid  down.  If  the  spinal  cord  be 
divided  in  the  neck,  the  blood-pressure  falls  to  about  40  mm. 
Hg  and  the  urinary  secretion  ceases.  On  the  other  hand, 
any  procedure  which  increases  the  pressure  and  velocity  of 
the  blood  in  the  glomerular  capillaries  is  attended  with 
augmented  flow  of  urine. 

The  condition  of  the  renal  circulation  in  such  experiments 
is  tested  by  placing  the  kidney  in  an  oncometer.  With  free 
venous  outflow,  every  increase  in  volume  of  the  kidney 
denotes  increased  blood-pressure  and  blood -flow  through  that 
organ. 


458 


PHYSIOLOGY 


The  parallelism 
will  be  evident  from 


of  vascular  conditions  and  urinary  flow 
the  following  table  of  results. 


Procedure 

Geueral 
blood- 
pressure 

Reual  vessels 

Kidney 
volume 

Urinary  flow 

Division  of   spinal  cord 
in  neck 

Falls  to 
40  mm. 

Eelaxed 

Shrinks 

Ceases 

Stimulation  of  cord 

Rises 

Constricted 

Shrinks 

Diminished 

Stimulation  of  cord  after 
section  of  renal  nerves 

Eises 

Passively 
dilated 

Swells 

Increased 

Stimulation     of      renal 

Unaffected 

Constricted 

Shrinks 

Diminished 

nerves 

Stimulation  of  splanch- 

Eises 

Constricted 

Shrinks 

Diminished 

nic  nerve 

Division  of  one  splanch- 

nic nerve : 

a.  In  dog 

b.  In  rabbit    . 

Unaffected 
Falls 

Dilated 
Eelaxed 

Swells  (?) 
Shrinks  (?) 

Increased 
Diminished 

Plethora 

Eises 

Dilated 

Swells 

Increased 

Haemorrhage 

Falls 

Constricted 

Shrinks 

Diminished 

Under  all  these  circumstances,  in  which  the  kidney 
increases  in  volume,  the  amount  of  urine  excreted  by  it  is 
increased.  But  we  have  here  two  factors,  either  of  which 
may  determine  an  increased  flow  of  urine— 1st,  increased 
blood-pressure  in  the  glomerulus  ;  and  2ndly,  increased  flow 
of  blood  through  the  kidney.  It  will  be  remembered  that  the 
flow  of  lymph  from  a  limb  is  markedly  increased  by  ligature 
of  the  veins  ;  and  this  increase  is  due  chiefly  to  the  enormous 
rise  of  pressure  that  takes  place  in  the  capillaries,  larger 
than  can  be  brought  about  by  the  constriction  of  the  arterioles 
in  other  parts  of  the  body.  If  the  secretion  of  urine  were 
similarly  dependent  on  the  intra-capillary  blood -pressure,  it 
might  be  expected  that  ligature  of  the  renal  vein  would  also 
cause  an  increase  in  the  urine  excreted.  This  is  not  the  case. 
Ligature  of  the  renal  vein  entirely  stops  the  secretion  of 
urine,  and  Heidenhain  concludes  therefore  that  the  glome- 
rular secretion  involves  the  activity  of  the  endothelial  cells 
of  the  capsule,  and  is  conditioned  not  by  the  pressure  but  by 
the  velocity  of  the  blood  through  the  glomerular  capillaries. 
This  operation  of  ligature  of  the  renal  vein  is  not  however 


FUNCTIONS   OF   THE   KIDNEYS   AND   SKIN 


459 


a  clean  experiment.  In  the  first  place  the  small  veins  of  the 
kidney  run  alongside  of  the  tubules  and  when  dilated  under 
the  influence  of  venous  congestion  may  press  upon  the  latter, 
occluding  their  lumen  and  so  stopping  the  flow  mechanically. 
Moreover  ligature  of  the  renal  vein  cannot  be  regarded  as 
equivalent  to  obstruction  of  the  eft'erent  vessels  of  the  glome- 
ruli. It  will  be  remembered  that  there  is  a  very  great 
difference  as  regards  the  effects  on  the  intestmes  between 
ligature  of  the  portal  vein  and  obstruction  of  the  inferior 
vena  cava  above  the  liver.  It  is  impossible  to  inject  the 
glomerular  capillaries  either  through  the  renal  portal  capil- 

FiG.  216. 


Bowman's 
C  .>.  p  s  u  I  e 


Diagram  (after  Morat)  to  illustrate  the  effect  of  active  changes  in 
the  vasa  afferentia  and  etferentia  on  the  pressure  in  the 
glomerular  capillaries.  If  the  vas  afferens  constricts,  the 
pressure  will  be  represented  by  the  lower  dotted  line.  On 
the  other  hand,  constriction  of  the  vas  efferens  would  raise  the 
pressure  in  the  glomerulus  till  it  almost  equalled  that  in 
the  renal  artery,  as  is  shown  by  the  upper  dotted  line. 

A,  arteries ;  G,  glomerular  capillaries  ;  C,  tubular  capillaries  ; 
V.  vein. 


laries  in  the  frog  or  through  the  renal  vein  in  the  mammal, 
and  in  view  of  the  presence  of  well-developed  muscular  fibres 
in  the  vasa  afferentia,  we  have  no  proof  that  any  given  rise 
of  pressure  produced  in  the  venous  side  of  the  tubular  capil- 
laries is  transmitted  effectively  to  the  glomerular  capillaries. 
It  is  a  common  experience  to  find,  in  injecting  a  kidney 
shortly  after  the  death  of  the  animal,  that  it  is  impossible 
to  get  any  of  the  injection  into  the  glomerular  capillaries, 
although  the  injection  may  flow  freely  through  all  the  vessels 
of  the  medulla. 

The    diagram  (Fig.   216)   will   serve  to  demonstrate  the 


460  PHYSIOLOGY 

great  importance  of  contraction  or  dilatation  of  the  mus- 
cular fibres  of  vas  afferens  or  vas  efferens  for  the  blood- 
pressure  in  the  glomerular  capillaries,  and  this  factor  is 
one  which  unfortunately  it  is  impossible  to  estimate  in  any 
experiment. 

It  stands  to  reason  however  that  the  velocity  of  the  blood 
must  be  at  least  as  important  as  the  blood-pressure  in  the 
glomeruli.  At  any  given  time  there  is  only  a  small  volume 
of  blood  in  the  glomeruli,  and  if  this  were  not  renewed  the 
transudation  through  the  epithelium  would  concentrate  it  to 
such  an  extent  that  transudation  would  be  no  longer  pos- 
sible under  any  pressure  which  the  capillaries  would  stand. 
In  any  rapid  formation  of  urine  therefore,  it  is  absolutely 
essential  that  there  should  be  a  rapid  renewal  of  blood  in  the 
glomerular  capillaries  as  well  as  an  adequate  blood-pressure. 

It  is  interesting  to  note  that,  if  the  renal  veins  be  ob- 
structed for  a  short  time,  the  urine  that  is  excreted  after 
the  removal  of  the  obstruction  contains  albumen,  showing  that 
the  short  deprivation  of  oxygen  undergone  by  the  cells  has 
injured  them,  and  that  they  are  in  consequence  no  longer  able 
to  prevent  the  passage  of  the  protein  constituents  of  the  plasma. 

Functions  of  the,  tuhules. — If  we  accept  the  view  as  to  the 
simple  character  of  the  glomerular  function  (a  view  that  must 
still  be  regarded  merely  as  a  working  hypothesis),  we  must 
assume  that  the  glomerular  secretion  is  an  almost  colourless 
fluid  having  the  same  proportion  of  salts  as  the  blood-plasma, 
and  like  this  containing  only  about  0*05  per  cent,  of  urea.  On 
its  way  down  the  tubules  this  fluid  is  converted  into  urine, 
containing  2  per  cent,  of  urea,  as  well  as  salts  in  proportions 
differing  widely  from  that  found  in  the  blood-plasma.  What 
is  the  nature  of  the  process  occurring  in  the  tubules  ?  Is  it 
one  of  absorption  as  Ludwig  assumed,  or  is  it  one  of  secre- 
tion, the  specific  constituents  of  the  urine  being  added  in 
large  quantity  to  the  watery  glomerular  fluid '?  A  definite 
decision  between  these  two  theories  is  not  at  present  pos- 
sible, although  there  is  no  doubt  that  in  its  original  form 
Ludwig's  theory  is  untenable.  Ludwig  ascribed  the  concen- 
tration to  processes  of  diffusion  occurring  between  the  fluid 
in  the  tubules  and  the  lymph  outside  the  tubules.  As  a 
matter  of  fact  if  urine  and  lymph  were  in  contact,  separated 
only  by  a  permeable  membrane,  the  urine  would  cause  a 
concentration    of    the   lymph,  since  its  osmotic    pressure    is 


FUNCTIONS   OF   THE   KIDNEYS   AND   SKIN  461 

very  much  higher  than  that  of  the  latter  fluid.  It  is  difficult 
to  believe  however  that  tlie  whole  of  the  urea  of  the  urine  is 
turned  out  by  a  process  of  filtration  in  the  glomeruli,  since,  in 
order  to  form  the  1,500  c.c.  of  urine  containing  30  grammes 
of  urea  per  day,  the  glomeruli  would  have  to  turn  out  60,000 
c.c.  of  urine,  of  which  58,500  would  be  reabsorbed  by  the 
tubules. 

We  must  see  therefore  what  evidence  there  is  of  a  secre- 
tory function  of  the  tubular  epithelium.  Older  observers 
described  crystals  of  uric  acid  in  the  tubular  epithelium  of 
the  bird's  kidney.  In  the  case  of  the  mammalian  kidney  it 
is  evidently  impossible  to  trace  such  a  soluble,  inert  body  as 
urea,  and  we  must  have  recourse  to  colouring  matters  on 
which  the  kidney  has  a  special  selective  activity  and  which 
can  be  detected  on  their  way  through  the  cells.  Such  a  sub- 
stance is  sodium  sulphindigotate.  If  this  be  injected  into 
the  blood-stream,  the  urine  within  a  minute  or  two  acquires 
an  intense  blue  colour,  although  the  blood  may  be  only 
slightly  tinged  with  tlie  dye.  If  during  this  period  of  secre- 
tion the  animal  be  killed  and  its  kidney  washed  through  with 
absolute  alcohol  in  order  to  fix  the  dye-stufif,  the  whole  of 
the  kidney  is  found  to  be  blue,  and  on  microscopic  section 
the  only  parts  free  from  the  dye  are  the  glomeruli.  In  order 
to  study  the  seat  of  excretion  of  the  indigo  we  must  stop  the 
glomerular  secretion  which  carries  the  blue  colour  to  all 
parts  of  the  tubule.  For  this  purpose,  before  the  injection, 
the  spinal  cord  is  divided  in  the  neck.  The  injection  then 
evokes  no  secretion,  but  on  fixing  the  kidney  in  alcohol  the 
cortex  alone  is  found  to  be  blue,  and  on  microscopic  section 
the  indigo  is  found  deposited  in  granules  in  the  lumen  and 
within  the  striated  epithelial  cells  of  the  first  and  second 
convoluted  tubules. 

Having  thus  proved  a  specific  secretory  activity  of  the 
striated  cells  of  the  convoluted  tubules  with  regard  to  indigo 
carmine,  it  is  a  reasonable  assumption  to  make  that  these 
cells  exercise  a  similar  function  in  respect  to  urea  and  pro- 
bably for  the  other  specific  constituents  of  the  urine. 

Even  if  we  grant  this  assumption,  it  seems  necessarj^  to 
allow  also  an  absorbing  function  to  the  tubules.  Thus  it  is 
found  that  during  the  diuresis  produced  by  injection  of 
mixtures  of  sodium  chloride  and  sulphate,  the  sulphate  is 
excreted   far    more   completely  than  the  chloride,  and  this 


462 


PHYSIOLOGY 


difference  is  exaggerated  if  we  favour  absorption  by  partial 
obstruction  of  the  ureter,  owing  to  the  greater  ease  of  absorp- 
tion of  the  chloride.     It  is  possible  that  certain  parts  of  the 


Fig.  217. 
Far  body 


Testis 


Kidney 


\tk,  -AoKia 


—  Vena  cava 

Renal  aKheries 


Renal  port-al 
Ant.  abdom.v. 

-Femoral  v. 
The  vascular  supply  to  the  kidney  in  the  frop;. 

tubules  are  secretory  while  other  partis  are  absorptive  in 
function,  but  we  are  far  as  yet  from  any  exact  knowledge  of 
the  part  played  by  each  segment  of  the  tubule  in  the  elabora- 
tion of  the  fully  formed  secretion. 

Fig.  218. 
Renal  art.  ^ 


Renal  portal 


vein 

Diagrammatic  representation  of  the  course  of  the  blood-flow  in  the 
frog's  kidney,  showing  the  double  blood-snpply  to  the  capillaries 
round  the  tubules. 

The  ideal  method  of  deciding  the  relative  functions  of  the  glomeruli  and 
tubules  would  be  to  collect  the  secretions  of  these  two  parts  separately.  In 
the  frog  and  newt,  the  kidneys  have  a  double  blood-supply,  the  glomerular 
arteries  being  branches  of  the  renal  artery,  whereas  the  urinary  tubules  derive 
their  blood  partly  from  the  vasa  efferentia  of  the  glomeruli,  but  partly  also  from 
a  branch  of  the  femoral  vein  which  forms  what  is  called  the  renal  portal  system 
(Figs.  217,  218). 


FUNCTIONS   OF   THE   KIDNEYS   AND   SKIN  463 

It  was  shown  by  Nnssbaum  that  ligature  of  the  renal  arteries  entirely  cut 
off  the  circulation  through  the  glomeruli  without  interfering  with  the  renal- 
portal  circulation ;  and  he  stated  that,  although  ligature  of  the  renal  arteries 
stops  the  urinary  secretion,  the  secretion  can  be  at  once  induced  by  injecting  a 
solution  of  urea  into  the  blood.  He  concluded  therefore  that  the  function 
of  the  tubules  is  to  secrete  urea  together  with  water.  On  the  other  hand,  he 
states  that  certain  substances,  such  as  sugar  and  peptone,  which  are  readily 
excreted  by  a  normal  kidney,  are  not  excreted  if  the  renal  arteries  have  been 
tied,  even  if  a  flow  of  urine  be  called  forth  by  the  simultaneous  injection  of  urea 
into  the  blood.  It  is  concluded  therefore  that  sugar  and  peptone  pass  into  the 
urine  through  the  glomerular  epithelium. 

A  recent  investigation  of  this  subject  by  Beddard  has  shown  however  that, 
while  Nussbaum's  anatomical  statements  are  correct,  his  conclusions  are 
probably  fallacious,  owing  to  the  absence  of  control  injections  of  the  frogs 
at  the  end  of  the  experiment.  It  is  very  easy  to  miss  some  small  vessel 
going  to  the  glomeruli  and  then  to  obtain  Nussbaum's  results  with  the 
injection  of  urea,  but  if  all  the  vessels  to  the  glomeruli  be  ligatured,  as  proved 
by  a  subsequent  injection,  the  secretion  of  urine  ceases  absolutely  and  cannot 
be  evoked  by  any  injection  of  urea.  It  seems  that  a  certain  supply  of  arterial 
blood  is  necessary  to  the  normal  life  of  the  tubular  epithelium,  since  this 
undergoes  fatty  degeneration  and  desquamates  in  consequence  of  the  occlusion 
of  the  glomeruli.  These  experiments  have  been  repeated,  an  adequate  supply 
of  oxygen  to  the  tubular  epithelium  being  provided  by  keeping  the  frog  in  an 
atmosphere  of  pure  oxygen.  Under  these  conditions  the  desquamation  of  the 
epithelium  does  not  take  place,  and  by  injection  of  urea,  a  small  flow  of  urine 
can  be  induced.     This  must  have  been  secreted  by  the  tubules. 

Begulation  of  urinary  secretion.^ln  most  other  glands 
of  the  body  we  have  seen  that  then-  activity  ^Yas  subject  to 
nervous  influences.  The  submaxillary  gland  is  supplied  by 
secretory  nerves,  stimulation  of  which  calls  forth  a  flow  of 
saliva  independently  of  any  change  in  the  blood-stream.  The 
conditions  in  the  kidney  however  are  diflerent.  The  func- 
tion of  this  organ  is  to  purify  the  blood  of  its  waste  products, 
and  hence  it  is  only  necessary  that  the  cells  should  react 
to  changes  in  the  composition  of  the  blood.  If  the  blood 
becomes  more  watery,  the  excess  of  water  must  be  turned 
out  into  the  urine ;  if  it  contains  too  much  urea  or  sugar, 
these  bodies  must  be  excreted,  in  order  that  the  blood  may 
act  as  a  normal  living  medium,  and  not  as  a  poison  to  the 
tissues  which  it  traverses.  There  is  indeed  no  evidence  of 
secretory  nerves  to  the  kidney.  The  urinary  secretion  is 
conditioned  only  by  the  composition  and  amount  of  blood 
supplied  to  it.  Injection  of  water  or  of  diuretics,  such  as 
sodium  acetate  or  urea,  into  the  blood,  causes  an  expansion  of 
the  kidney  from  dilatation  of  its  vessels,  and  increased  flow  of 
urine,  until  the  blood  is  restored  to  its  normal  composition. 


464  PHYSIOLOGY 

The  nervous  system  can  influence  this  secretion  of  urine, 
but  only  by  its  action  on  the  vessels.  In  this  way  may 
be  explained  the  copious  flow  of  dilute  urine  which  may 
occur  under  the  influence  of  emotions  or  in  hysteria. 
Increased  secretion  of  urine  brought  about  by  the  applica- 
tion of  cold  to  the  skin  may  be  due  to  reflex  dilatation  of  the 
renal  blood-vessels. 

In  the  dog  the  vaso-motor  nerves  to  the  kidney  pass 
from  the  spinal  cord  chiefly  through  the  anterior  roots  of  the 
11th,  l'2th,  and  13th  dorsal  nerves,  and  stimulation  of  the 
peripheral  ends  of  these  roots  causes  shrinking  of  the  kidney. 
If  slowly  repeated  rhythmical  stimulation  instead  of  the  ordi- 
nary faradic  current  be  applied  to  these  nerves,  swelling  of 
the  kidney  may  be  produced,  showing  that  these  roots  also 
contain  vaso-dilator  fibres. 

On  tlie  Work  done  by  the  Kidney 

I  have  already  mentioned  certain  arguments  against  the 
hypothesis  that  the  urine  is  separated  from  the  blood  circu- 
lating in  the  kidney  by  a  simple  process  of  filtration.  A  still 
stronger  argument  however  is  furnished  by  the  fact  that  we 
can  measure  the  work  done  by  the  kidney  in  the  secretion 
of  urine,  and  find  that  it  is  very  much  greater  than  could 
be  accomplished  by  the  pressure  of  the  blood  in  the  renal 
capillaries. 

The  measurement  of  the  work  done  by  the  kidney  depends 
on  a  determination  of  the  osmotic  pressures  of  the  blood-plasma 
and  urine  respectively.  Before  describing  these  results,  it 
will  be  necessary  to  say  a  few  words  as  to  what  is  meant  by 
the  term  '  osmotic  pressure.' 

It  is  well  known  that,  if  a  bladder  containing  strong  salt 
solution  be  placed  in  a  vessel  of  distilled  water,  water  passes 
into  the  bladder  by  diffusion  or  osmosis,  so  that  the  bladder 
swells  and  becomes  tense.  A  manometer  connected  with  the 
bladder  will  show  a  considerable  rise  of  pressure  (osmotic 
pressure).  It  is  evident  however  that  we  cannot  expect  under 
these  conditions  to  get  the  total  possible  rise  of  pressure 
in  the  bladder ;  since  the  salt  diffuses  out  of  the  bladder 
while  the  water  is  diffusing  in,  and  moreover  the  animal 
membrane  permits  of  a  distinct  filtration,  i.e.  leaks,  as  soon 
as  the  pressure  within  it  has  attained  a  certain  height.     It  is 


FUNCTIOxNS   OF   THE   KIDNEYS   AND   SKIN 


465 


necessary  then,  in  order  to  measure  the  osmotic  pressure  of  a 
solution,  to  enclose  it  in  some  vessel  whose  walls  will  only 
allow  of  the  passage  of  water,  and  will  not  permit  salt  to  pass 
out  either  by  diU'usion  or  by  filtration.  Such  a  vessel  may 
be  made  by  washing  out  a  porous  cell,  first  with  copper  sul- 
phate and  then  with  potassium  ferrocyanide.  An  insoluble 
precipitate  of  copper  ferrocyanide  is  deposited  in  the  pores  of 
the  earthenware,  and  it  is  found  that  these  now  allow  only 
water  to  pass  through,  and  are  perfectly  impermeable  to  dis- 
solved salts.  If  we  arrange  such  a  cell  as  in  the  diagram 
(Fig.  219),  fill  it  with  1  per  cent.  NaCl  solution,  and  then 

Fig.  219. 


— MerciaicdMcmomefer 


OiderTessel 

coniainiricr 
Distilled  Watet: 


Inner  I^sscl  (Scmipenneahle) 
cordaintng  l%,Jm..Ci.  Solution. 


suspend  it  in  distilled  water,  we  find  that  water  diffuses  in 
until  the  pressure,  as  shown  by  the  attached  manometer, 
has  attained  a  great  height.  The  osmotic  pressure  of  a  1  per 
cent.  NaCl  solution  is  equal  to  about  5,000  mm.  Hg.  If  by 
artificial  means  we  increase  the  pressure  in  the  cell  above 
this  height,  water  will  be  pressed  through  the  semi-permeable 
walls  of  the  cell,  and  the  solution  will  become  more  concen- 
trated. In  order  then  to  make  a  1  per  cent.  NaCl  solution 
more  concentrated,  we  must  employ  a  pressure  greater  than 
5,000  mm.  Hg. 

30 


46G  PHYSIOLOGY 

Now  it  is  found  that  the  osmotic  pressures  of  various  sohi- 
tioiis  depend,  not  on  the  nature  of  the  dissolved  substance, 
but  merely  on  the  number  of  molecules  present  in  solution. 
The  osmotic  pressure  of  any  solution  is  in  fact  equal  to  the 
pressure  which  the  dissolved  substance  would  exert  if  it 
occupied  the  same  space  in  the  form  of  a  gas. 

Hence,  if  we  can  determine  the  osmotic  pressures  of  the 
blood-plasma  and  of  the  urine,  we  can  estimate  what  w^ork 
must  be  done  by  the  kidney  cells  in  order  to  separate  from  the 
blood-plasma  a  fluid  having  the  osmotic  pressure  of  the  urine. 

We  may  take  as  example  an  instance  quoted  by  Dreser  in 
which  200  c.c.  urine  had  been  secreted.  The  blood-plasma 
in  this  case  had  an  osmotic  pressure  equal  to  a  0'92  per  cent. 
NaCl  solution.  The  urine  had  an  osmotic  pressure  equal  to 
a  4*0  per  cent.  NaCl  solution.  It  may  be  shown  mathemati- 
cally that  in  this  case  the  kidney  had  performed  37  kilogram - 
metres  of  work  in  the  secretion  of  the  200  c.c.  urine.  Very 
interesting  is  the  determination  in  this  way  of  the  maximum 
force  of  the  kidney.  In  one  case  in  which  a  cat  had  been 
deprived  of  water  for  three  days,  the  urine  was  so  concen- 
trated that  it  was  equivalent  to  an  8  per  cent,  salt  solution. 
The  blood-plasma  in  the  same  animal  had  an  osmotic  pressure 
equal  to  I'l  per  cent.  NaCl.  The  difference  of  osmotic  pres- 
sures in  this  case  was  equal  to  498  metres  of  water,  so  that 
the  kidney  had  separated  the  urine  from  the  blood  against  a 
pressure  of  49,800  grams  per  square  centimetre.  The  abso- 
lute force  of  human  muscle  {i.e.  the  weight  it  can  just  raise) 
is  8,000  grams  per  square  centimetre  ;  hence  we  see  that  the 
mammalian  kidney  can  exert  a  force  six  times  greater  than 
the  maximum  performance  of  voluntary  muscle. 

Some  observations  of  Bradford  show  that  we  have  not  exhausted  the 
subject  of  the  functions  of  the  kidney  when  we  have  described  its  action  as  an 
excreting  organ.  If  in  dogs  one  kidney  be  first  excised,  and  at  a  later  period 
half  or  two-thirds  of  the  other  kidney,  it  is  found  that  the  urine  after  the 
second  operation  is  largely  increased  in  quantity,  and  contains  much  more  urea 
than  it  did  under  normal  circumstances.  This  urea  comes  from  the  disintegra- 
tion of  the  nitrogenous  tissues,  since  the  animal  wastes  rapidly  and  dies  in  a 
few  weeks.  An  explanation  is  yet  wanting  for  the  paradoxical  fact  that  an 
animal  with  one-fourth  its  normal  amount  of  kidney  substance  should  form  and 
excrete  double  the  normal  amount  of  urea.  It  is  evident  that  the  kidneys 
play  an  important  and  hitherto  unlooked-for  part  in  nitrogenous  metabolism, 
but  we  are  not  yet  in  possession  of  sufticient  facts  to  decide  the  exact  extent 
and  nature  of  this  function. 


FUNCTIONS   Oi<    THE   KIDNEYS    AND   SKIN  4CV, 


Section  B 
MICTURITION 

The  urine  is  secreted  continuously,  although  the  amount 
secreted  may  vary  from  time  to  time  according  to  the  con- 
dition of  the  animal.  Partly  through  gravity,  partly  through 
the  pressure  under  which  it  is  secreted,  the  urine  is  driven 
on  through  the  ureters.  If  these  be  occluded,  the  secretion 
of  urine  continues  until  its  pressure  reaches  60  to  80  mm.  Hg, 
when  it  ceases.  This  pressure  is  sufficient  to  distend  widely 
the  up])er  part  of  the  ureters  and  the  pelvis  of  the  kidney. 
In  the  ureters,  which  are  muscular  tubes  lined  with  transitional 
epithelium,  the  urine  is  driven  on  by  peristaltic  contractions, 
which  travel  from  the  upper  to  the  lower  part  of  the  ureter. 
These  contractions  occur  from  three  to  ten  times  a  minute, 
and  seem  to  originate  in  the  muscular  substance  of  the  ureter 
itself,  since  they  are  to  be  seen  in  an  excised  ureter.  The 
urine  in  this  way  gradually  accumulates  in  the  bladder  ;  its 
reflux  into  the  ureters  is  prevented  by  the  oblique  manner  in 
which  these  enter  the  bladder,  a  sort  of  valvular  opening  being 
thus  formed.  At  intervals  the  urine  that  is  collected  in  the 
bladder  is  expelled  by  contraction  of  its  muscular  wall.  This 
act  of  micturition  is  in  the  .young  child  purely  reflex,  and 
dependent  on  the  tension  in  the  bladder.  With  advancing 
age  however  the  individual  acquires  more  or  less  voluntary 
control  over  the  reflex  act.  It  will  be  convenient  to  consider 
first  the  purely  reflex  act  of  micturition. 

The  muscular  wall  of  the  bladder  is  generally  described  as 
consisting  of  three  coats,  an  external  longitudinal,  sometimes 
known  as  the  detrusor,  and  chiefly  marked  on  the  anterior 
and  posterior  surfaces,  a  middle  coat  of  circular  fibres,  which 
are  better  developed  towards  the  base  of  the  bladder,  and  a 
very  incomplete  internal  coat  of  longitudinal  fibres.  In  the 
bladder  no  distinct  demarcation  can  be  made  between  the 
various  coats,  and  a  bundle  of  fibres,  which  is  at  first  longi- 
tudinal, may  dip  inwards  and  run  in  a  circular  direction.  No 
distinct  thickening  of  the  circular  coat  to  form  a  sphincter 
can  be  made  out  at  the  vesico-urethral  orifice,  and  on  this 


468  PHYSIOLOGY 

account  the  circular  muscle-fibres  surrounding  the  prostate 
have  been  described  as  an  external  spliiucter  vrsiccc,  and 
have  been  regarded  as  the  chief  sphincter  of  the  bladder. 
This  however  would  leave  the  female  sex  entirely  without  a 
sphincter.  Moreover  in  many  animals  the  prostate  is  con- 
fined to  the  dorsal  side  of  the  urethra,  and  does  not  run  com- 
pletely round  it.  It  has  therefore  been  suggested  that  the 
urine  is  retained  by  the  mechanical  arrangements  at  the  neck 
of  the  bladder.  Thus  if  fluid  be  injected  into  the  bladder 
through  one  of  the  ureters,  a  point  is  finally  reached  at  an 
internal  pressure  of  about  120  cm,  of  water,  at  which  the 
fluid  begins  to  escape  from  the  urethra.     The  same  result  is 

Fig.  220. 


Ureter--- 


Prosrate- 

Memb.' 


observed  if  the  experiment  be  tried  onia  dead  animal,  but  in 
the  latter  case  the  urethra  does  not  contract  after  the  passage 
of  the  fluid,  and  so  remains  patent  and  filled  with  fluid.  If 
therefore  the  experiment  be  tried  a  second  time,  a  very  small 
pressure  suffices  to  bring  about  an  escape  of  urine  from  the 
urethra.  On  this  account  also  the  direction  of  the  tension  of 
the  bladder-walls  is  of  considerable  importance  in  determining 
the  amount  of  pressure  necessary  to  overcome  the  resistance 
of  the  urethral  orifice.  If  the  abdomen  be  opened,  consider- 
able pressure  may  be  applied  to  the  bladder  in  a  downward 
direction  without  any  escape  of  urine,  whereas  if  the  bladder 
be  taken  in  the  hand  and  drawn  up,  comparatively  slight 
pressure  serves  to  expel  its  contents.  When,  as  usually  occurs, 
the  expelling  force  is  rej)resented  by  the  contracting  walls  of 


FUXCTIONS   OF   THE   KIDNEYS   AM)   SKIN  460 

the  bladder  itself,  the  circulai-  and  longitudinal  fibres  both 
co-operate  in  straightening  the  vesico-urethral  canal  and 
forcing  a  tluid  wedge  into  the  beginning  of  the  urethra. 

A  recent  investigation  by  Kalischer  of  the  structures  at  the  neck  of  the 
bladder  has  brought  to  light  the  existence  of  two  distinct  structures,  both 
of  which  must  play  some  part  in  the  normal  retention  of  urine.  The  first 
of  these  is  a  dense  ring  of  unstriated  muscular  fibres,  the  sphincter  frigonalis, 
which  encircle  the  beginning  of  the  urethra  in  an  oblique  direction,  as  indicated 
by  the  dotted  line  s  s  in  Fig.  220,  and  are  continuous  with  the  muscle-fibres  of 
the  trigonum  (the  space  between  the  openings  of  the  ureters).  These  fibres  are 
quite  distinct  from  the  circular  fibres  of  the  bladder. 

Besides  this  involuntary  sphincter,  Kalischer  describes  another,  consisting 
of  striated  fibres  which  form  a  complete  circle  round  the  membranous  portion 
of  the  urethra,  and  an  incomplete  ring  round  the  anterior  part  of  the  prostate. 
This  is  the  sphincter  urogeniiaUa.  Both  structures,  it  will  be  observed,  belong 
not  to  the  bladder  but  to  the  urethra,  of  which  the  trigonum  is  genetically 
a  part. 

We  must  now  inquire  into  the  mechanism  of  the  act  of 
expulsion.  As  the  urine  slowly  trickles  into  the  bladder,  this 
viscus  first  relaxes  to  accommodate  the  fluid,  so  that  its  con- 
tents increase  without  a  corresponding  rise  of  intra-vesical 

Fig.  221. 


U,  B. 


20"  ^ 

-VWl  WAMWVAA,WA  VW^UAAM\M/V\MMM/\\AMAAVv^\naAVv'vAAA/ 
Tracings  of  rhythmic  contractions  of  urinary  bladder.     (Sherrington.) 

pressure.  With  increasing  distension  however  the  pressure 
in  the  bladder  begins  to  rise,  and  this  increased  tension  on 
the  muscular  walls  has  the  ordinary  excitatory  effect  on  the 
muscle-fibre.  Slow  rhythmic  contractions  of  the  bladder 
make  their  appearance  and  increase  in  force  with  increasing 
tension  on  the  muscular  walls  (Fig.  221). 

The  exact  point  at  which,  with  increasing  pressure,  this  excitatory  effect 
begins,  depends  largely  on  the  rate  at  which  the  pressure  is  raised.  A  bladder 
which  normally  would  hold  250  c.c.  of  fluid,  will  not  hold  more  than  100  c.c.  if 
injected  rapidly  by  means  of  a  syringe.     In  each  case  micturition  would  begin 


470  PHYSIOLOGY 

or,  in  the  conscious  animal,  feelings  of  distress  would  occur,  at  the  same  intra- 
vesical pressure,  althouj^h  the  anrounts  of  fluid  in  the  two  cases  are  so  very 
different. 

These  rliytlimic  contractions,  causing  a  waxing  and  waning 
of  intra-vesical  pressure,  start  impulses  which  travel  up  the 
afferent  nerves  of  the  bladder  to  the  lumbo-sacral  cord,  and 
here  a  continual  summation  of  these  impulses  goes  on,  until 
finally,  as  the  bladder  is  contracting,  the  stored-up  impulses 
break  through  all  resistance  and  give  rise  to  a  reflex  discharge 
down  the  efferent  motor  nerves  of  the  bladder.  A  strong 
contraction  of  all  the  muscular  fibres  of  the  bladder  is  pro- 
duced, causing  a  rise  of  pressure  to  180  or  even  150  cm.  of 
water,  which  overcomes  the  resistance  of  the  tissues  at  the 
neck  of  the  bladder  and  continues  until  this  viscus  is  emptied. 
This  evacuation  may  be  aided  by  an  associated  contraction 
of  the  abdominal  muscles.  Whether  or  not  these  events  are 
accompanied  by  an  active  relaxation  of  the  '  sphincter '  or 
*  sphincters  '  is  still  undecided.  At  the  end  of  the  act  the  last 
drops  are  expelled  from  the  urethra  by  rhythmical  contractions 
of  the  perinaeal  muscles,  especially  the  accelerator  urina? 
and  levator  ani,  thus  emptying  this  canal  and  restoring  the 
sphincter  action  of  the  tissues  at  the  neck  of  the  bladder. 

The  nerve-supply  of  the  bladder  is  shown  in  the  accom- 
panying diagram  (Fig.  222).  It  will  be  seen  that  it  receives 
nerves  from  two  sources  :  firstly,  from  the  upper  lumbar 
nerves ;  and  secondly,  from  the  second  and  third  sacral 
nerves.  The  fibres  from  the  latter  source  run  direct  to  the 
bladder  in  the  nervi  erigentes.  Those  from  the  upper  lumbar 
cord  have  a  more  circuitous  course,  by  the  rami  coimminicantes 
to  the  sympathetic  chain,  thence  to  the  collection  of  ganglion 
cells  surrounding  the  inferior  mesenteric  artery,  and  from 
this  by  the  two  hypogastric  nerves  to  the  bladder. 

Stimulation  of  the  upper  set  of  nerves  causes  a  feeble 
contraction  of  the  bladder-wall  which,  if  one  hypogastric 
nerve  be  stimulated,  is  confined  to  the  stimulated  side, 
and  is  always  most  marked  at  the  base  of  the  bladder. 
Excitation  of  the  pelvic  nerves,  on  the  other  hand,  causes  a 
strong  contraction  of  the  bladder,  which  is  generally  sufficient 
to  overcome  the  resistance  of  the  sphincter,  and  so  bring  about 
micturition.  In  many  animals  the  upper  nerve-supply  also 
carries  some  inhibitory  fibres  to  the  bladder-wall. 


FUNCTIONS   OF   THE    KIDNEYS   AND   SKIN 


471 


The  sensory  nerves  of  the  bladder  probably  run  in  the 
upper  (sympathetic)  supply. 

According  to  some  authors  these  two  sets  of  fibres  are  antagonistic  in 
function.  Stimulation  of  tlie  liypogastric  nerves  is  said  to  cause  contraction  of 
the  circular  fibres  of  the  bladder-wall,  and  therefore  increased  contraction  of  the 
sphincter  vesicJB,  whereas  stimulation  of  the  nervi  erigentes  causes  relaxation  of 


Fig.  222. 


Sujf  mes.  ganglion. — • 


Jnf:  mesenteric 
ganglion, 


Jfypogdjtric 
Hemes. 


Hecftfm.  — 


gadder. 


''.LumhaT  jverfe^ 


':-/>ympal}idio  chain. 


•Lumhar  mrve3. 


--.J.EmSacral 


"    _;  A'eri'i  eriffenies. 

tectum. 
Diagram  of  nerve-supply  to  bladder.     (Nawrocki  and  Skabitchewsky.) 


the  sphincter  and  a  strong  contraction  of  the  detrusor  urin®.  The  sacral  fibres 
are  therefore  those  which  are  most  important  for  the  act  of  micturition.  The 
whole  question  of  the  behaviour  of  the  sphincter  however  is  in  need  of  renewed 
investigation,  in  view  of  the  anatomical  results  mentioned  above. 

In  the  adult  the  processes  of  retention  and  evacuation  of 
urine  are  modified  and  controlled  by  voluntary  effort.  The 
normal  action  of  the  sphincter  mechanism  may  be  aided  by 
the  contraction  of  the  perineal  muscles  which  keep  the 
urethra   closed.     The   reflex   process  of   evacuation  may  be 


47'2  PHYSIOLOGY 

set  in  motion  by  voluntary  contraction  of  the  abdominal 
muscles,  by  which  the  pressure  in  the  bladder  is  increased 
and  the  normal  sphincter  action  overcome.  It  is  probable 
too  that  the  individual  has  a  certain  degree  of  voluntary 
power  over  the  unstriated  muscles  of  the  bladder,  and  that 
the  contraction  of  the  muscular  wall  may  be  directly  aug- 
mented by  impulses  proceeding  from  the  cortex  to  the  upper 
part  of  the  lumbar  cord. 

This  view  is  favoured  by  the  fact  that  stimulation  of 
the  crus  cerebri  has  been  observed  to  cause  contraction  of 
the  detrusor  urinae.  In  this  experiment  the  abdomen  was 
opened,  so  there  could  be  no  question  of  the  contraction  of 
abdominal  muscles. 


FUNCTIONS   OF   THE   KIDNEYS   AND   SKIN  473 


Section  4 
THE     SKIN 

The  skin,  which  covers  the  whole  external  surface  of  the 
body,  consists  of  a  layer  of  stratified  squamous  epithelium, 
the  epidermis,  resting  on  a  layer  of  fibrous  tissue,  the  dermis. 
The  epidermis  is  composed  of  several  layers. 

(1)  The  stratum  Malpighii,  resting  on  the  papillae  of  the 
dermis,  and  composed  of  several  layers  of  cells  which, 
columnar  below,  become  polygonal  and  flattened  towards  the 
surface. 

(2)  The  stratuvi  granulosum,  one  or  two  layers  of 
flattened  cells  with  many  eleidin  granules. 

(3)  The  stratum  lucidum,  a  layer  of  flattened  nucleated 
hyaline  cells. 

(4)  The  stratum  corneum,  produced  by  the  conversion  of 
the  cells  of  the  subjacent  layers,  and  consisting  of  horny 
scales. 

Keratin,  which  forms  the  main  part  of  the  horny  layer  of  the  skin  as  well 
as  of  the  skin  appendages,  such  as  nails,  hairs,  wool,  feathers,  horns,  is  an 
insoluble  resistent  body  closely  allied  to  the  proteins.  On  heating  with  acids  it 
gives  the  same  decomposition  products  as  the  proteins,  viz.  leucine,  tyrosine,  and 
other  amino-acids  as  well  as  the  hexone  bases.  It  gives  the  xantho-proteic 
and  Millon's  reactions,  but  is  distinguished  from  other  proteins  by  the  high 
percentage  of  sulphur  contained  in  its  molecule  (2-5  to  5  per  cent.).  The 
greater  part  of  the  sulphur  is  present  in  the  form  of  cystine. 

The  dermis  consists  of  a  close  meshwork  of  interlacing 
white  fibres,  together  with  connective  tissue  cells,  blood- 
vessels, lymphatics,  and  nerves.  At  its  under  surface  it 
passes  insensibly  into  the  subcutaneous  tissue,  which  also 
consists  of  interlacing  fibrous  bundles,  but  with  coarse 
meshes,  giving  a  loose  texture  to  the  tissue.  These  meshes 
often  contain  a  large  amount  of  adipose  tissue. 

Two  forms  of  glands  are  found  in  the  skin,  the  sebaceous 
and  the  sweat  glands.     These  may  be  considered  separately. 

Sebaceous  glands.— These  always  occur  in  connection  with 
the  hair  follicles.  The  hairs  are  formed  by  a  transformation 
of  certain  cells  of  the  stratum  Malpighii,  which  have  grown 
down  into  the  dermis  or  subcutaneous  tissue  to  form  a  hair 
follicle.     The  hairs  are  always  set  obliquely.     At  the  neck  of 


474  rHYsioLOtJY 

the  follicle,  where  the  stratum  conieum  of  the  surface  skin 
comes  to  an  end,  we  find  the  openings  of  the  sebaceous  glands. 
These' are  saccular,  with  a  definite  basement  membrane  which 
is  lined  by  a  layer  of  columnar  cells  not  differing  greatly  from 
the  lowermost  cells  of  the  Malpighian  layer.  The  lumen  of 
the  gland  is  however  also  filled  with  cells,  which  are  large 
and  swollen,  staining  feebly  with  hfematoxylin,  and  presenting 
quantities  of  fat  granules.  These  degenerated  cells  are 
the  secretion.  Instead  of  the  protoplasm  of  the  cell  manu- 
facturing and  storing  up  the  secretion,  as  is  the  case  with 
salivary  glands,  it  is  itself  converted  into  sebum,  the  cells  so 
used  up  and  destroyed  being  replaced  by  proliferation  of  the 
cells  nearest  the  basement  membrane. 

The  secretion  is  composed  of  various  neutral  fats  mixed 
with  fatty  acids,  and  has  an  acid  reaction.  It  is  interesting 
to  note  that  a  considerable  amount  of  the  fatty  acids  are  in 
combination,  not  with  the  tribasic  alcohol  glycerin,  but  with 
the  monatomic  alcohol  cholesterin.  These  cholesterin  fats, 
which  are  present  also  to  a  large  extent  in  lanolin  or  wool 
fat,  have  the  advantage  of  not  decomposing  readily,  so 
serving  as  an  efficient  protection  to  the  skin, 

Eunning  from  the  superficial  part  of  the  dermis  to  the 
under  surface  of  the  hair  follicle  is  a  bundle  of  smooth 
muscle-fibres,  the  arrector  pili.  Contraction  of  these  muscles 
can  be  excited  by  direct  application  of  cold  (as  in  produc- 
tion of  '  goose  skin ')  or  by  the  nervous  system  through  the 
pilomotor  nerves,  and  results  in  erection  of  the  hairs.  As 
these  muscles  contract,  they  press  at  the  same  time  on  the 
sebaceous  glands  and  squeeze  out  some  secretion  on  to  the 
root  of  the  hair. 

The  course  of  the  pilomotor  nerves  has  been  very  thoroughly  worked  out  by 
Langley.  It  may  be  stated  as  a  general  rule  that  they  follow  the  same  course 
as  the  vaso- constrictor  nerves,  leaving  the  spinal  cord  by  the  anterior  roots 
from  second  dorsal  to  third  lumbar,  and  passing  as  fine  medullated  fibres  into 
the  sympathetic  chain  by  the  white  rami  communicantes.  In  the  ganglia  they 
end,  and  new  relays  of  non-medullated  fibres,  starting  from  the  ganglion  cells  of 
the  sympathetic  chain,  go  by  the  grey  rami  to  the  nerve-roots,  in  which  they 
pass  to  their  destination  in  the  skin. 

The  sweat-glanch  are  composed  of  coiled  tubes  situated 
in  the  subcutaneous  tissue.  Their  secreting  portion  forms 
about  half  the  coil  and  is  bounded  by  a  basement  membrane 
and  lined   by  a  single  layer  of  cubical  cells.     Between   the 


FUNCTIONS   OF   THE   KIDNEYS   AND    SKIN  475 

cells  and  the  basement  membrane  is  a  layer  of  spirally 
arranged  smooth  muscle-fibres,  which  are  especially  well 
marked  in  the  glands  of  the  axilla.  The  duct  is  lined  by 
two  or  three  layers  of  cells  until  it  arrives  at  the  epidermis, 
where  it  is  continued  to  the  surface  as  a  simple  corkscrew 
channel  between  the  epithelial  cells. 

Sweat,  the  secretion  of  the  sweat-glands,  is  a  clear  colour- 
less fluid,  with  peculiar  odour  and  a  salt  taste.  It  is  gene- 
rally acid  in  reaction,  from  the  admixture  of  the  secretion  of 
the  sebaceous  glands.  If  the  skin  from  which  it  is  obtained 
be  first  thoroughly  cleansed,  the  sweat  that  is  subsequently 
collected  has  a  neutral  or  slightly  alkaline  reaction.  It  con- 
tains a  few  epithelial  scales  derived  from  the  skin,  and  about 
2  per  cent,  solids,  of  which  sodium  chloride  makes  up  the 
greater  part.  Traces  of  fatty  acids — formic,  butyric,  and 
propionic  -are  also  present.  Under  pathological  conditions 
urea,  sugar,  and  other  substances  are  found.  Many  drugs 
after  administration  reappear  in  the  sweat. 

The  secretion  of  sweat  is  constantly  going  on.  If  only  a 
small  amount  is  formed,  it  is  at  once  evaporated,  and  goes  oil" 
into  the  atmosphere  as  insensible  perspiration.  If,  on  the 
other  hand,  the  amount  secreted  be  large,  or  the  surrounding 
atmosphere  moist  so  that  evaporation  cannot  easily  take 
place,  the  sweat  collects  on  the  surface  of  the  body  as  sensible 
perspiration. 

The  quantity  of  perspiration  given  off  is  considerable,  but 
varies  so  much  that  it  is  impossible  to  give  an  average  figure 
for  the  amount.  It  is  increased  by  imbibition  of  large  quan- 
tities of  fluid,  especially  if  warm,  by  a  warm  atmosphere,  or 
by  anything  which  tends  to  increase  the  amount  of  heat 
formed  in  the  body,  such  as  muscular  exercise. 

The  fact  that  sweating  is  so  constantly  associated  with  a 
warm  skin  seemed  at  first  to  show  that  the  secretion  of  this 
fluid  was  called  forth  by  the  increased  supply  of  blood  to  the 
surface  of  the  body  and  to  the  sweat-glands.  This  view 
however  is  negatived  by  the  fact  that  sweating  may  coincide 
with  a  pale  ansemic  skin,  as  in  the  cold  sweats  of  phthisis  or 
of  the  death  agony,  or  associated  with  mental  emotion, 
especially  extreme  fear.  The  activity  of  these  glands,  as  of 
the  salivary  glands,  is  under  the  control  of  the  central  ner- 
vous system.     Stimulation  of  the  peripheral  end  of  the  cut 


470  PHYSIOLOGY 

sciatic  nerve  of  the  cat  causes  abundant  secretion  of  sweat 
on  the  toes  supplied  by  that  nerve.  If,  on  the  other  hand, 
the  sciatic  nerve  on  one  side  be  cut,  and  the  cat  asphyxiated, 
sweating  occurs  in  the  toes  of  the  other  three  limbs,  but  not 
in  the  limb  the  nerve  of  which  has  been  cut.  Stimulation 
of  the  sciatic  nerve  causes  contraction  of  the  blood-vessels, 
so  that  the  secretion  of  sweat  cannot  in  this  case  be  deter- 
mined by  an  increased  blood-supply.  We  can  indeed  excite 
secretion  of  sweat  by  stimulating  the  sciatic  nerve  of  an 
amputated  leg. 

Secretion  of  sweat  may  also  be  excited  reflexly.  Pungent 
substances  taken  into  the  mouth  may  cause  abundant  per- 
spiration on  the  face. 

The  course  of  the  sweat-neives  in  the  cut  has  been  investigated  by  several 
observers,  especially  by  Langley.  In  this  animal  the  hairless  pads  of  the  feet 
are  the  only  parts  of  the  body  where  sweat  is  secreted.  The  sweat-nerves  to  the 
hind  foot  leave  the  cord  chiefly  by  the  anterior  roots  of  the  first  two  lumbar 
nerves,  and  passing  along  the  white  rami  to  the  sympathetic  chain  make  con- 
nections with  cells  in  the  sixth  and  seventh  lumbar  and  first  two  sacral  ganglia. 
From  these  ganglia  grey  rami  carry  the  impulses  along  to  the  corresponding 
spinal  nerve-roots  and  to  the  sciatic  nerve. 

The  sweat-nerves  to  the  fore-limbs  leave  the  cord  by  the  sixth,  seventh,  and 
eighth  dorsal  nerve-roots,  and  all  have  their  cell-stations  in  the  stellate  ganglion, 
whence  non-medullated  fibres  carry  the  impulses  to  the  nerves  of  the  brachia, 
plexus  and  so  to  the  paw.  The  nerve-cell  connections  of  these  fibres  have  been 
determined  by  the  nicotine  method  already  described  (p.  2(35). 

The  sweat-glands  may  likewise  be  affected  peripherally. 
Injection  of  pilocarpine  calls  forth  secretion  of  sweat  between 
the  toes,  even  after  the  sciatica  have  been  cut.  It  is  found 
that  the  secretion  produced  by  stimulation  of  the  sciatic  is 
much  increased  by  warming  the  air  surrounding  the  toes. 
The  importance  of  this  secretion  for  the  regulation  of  the 
body-temperature  will  be  spoken  of  in  the  next  chapter. 

Cutaneous  respiration. — In  the  frog,  a  large  amount  of 
gaseous  interchange  takes  place  through  the  skin,  so  that  the 
animal  may  live  a  considerable  time  after  the  extirpation  of 
the  lungs.  In  man,  the  skin  and  dead  cuticle  are  much  too 
thick  to  allow  any  great  interchange  to  take  place  between 
the  gases  of  the  blood  and  the  surrounding  atmosphere.  It 
is  reckoned  that  on  the  average  from  5  to  8  grms.  of  CO^  are 
given  off  by  a  man  from  the  skin  in  twenty-four  hours,  and 
a  considerably  smaller  amount  of  oxygen  is  absorbed  through 
the  same  agency. 


FUNCTIONS   OF  THE   KIDNEYS   AND   SKIN  477 

It  was  formerly  tliought  thai  various  poisonous  products 
were  excreted  with  the  sweat,  and  that  retention  of  these  in 
the  body  might  give  rise  to  sjanptoms  of  poisoning.  It  seems 
however  that  if  a  man's  skm  be  clean,  the  sweat  is  perfectly 
innocuous,  and  it  is  found  that  a  man  may  be  varnished  all 
over  without  suffering  much  harm.  In  rabbits,  which  do  not 
sweat,  varnishing  the  body  all  over  causes  rapid  death  of  the 
animal,  but  this  death  is  due  simply  to  excessive  loss  of  heat, 
and  may  be  prevented  b}^  wrapping  the  animal  m  cotton  wool. 
Varnishing  this  animal  seems  to  cause  dilatation  of  all  the 
superficial  capillaries,  and  hence  a  great  discharge  of  heat 
from  the  surface  of  the  body.  The  only  function  of  any 
importance  therefore,  that  can  be  ascribed  to  the  secretion 
of  sweat,  is  the  regulation  of  the  heat  discharge  from  the 
bodv. 


478  PHYSIOLOGY 


CHAPTER   XI 

FATE    OF    FOODSTUFFS    IN    THE    ORGANISM- 
METABOLISM 

Section  1 
PEOTEIN  METABOLISM 

Having  studied  the  paths  by  which  the  foodstuffs  are 
absorbed  and  waste  products  removed  from  the  body,  it  re- 
mains to  inquire  into  the  changes  that  take  place  in  the  food 
after  its  absorption.  We  assume  from  our  experience  with 
the  various  tissues,  especially  muscle,  that  the  combustible 
parts  of  the  food  are  built  up,  together  with  oxygen,  into  the 
living  protoplasm  of  the  cell  to  form  a  highly  unstable 
molecule  with  large  potential  energy.  In  the  breakmg  down 
of  this  molecule  there  is  a  rearrangement  of  atoms  to  form 
more  stable  compounds,  the  carbon  and  oxygen  combining 
into  carbon  dioxide  with  the  evolution  of  energy,  which  may 
be  displayed  either  as  heat  or  work. 

We  are  still  far  from  being  able  to  follow  the  changes  that 
the  foodstuffs  undergo  on  entering  into  the  living  protoplasmic 
molecule.  Up  to  this  point  however  we  can,  though  with 
many  gaps  in  our  chain  of  facts,  follow  the  fate  of  foodstuffs 
in  the  body  ;  and  a  short  account  of  these  facts,  which  are 
grouped  together  under  the  term  'metabolism,'  is  the  object  of 
the  present  chapter. 

In  all  experiments  on  metabolism  we  must  be  able  to 
make  an  exact  comparison  of  the  Income  and  Output  of  the 
body.  To  this  end  the  food  must  be  weighed  and  analysed, 
and  the  oxygen  taken  in  measured  by  means  of  some  respira- 
tory apparatus.  The  output  of  the  body  includes  the  carbon 
dioxide  and  water  expired  by  the  lungs  ;  the  urine,  contain- 
ing chiefly  the  nitrogenous  excreta  ;  the  faeces  ;  the  carbon 
dioxide  and  water  given  out  by  the  skin  as  perspiration,  and  a 


FATE    OF   FOOD.sTUFFS   IX   THE    ORGANIS^NI  479 

slight  loss  dependent  on  the  wearing  away  of  the  cuticle.  In 
a  balance-sheet  of  the  organism  the  faeces  should  be  subtracted 
from  the  income,  since  they  represent  the  undigested  parts 
of  the  food. 

It  has  been  thought  that  in  a  normal  animal  the  excreta 
may  possibly  have  a  twofold  origin,  and  may  come  partly 
from  the  breakmg  down  of  the  living  protoplasm  of  the  body, 
partly  from  the  direct  oxidation  of  the  food  that  is  continu- 
ally being  taken  in.  It  must  be  remembered  however  that 
we  have  no  evidence  of  any  oxidative  destructive  changes  in 
the  tissue  juices  or  blood,  and  that  the  whole  weight  of  ex- 
periment points  to  the  cells  being  the  sole  seat  of  all  meta- 
bolic changes.  It  will  be  convenient  to  consider  first  the 
simplest  condition,  in  which  the  animal  takes  no  food,  in 
order  that  we  may  have  only  the  metabolism  of  the  living 
tissues  themselves  to  deal  with. 

In  starving  animals  the  income  of  the  body  is  limited  to 
the  inspired  oxygen  and,  in  most  experiments,  to  a  certain 
amount  of  water.  During  an  experiment  of  this  sort,  the 
animal  is  weighed  every  day,  and  the  amount  of  nitrogen 
excreted  and  carbon  dioxide  given  off  by  the  lungs  carefully 
estimated.  Preliminary  experiments  have  shown  us  the 
amount  of  nitrogen  contained  in  muscle,  so  that  from  the 
amount  of  nitrogen  excreted  we  can  estimate  the  degree  of 
disintegration  of  the  muscular  tissues  that  has  gone  on. 
From  the  quantity  of  CO2  eliminated  we  can  determine  the 
loss  of  the  carbonaceous  part  of  the  body.  This  may  be 
regarded  as  composed  entirely  of  fat,  since  the  amount  of 
glycogen  and  carbohydrates  in  the  body  is  small  in  com- 
parison with  the  fat  present.  These  two  amounts  (proteins  +  fat 
lost)  subtracted  from  the  daily  loss  of  weight  leave  a  remainder 
which  represents  the  output  of  water.  Adult  animals,  supplied 
with  water  only,  live  for  four  or  five  weeks.  During  this 
time  they  suffer  gradual  loss  of  weight,  and  at  death  have 
lost  about  50  per  cent,  of  their  weight.  The  excretion  of 
CO2  and  water  sinks  continually  until  death  takes  place.  The 
urea  excretion  falls  considerably  within  the  first  four  or  five 
days,  and  then  remains  almost  constant  at  a  low  level  for 
about  four  weeks.  At  the  end  of  the  fourth  week  there  may 
be  a  sudden  rise  in  the  amount  of  urea  excreted.  At  this 
])eriod  every  trace  of   fat   has   disappeared    from    the  body. 


480  PHYSIOLOaY 

The  animal  has  no  further  store  of  carbonaceous  material  to 
draw  upon,  and  so  must  consume  the  protein  of  its  tissues  in 
order  to  su])ply  the  necessary  proportion  of  heat  and  work. 
In  the  emaciation  consequent  upon  starvation  it  is  observed 
that  the  tissues  whose  energies  are  most  necessary  for  the 
carrying  on  of  the  vital  functions  suffer  least.  Thus  the 
heart  and  the  central  nervous  system  lose  only  3  per  cent, 
of  their  weight.  Of  the  fat,  on  the  other  hand,  97  per  cent, 
disappears,  of  the  muscle  30  per  cent.  ;  and  a  considerable 
decrease  of  weight  is  also  observed  in  the  bones,  liver,  blood, 
and  alimentary  canal. 

The  nitrogen  that  is  eliminated  during  starvation  must 
necessarily  arise  from  the  disintegration  of  the  proteins  of 
the  body.  As  we  have  just  seen,  this  amount  during  certain 
periods  of  starvation  is  fairly  constant  from  day  to  day.  It 
might  be  thought  that,  if  an  amount  of  protein  were  given  to 
the  animal  containing  a  proportion  of  nitrogen  equivalent  to 
that  which  the  starving  animal  was  excreting,  the  loss  of 
nitrogen  to  the  body  would  be  checked,  the  loss  of  nitrogen 
in  the  urine  being  replaced  in  the  tissues  by  the  nitrogen  of 
the  food.  This  is  however  not  the  case.  After  the  adminis- 
tration of  the  protein  to  the  starving  animal,  the  quantity  of 
urea  excreted  is  almost  doubled,  showing  that  nearly  the 
whole  of  the  protein  taken  in  is  disintegrated  within  twenty- 
four  hours  and  excreted  with  the  urine.  In  order  to  pro- 
duce a  condition  in  which  the  amount  of  nitrogen  eliminated 
is  equal  to  the  amount  of  nitrogen  taken  in  with  the  proteins 
of  the  food,  it  is  necessary  to  give  the  animal  at  least  two 
and  a  half  times  the  amount  of  protein  corresponding  to 
the  nitrogen  that  is  excreted  during  starvation.  In  this  case 
the  animal  is  said  to  be  in  a  condition  of  'nitrogenous 
equilibrium.' 

The  condition  of  the  protein  metabolism  in  an  animal 
that  is  living  on  a  purely  protein  diet  and  is  in  a  state  of 
nitrogenous  equilibrium  deserves  a  little  further  consideration. 
The  fact  that,  to  maintain  the  animal  in  this  condition,  two 
and  a  half  times  as  much  protein  is  required  as  is  necessary 
to  replace  disintegrated  nitrogenous  tissues  in  the  body,  has 
been  regarded  as  showing  that  not  all  the  protein  in  the  food 
can  be  devoted  to  this  object.  It  has  been  supposed  by  Yoit 
that  the  protein  taken  in  with  the  food  has  a  twofold  destina- 


FATE   OF   FOODSTUFFS   IN   THE    ORGANISM  481 

tion  in  the  body,  part  of  it  going  to  supply  the  tissue  waste, 
and  being  built  up  into  the  living  protoplasm  of  the  tissues 
(morphotic  or  tissue  protein)  ;  while  the  other  and  probably 
greater  moiety  passes  into  the  juices  that  bathe  the  proto- 
plasmic elements  of  the  cells,  and  is  rapidly  broken  up  and 
oxidised  there  without  at  any  time  forming  an  integral  part 
of  the  protoplasm.     This  is  spoken  of  as  circulating  protein. 

I  have  however  already  drawn  attention  to  the  numerous 
experiments  on  the  subject,  carried  out  chiefly  by  Pfliiger 
and  his  pupils,  all  of  which  tend  to  prove  that  the  sole  seat 
of  oxidative  processes  in  the  body  is  the  living  cell. 

Some  experiments,  which  were  carried  out  by  Schonclorff  under  Pfliiger's 
guidance,  are  of  especial  interest  in  this  question.  According  to  Voit  the 
greater  excretion  of  in-ea  in  a  protein-fed  animal  is  due  to  the  fact  that  there 
is  an  increased  circulation  of  a  fluid  that  is  rich  in  proteins  round  the  cells. 
According  to  Pfliiger's  views  however,  the  presence  of  a  greater  or  less  amount 
of  protein  in  the  nourishing  medium  would  not  be  the  determining  factor  for 
the  amount  of  urea  formed,  which  would  be  regulated  simply  and  solely  by  the 
condition  of  the  cells  themselves.  To  decide  this  point,  defibrinated  dog's  blood 
was  led  alternately  through  the  hind  limbs  and  the  liver  of  another  dog,  in 
order  to  get  the  products  of  metabolism  of  the  limb  tissues  and  then  convert 
them  into  urea  by  passing  the  blood  through  the  liver. 

1.  In  one  set  of  experiments  the  blood  from  a  dog  that  had  been  starved  for 
flve  days  was  led  through  the  organs  of  a  well-fed  dog.  In  these  experiments 
Schondorff  found  that,  without  exception,  the  urea  in  the  blood  was  largely 
increased  at  the  end  of  the  experiment. 

2.  In  a  second  series  of  experiments  the  blood  of  a  fasting  animal  was  led 
through  the  hind  limbs  and  liver  of  a  fasting  animal.  In  these  the  amount  of 
urea  in  the  blood  was  unaltered. 

3.  In  a  third  set  blood  of  a  well-fed  animal  was  led  through  organs  and  liver 
of  a  fasting  animal.     In  these  cases  the  amount  of  urea  was  always  diminished. 

It  was  concluded  from  these  experiments  that  the  extent  of  protein  metabolism 
depends  on  the  nutritive  condition  of  the  cell  and  not  on  the  condition  of  the 
protein  contained  in  the  circulating  tissue  juices. 

We  must  therefore  conclude  that  the  whole  of  the  protein 
metabolism  takes  place  in  the  living  cells,  and  not  in  the  l^^mph 
or  blood-stream.  How  far  it  is  necessary  for  all  the  protein 
to  be  built  up  into  tissue  protein  it  is  however  difficult  to 
say.  We  may  at  any  rate  adopt  Voit's  view  as  to  the  twofold 
destination  of  protein,  provided  that  we  introduce  certain 
modifications  into  his  description  of  the  events  that  take  place. 

Protein,  as  indeed  all  food,  has  a  twofold  object.  In  the  first 
place  the  whole  of  the  activities  of  the  body  are  associated 
with  the  discharge  of  energy.  The  source  of  this  energy  is  the 
food,  and,  so  far  as  we  can  tell,  the  value  of  a  given  foodstuff 

Bl 


482  PHYSIOLOGY 

for  this  purpose  is  determined,  apart  from  its  digestibility, 
solely  by  its  heat  value,  i.e.  the  amount  of  energy  it  will  set 
free  in  combining  with  oxygen  to  form  the  end  products  of  its 
metabolism  in  the  body. 

The  heat-values,  i.e.  the  amount  of  heat  evolved  by  the 
combustion  of  one  gram  of  the  substance,  for  the  three  main 
classes  of  foodstuffs  are  as  follows  — 

Fats  .         .         .         .9-3  calories 

Proteins    .         .         .         .     5'5         ,, 
Carbohydrates  .         .         .     4*0         ,, 

In  the  body,  while  the  combustion  of  fats  and  carbohydrates 
is  complete,  part  of  the  protein  molecule  is  oxidised  only 
as  far  as  urea,  which  has  a  heat-equivalent  of  2-5  calories. 
Since  one  gram  of  protein  gives  rise  to  0-3  gram  of  urea,  it 
is  necessary,  in  order  to  get  the  real  heat-value  of  protein 
in  the  body,  to  subtract  0-8  from  the  above  figures,  giving 
a  corrected  value  for  proteins  of  4- 7  calories.  In  man  and 
herbivora  the  total  needs  of  the  body  cannot  be  satisfied  on 
a  purely  protein  diet.  In  the  normal  diet  given  on  page  508, 
the  protein  represents  only  about  one-sixth  of  the  total 
energy  of  the  food. 

The  calorie  here  employed  is  the  amount  of  heat  which  is 
necessary  to  raise  the  temperature  of  one  kilogram  of  water 
one  degree  Centigrade. 

As  a  source  of  energy  it  is  apparently  a  matter  of  indiffer- 
ence whether  the  organism  is  supplied  with  proteins,  fats,  or 
carbohydrates. 

Far  otherwise  is  it  with  the  other  destination  of  protein. 
The  greater  part  of  the  living  structure  of  the  body  is  com- 
posed of  proteins,  or  of  more  complex  nitrogenous  bodies  in 
the  building  up  of  which  the  proteins  play  a  preponderating 
part.  In  the  young  and  growing  animal  these  tissues  are 
constantly  being  added,  and  the  raw  material  for  growth  can 
be  supplied  by  protein,  and  by  protein  only.  Moreover,  in  the 
period  of  adult  life,  when  the  body  is  neither  gaining  nor  losing 
weight,  every  vital  act  is  associated  with  a  certain  degree  of 
what  we  may  term  '  wear  and  tear '  of  the  living  structure. 
Few  of  the  cells  of  the  body  have  a  life  coterminous  with  that 
of  the  whole  organism.  Most  of  the  cells  are  continually 
dying  and  being   replaced  by  fresh  ones.     For    this  mitri- 


FATE   OF   FOODSTUFFS   IX   THE   ORGANISM  483 

tional  metabolism  a  supply  of  proteins  in  the  food  is  an 
absolute  necessity.  Every  diet  therefore  must  contain  a 
certain  minimum  amount  of  protein  to  supply  the  nutritional 
needs  of  the  organism,  while  the  energy  requirements  can  be 
supplied  at  the  expense  of  either  proteins,  fats,  or  carbo- 
hydrates. 

The  physiological  value,  however,  of  these  three  classes  is 
not  entirely  expressed  by  their  heat  values.  The  organism 
has  the  power  of  storing  up  any  excess  of  fat  or  carbohydrate 
above  its  energy  requirements  in  the  form  of  fat,  which  can 
be  utilised  for  the  future  activities  of  the  organism.  Its 
power  of  storing  up  protein  is  however  extremely  limited. 
In  some  animals  overfeeding  with  protein  may  produce  a 
growth  of  muscles,  which  may  be  regarded  as  forming,  in 
some  sense,  a  protein  storehouse.  In  most  animals,  including 
man,  this  power  is  practically  absent.  Load  the  animal  as 
we  will  with  protein,  two  days  of  starvation  will  suffice  to 
exhaust  any  store  of  protein  or  nitrogenous  materials,  and 
the  excretion  of  urea  sinks  to  the  starvation  minimum,  its 
amount  being  determined  by  the  size  and  activities  of  the 
animal  and  the  amount  of  fat  available  for  the  supply  of  the 
energy  requirements  of  the  body.  On  the  other  hand,  feeding 
the  animal  with  excess  of  protein  only  leads  to  increased 
excretion  of  urea.  If  for  instance  a  man  were  taking  10 
grams  of  nitrogen  in  the  form  of  protein  in  the  day  with  a 
sufficiency  of  fats  and  carbohydrates  to  maintain  his  weight 
constant,  he  would  probably  excrete  also  10  grams  (9  grams 
in  the  urine,  1  gram  in  the  faeces),  and  would  therefore  be  in 
a  state  of  nitrogenous  equilibrium.  On  doubling  the  protein 
intake,  the  nitrogenous  excretion  would  rise  in  proportion,  i.e. 
to  20  grams,  and  the  man  would  remain  in  a  state  of  nitro- 
genous equilibrium  however  much  his  protein  intake  were 
increased.  The  increased  protein  diet  would,  however,  raise 
his  energy  income  above  his  daily  requirements.  A  certain 
amount  of  the  fat  and  carbohydrate  would  therefore  escape 
oxidation  and  w^ould  be  stored  up  in  the  form  of  fat,  and  the 
man  would  therefore  increase  in  weight.  It  is  impossible  in 
man  to  push  this  experiment  very  far,  since  a  large  excess 
of  protein  diet  gives  rise  to  digestive  disturbances,  and  the 
experiment  has  to  be  discontinued. 

It  is  evident  that,  given  a  sufficiency  of  fatty  and  carbo- 


484  PHYSIOLOGY 

hyclniie  food,  the  minimum  protein  possible  in  any  diet  repre- 
sents the  nutritional  needs  of  the  body  for  nitrogen.  Many 
experiments  have  been  made  to  determine  this  minimum.  It 
probably  varies  considerably  in  different  individuals.  Just  as 
some  machines  undergo  greater  wear  and  tear,  and  others 
require  a  greater  consumption  of  coal  to  perform  a  certain 
amount  of  work,  so  it  is  with  animals  and  men,  and  it  would 
be  dangerous  to  lay  down  rigid  laws  for  diet  from  observations 
on  a  few  individuals.  The  minimum  amount  of  protein 
required  in  any  diet  may  probably  be  put  at  about  GO  grams, 
though  some  individuals  may  keep  in  good  health  on  a  still 
smaller  amount.  Any  protein  above  this  amount  may  be 
regarded  as  concerned,  together  with  fats  and  carbohydrates, 
in  supplying  the  energy  requirements  of  the  body. 

The  minimum  need  for  protein  cannot  be  determined  from 
experiments  on  fasting  animals.  In  these  there  is  often  a 
deficiency  of  fat,  and,  after  a  few  days,  of  carbohydrate,  and 
the  protein  metabolism  may  therefore  be  concerned,  not  merely 
in  maintaining  the  nutrition  of  the  working  tissues,  but  also 
in  supplying  a  certain  amount  of  the  energy  required  for  the 
different  activities  of  the  body. 


FATE   OF   FOODSTUFFS   IN   THE   OKGANISM  485 

Section  2 
FOEMATION   OF   FAT 

If,  while  the  animal  is  in  a  state  of  nitrogenous  equi- 
librium, larger  amounts  of  fat  or  carbohydrates  be  given  than 
are  necessary  for  its  daily  consumption,  the  animal  increases 
in  weight,  and  the  excess  of  carbonaceous  material  is  deposited 
in  its  body  in  the  form  of  fat.  On  a  purely  protein  diet  no 
very  large  amount  of  fat,  if  any,  is  ever  deposited  in  the  body. 
A  formation  of  fat  from  protein,  though  not  proved  under 
normal  circumstances,  has  been  thought  to  occur  under  patho- 
logical conditions  in  the  higher  animals  and  in  many  lower 
organisms.  Thus  if  dogs,  that  have  been  starved  till  all  fat 
has  disappeared  from  the  body,  be  poisoned  with  phosphorus, 
a  large  increase  in  the  nitrogen  excretion  is  observed,  and 
when  the  animal  dies  all  its  organs  are  found  to  be  in  a  state 
of  fatty  degeneration,  and  to  contain  two  or  three  times  the 
normal  amount  of  fat.  In  this  case  there  is  apparently  a 
splitting  up  of  the  protein  molecules  of  the  tissues  into  a 
nitrogenous  moiety,  which  is  excreted,  and  a  carbonaceous 
moiety,  which  is  retained  in  the  cells  in  the  form  of  fat.  It 
is  more  probable,  however,  that  the  fat  deposited  in  the  cells 
is  the  product  of  conversion  of  carbohydrate  or  fat  molecules 
transported  from  other  cells  of  the  body.  Evidence  in  favour 
of  the  fat  in  fatty  degeneration  of  the  liver  being  deposited 
rather  than  formed  in  situ  is  afforded  by  the  fact  that,  if  the 
poisoned  animals  be  fed  with  abnormal  fats,  these  fats  are 
found  forming  part  of  the  fat  in  the  degenerated  tissue.  In 
the  ripening  of  cheese,  which  is  accomplished  by  the  agency 
of  low  organisms,  there  is  a  conversion  of  protein  into  fat.  If 
the  eggs  of  fly-maggots  be  allowed  to  develop  on  a  blood-clot, 
the  maggots,  when  full-grown,  will  be  found  to  contain  ten 
times  as  much  fat  as  there  was  previously  in  the  blood-clot 
and  eggs  together  ;  so  that  in  this  case  the  maggots  have 
been  able  to  convert  the  protein  of  the  blood-clot  into  fat. 
We  must  conclude  therefore  that  proteins  may  be  converted 
into  fat  in  the  living  organism,  although  it  is  very  doubtful 
whether  any  such  conversion  takes  place  in  the  higher  animals 
under  normal  conditions. 


486  PHYSIOLOGY 

It  was  long  doubted  wlietlier  the  fat  in  the  food  could  be 
directly  deposited  in  tlie  body  as  such.  It  was  supposed 
that  the  fat  in  the  food  exerted  merely  a  sparing  effect  on 
the  fat  in  the  body  formed  from  the  proteins— that  the  fats 
absorbed  from  the  alimentary  canal  served  to  supply  the 
organism  with  an  oxidisable  material,  and  so  shielded  from 
oxidation  the  fat  in  the  tissues  that  had  been  formed  from 
protein.  We  have  however  conclusive  evidence  that  the 
fats  taken  in  with  the  food  can  be  deposited  in  the  body 
as  such.  Thus  two  dogs  were  fed,  one  with  linseed  oil,  the 
other  with  mutton  suet,  for  a  considerable  period.  The  fat 
in  the  tissues  of  the  former  was  liquid  at  0°  C,  while  the  fat 
of  the  latter  had  a  melting-point  at  above  50°.  In  another 
experiment,  in  which  a  dog  had  been  fed  with  colza  oil, 
erucic  acid,  which  is  an  ingredient  of  colza  oil,  but  absent 
from  animal  fat,  was  found  in  the  fat  of  the  dog  after 
death. 

Not  only  are  neutral  fats  thus  absorbed  from  the  intestine 
and  deposited  in  the  body,  but  also  fatty  acids.  If  these  be 
administered  to  an  animal  the  greater  part  is  absorbed,  and 
it  is  found  that  in  the  chyle  of  the  thoracic  duct  nearly  the 
whole  of  the  fat  is  present  as  a  neutral  fat  and  not  as  a  fatty 
acid,  showing  that,  in  the  passage  of  the  fat  from  the  intes- 
tine through  the  wall  of  the  villi  into  the  lacteals,  there 
has  been  a  synthesis  of  fatty  acid  with  glycerin.  This  is  an 
interesting  fact,  since  glycerin  is  at  no  time  found  free  in  the 
animal  body,  although  we  see  that  the  epithelial  cells  of 
the  intestine  can  supply  sufficient  of  it  to  unite  with  nearly 
the  whole  of  the  fatty  acid  absorbed. 

Long  experience  has  shown  the  farmer  the  value  of  carbo- 
hydrates as  fattening  food.  As  in  the  case  of  fats,  the 
question  has  arisen  whether  the  carbohydrates  are  converted 
into  fat,  or  whether  they  have  only  a  sparing  influence  on 
the  hypothetical  fat  formed  from  the  proteins  of  the  food. 
That  the  former  is  the  case  is  shown  by  the  following  ex- 
periment. Two  young  pigs,  ten  weeks  old,  of  the  same 
litter,  with  approximately  equal  weights,  were  taken.  One 
was  killed,  and  the  fat  and  total  nitrogen  in  the  body 
estimated.  From  the  amount  of  nitrogen  the  maximum 
possible  quantity  of  proteins  present  was  calculated.  The 
second  was  fed  on  barley  for  four  months.     The  barley  was 


FATE   OF   FOODSTUFFS   IN   THE   OROxANISM  487 

measured  and  analysed,  as  well  as  the  amount  of  undigested 
fat  and  protein  that  passed  through  the  animal.  At  the 
end  of  the  four  months  the  second  animal  was  killed  and 
analysed.  It  was  found  that  the  animal  contained  1'5G 
kilos  more  protein,  and  86  kilos  more  fat.  It  had  taken 
up  with  the  food  7'49  kilos  more  protein,  and  0'66  kilo  fat. 
If  we  subtract  the  protein  added  to  the  body  (1"56)  from 
that  taken  up  with  the  food  (7*49),  there  is  a  remainder  of 
5-93  kilos  which  might  possibly  have  given  rise  to  fat. 
But  7*9  kilos  of  fat  had  been  added  in  the  body— a  far 
larger  amount  than  could  pos.sibly  have  arisen  from  the 
maximum  amount  of  protein  left  over  for  the  purpose.  At 
least  5  kilos  of  fat  in  this  experiment  must  have  been 
derived  from  the  direct  conversion  of  the  carbohydrates  of 
the  food.  We  must  conclude  that  fat  can  be  formed  directly 
from  carbohydrates,  although  how  and  where  this  conversion 
takes  place  is  at  present  quite  unknown.  We  have  however 
parallel  instances  in  the  formation  of  butyric  and  other  acids 
of  the  fatty  acid  series  from  sugar  by  means  of  certain 
organised  ferments. 

As  w^e  sliould  expect,  peptones  may  be  used  entirely  to 
replace  the  proteins  of  the  food,  and  the  animal  will  maintaiii 
or  even  increase  its  weight  on  such  a  diet. 

Gelatin  cannot  take  the  place  of  proteins  in  the  food. 
As  we  have  seen,  it  differs  from  ordinary  proteins  in  certain 
important  chemical  relationships.  If  given  in  the  food,  it 
has,  like  carbohydrate  and  fat,  a  sparing  effect  on  the  protein, 
so  that  nitrogenous  equilibrium  is  attained  with  a  smaller 
amount  of  protein  than  would  be  the  case  if  no  gelatin  were 
given.  If  a  dog  be  fed  on  gelatin  and  fat,  the  excess  of  the 
nitrogen  excreted  over  the  nitrogen  taken  in  is  less  than 
when  the  same  dog  is  fed  on  fat  alone,  showing  that  the 
gelatin  has  sheltered  from  disintegration  some  protein  con- 
stituents of  the  body.  It  has  been  said  that  gelatin  can  take 
the  place  of  circulating,  not  of  tissue  protein.  It  would 
probably  be  more  correct  to  say  that,  containing  as  it  does 
only  certain  of  the  ordinary  constituents  of  the  protein 
molecule  (tryptophane  and  tyrosine,  for  example,  being 
absent),  it  can  only  partially  replace  protein  in  supplying  the 
nutritional  needs  of  the  tissues. 

Since  fat  represents  a  store  of  food  in  the  body,  -it  is 


488  PHYSIOLOGY 

evident  that  a  reduction  of  the  fat  can  be  effected  only  in  two 
ways,  either  by  increasing  the  energy  demands  of  the  body, 
as  by  muscular  work,  or  by  diminishing  the  energy  supply  by 
lessening  the  food  supply,  i.e.  by  starvation.  A  man  of  70 
kilos  doing  a  moderate  amount  of  work  needs  about  40 
caolries  per  kilo  body-weight  each  day,  i.e.  his  diet  must 
have  a  heat-value  of  about  3,000  calories.  If  this  supply  be 
reduced,  he  will  begin  to  use  up  the  supply  of  energy  repre- 
sented by  the  fat  of  his  body  ;  and  all  the  obesity  cures  take 
advantage  of  this  fact.  In  some  all  the  foodstuffs  are  dimi- 
nished at  the  same  time  ;  in  others,  as  in  the  Banting  cure, 
the  man  is  allowed  to  eat  as  much  lean  meat  as  possible,  only 
fats  and  carbohydrates  being  restricted.  In  this  case  a  very 
rapid  diminution  of  body-fat  results.  This  diminution  has 
been  ascribed  to  a  specific  stimulating  effect  of  protein  on 
metabolism.  Such  an  explanation  is  unnecessary.  A  man 
who  is  eating  even  five  pounds  of  lean  meat  a  day  is  being 
starved.  "Whereas  his  daily  requirements  are  3,000  calories, 
he  is  getting  with  his  food  only  2,300  calories,  and  as  a 
matter  of  fact  no  man  can  continue  to  eat  such  large  quan- 
tities of  meat.  Three  pounds  of  meat  a  day,  a  more  usual 
quantity,  would  have  a  heat- value  of  about  1,400  calories, 
and  it  is  not  surprising  that  the  organism  makes  good  the 
large  deficit  by  feeding  on  the  stored-up  fat  of  the  body. 


FATE   OF  FOODSTUFFS   IN    THE   OKGANISM  489 


Section  3 
HISTORY   OF   CARBOHYDEATES   IN   THE   BODY 

Normally,  the  blood  of  man  and  dogs  contains  from  0*05 
per  cent,  to  0"15per  cent,  of  sugar  (dextrose).  If  this  amount 
be  artificially  increased  by  the  injection  of  sugar  into  the 
blood,  it  is  found  that,  as  soon  as  the  amount  of  sugar  rises 
above  0*2  per  cent.,  the  excess  is  eliminated  by  the  kidneys 
and  appears  in  the  urine.  After  a  meal  rich  in  carbohydrates, 
such  as  the  Irishman's  mess  of  potatoes,  a  very  large  amount 
of  sugar  passes  into  the  blood  of  the  portal  vein.  Several 
hundred  grams  however  of  carbohydrates  may  be  ingested 
without  any  sugar  appearing  in  the  urine.  Again,  in  the 
interval  between  meals  when  no  sugar  is  passing  into  the 
blood,  the  amount  of  sugar  remains  constant,  although  we 
have  reason  to  believe  that  it  is  incessantly  being  used  up  by 
the  muscles  and  other  tissues  of  the  body.  There  must  be 
some  means  therefore  by  which  the  overloading  of  the  blood 
with  sugar  is  guarded  against,  at  the  same  time  that  the 
sugar  percentage  of  the  blood  is  maintained  constant  during 
periods  of  temporary  starvation.  This  function  is  subserved 
by  the  liver,  the  great  chemical  factory  of  the  body.  This 
is  shown  by  the  fact  that,  if  a  solution  of  dextrose  be  slowly 
injected  into  a  mesenteric  vein,  no  sugar  appears  in  the 
urine,  whereas  glycosuria  is  at  once  produced  if  the  injection 
be  made  into  the  jugular  vein. 

Just  as  plants  have  the  power  of  transforming  sugar  into 
starch,  which  is  deposited  as  a  reserve  material  in  their 
tubers  and  similar  organs,  so  the  liver  has  the  power  of 
seizing  upon  the  excess  of  sugar  passing  through  its  capil- 
laries and  transforming  it  into  a  colloidal  substance,  which  is 
deposited  in  the  meshes  of  the  cell-protoplasm.  This  colloidal 
substance  belongs  to  the  group  of  starches,  and  is  called 
glycogen  or  animal  starch. 

Preparation  of  glycogen. — A  rabbit  is  well  fed  with  carrots 
or  arrowroot  for  a  couple  of  days.  It  is  then  killed  by 
decapitation,  the  liver  cut  out  and  thrown  into  boiling  water, 


490  PHYSIOLOGY 

and  boiled  for  about  ten  minutes.  The  pieces  are  then 
ground  up  with  sand  to  a  line  paste,  returned  to  the  same 
water,  and  boiled  for  half  an  hour.  While  the  mixture  is 
still  boiling,  a  few  drops  of  acetic  acid  are  added  until  the 
reaction  is  very  faintly  acid.  In  this  way  practically  all  the 
proteins  are  coagulated.  The  mixture  is  then  thrown  on  a 
filter,  and  an  opalescent  fluid  runs  through  containing  only 
the  merest  traces  of  protein.  From  this  fluid  the  glycogen  is 
precipitated  as  a  white  amorphous  powder  by  addition  of  two 
volumes  of  90  per  cent,  alcohol.  It  may  be  purified  by  solu- 
tion in  water  and  reprecipitation  by  alcohol ;  it  is  finally 
washed  with  alcohol  and  ether  and  dried.  It  may  be  freed 
from  adherent  traces  of  protein  by  boiling  with  alcoholic 
solution  of  potash,  in  which  the  glycogen  is  insoluble. 

The  glycogen  so  obtained  is  a  white  powder,  free  from 
taste  or  smell,  dissolving  in  hot  or  cold  water  to  form  a 
strongly  opalescent  solution.  With  iodine  its  solutions  give  a 
port-wine  coloration.  It  may  be  distinguished  from  erythro- 
dextrin  by  the  fact  that  it  is  precipitated  on  saturation  with 
ammonium  sulphate,  or  by  60  per  cent,  alcohol,  whereas 
dextrin  needs  nearly  90  per  cent,  alcohol  for  its  precipitation. 
Its  relationship  to  starch  is  shown  by  its  composition 
« (C,iII|„0-,)  and  by  the  fact  that  on  hydrolysis  it  yields  dextrins, 
maltose,  and  finally  dextrose. 

In  the  liver  treated  in  the  manner  described  above  we  find 
only  the  merest  traces  of  sugar.  If  however  the  liver  be  left 
for  some  hours  after  the  death  of  the  animal  before  extraction 
with  boiling  water,  the  extract  will  be  found  to  contain  much 
less  glycogen,  but  very  large  quantities  of  sugar.  We  see 
that  after  death  a  process  takes  place  in  the  liver  by  which 
the  glycogen  is  converted  into  sugar.  This  sugar  is  dextrose. 
The  conversion  may  take  place  even  after  the  cells  have  been 
subjected  to  the  action  of  absolute  alcohol  for  a  considerable 
time.  This  fact  shows  that  the  conversion  is  due  to  the 
presence  in  the  liver  of  some  substance  which  may  act  as  an 
amylolytic  ferment,  converting  starch  or  glycogen  into  sugar. 
This  ferment  is  destroyed  by  heat,  and  it  is  on  this  account 
that  the  liver  is  thrown  into  boiling  water  in  order  to  obtain 
the  maximum  yield  of  glycogen. 

Circumstances  influencing  the  formation  of  glycogen. — 
The  amount  of  glycogen  present  in  the  liver  at  any  given 


FATE   OF   FOODSTUFFS   IN   THE   ORGANISM  491 

time  is  intimately  dependent  on  the  food  taken.  If  an  animal 
be  starved,  the  glycogen  in  the  liver  diminishes  quickly  at 
first,  and  more  slowly  afterwards.  After  prolonged  starvation 
the  liver  contains  only  the  merest  traces.  If  a  rabbit,  deprived 
of  glycogen  b}^  starvation,  be  given  a  meal  of  carbohydrates 
and  killed  a  few  hours  later,  the  liver  will  be  found  to  contain 
large  quantities  of  glycogen. 

Although  carbohj'drates  furnish  the  chief  material  for 
the  manufacture  of  glycogen,  yet  we  have  evidence  that  the 
organism  is  able  to  form  glycogen  out  of  proteins.  If  a  dog, 
deprived  of  glycogen  by  long  starvation  and  work,  be  fed  for 
a  few  days  on  a  diet  of  washed  fibrin  perfectly  free  from 
carbohydrates,  the  liver  will  be  found  to  contain  a  fair 
quantity  of  glycogen,  though  the  amount  is  many  times  less 
than  that  formed  on  a  diet  of  carbohydrates. 

Fats  have  no  influence  on  the  formation  of  glycogen.  The 
glycogen  disappears  as  rapidly  in  an  animal  fed  on  fat  alone 
as  in  starvation. 

Much  more  efficacious  than  starvation  in  causing  dis- 
appearance of  the  hepatic  glycogen  is  muscular  work.  If  a 
dog  be  starved  for  a  day  and  be  then  made  to  drag  a  heavy 
milk-cart  about  all  the  next  day,  there  will  be  found  only  the 
merest  traces  of  glycogen  in  its  liver. 

Glycogen  also  occurs  in  the  muscles,  where  it  probably 
serves  as  a  local  supply  of  reserve  material  for  the  furnishing 
of  muscular  energy.  The  glycogen  must  be  conveyed  from 
the  liver  to  the  muscles  and  other  organs  of  the  body  in  the 
form  of  sugar,  since  we  cannot  detect  any  glycogen  in  the 
Idood.  The  importance  of  glycogen  as  a  reserve  material  is 
shown  by  the  fact  that  it  is  present  in  very  large  quantities 
in  embryonic  muscle,  at  a  period  when  the  formation  of  new 
muscular  fibres  is  going  on  most  intensely. 

We  may  also  look  upon  glycogen  as  a  source  of  heat.  If 
the  temperature  of  a  rabbit  be  lowered  by  immersing  it  in  a 
cold  bath,  the  glycogen  is  found  to  have  disappeared  from  the 
liver  after  a  few  hours. 


492  PHYSIOLOGY 


Glycosuria 


Under  certain  circumstances  the  power  of  the  liver  to 
store  up  the  glycogen  may  be  temporarily  destroyed.  If  a 
puncture  {piqure,  or  diabetic  puncture)  be  made  in  the  floor 
of  the  fourth  ventricle  between  the  nuclei  of  the  auditory  and 
vagus  nerves,  the  animal  for  the  next  twenty-four  or  thirty- 
six  hours  will  suffer  from  glycosuria ;  that  is  to  say,  sugar 
will  pass  into  the  urine.  At  the  end  of  this  time  the  liver 
will  be  found  to  be  quite  free  from  glycogen.  If,  on  the  other 
hand,  the  animal  be  first  deprived  of  glycogen  by  starvation 
and  work,  the  diabetic  puncture  will  be  without  eflect,  showmg 
that  the  glycosuria  is  due  to  the  rapid  conversion  of  the 
hepatic  store  of  glycogen  into  sugar.  This  is  turned  out  into 
the  blood-stream,  where  it  raises  the  percentage  amount  of 
sugar  so  high  that  the  excess  is  excreted  by  the  kidney-cells 
and  appears  in  the  urine.  Temporary  glycosuria  may  be 
brought  about  by  various  other  means,  such  as  poisoning  by 
strychnine  or  curare.  After  administration  of  chloral  and 
some  other  drugs,  a  reducing  body  appears  in  the  urine, 
which  was  formerly  thought  to  be  dextrose.  It  has  been 
proved  however  that  in  this  latter  case  the  reducing  body  is 
not  dextrose,  but  an  oxidation-product  of  dextrose,  glycuronic 
acid  (C,;H„0„). 

It  is  interesting  to  note  that,  in  the  formation  of  glycuronic  acid,  the 
oxidation  has  attacked  the  end  of  the  molecule  furthest  away  from  the  aldehyde 
COH  group,  perhaps  pointing  to  the  fact  that  the  sugar  molecule  is  attached  to 
the  protoplasmic  molecule  by  this,  its  most  unstable  group.     The  formula  of 

COOH 
glycuronic  acid  is  (CHOH)^.     It  is  apparently  being  constantly  formed  in  the 

COH. 
body  in  the  oxidation  of  dextrose,  although  special  means  are  necessary  to  fix 
it  in  this  stage  before  it  has  undergone  further  decomposition.  Thus  phenol 
when  administered  to  a  dog  is  normally  excreted  in  ethereal  combination  with 
sulphuric  acid.  If  however  the  dog  be  fed  on  a  farinaceous  diet  poor  in  proteins 
the  administration  of  phenol  is  followed  by  its  excretion  in  combination  with 
glycuronic  acid,  i.e.  an  acid  derived  from  carbohydrate  metabolism  instead  of 
one  derived  from  the  metabolism  of  proteins. 

The  results  of  the  administration  of  phloridzin  throw  some 
light  on  the  carbohydrate  metabolism  of  the  body.  Phloridzhi 
is  a  glucoside  found  in  the  root  cortex  of  apple  and  cherry 
trees.     If  a  certain    amount   of  this    be   administered    to  a 


F^TE  OF  FOODSTUFFS  IN  THE  OEGANISM       493 

dog,  sugar  appears  in  the  urine  after  a  few  hours,  and  the 
glycosuria  lasts  for  two  or  three  days.  If  the  dog  be  killed  at 
the  end  of  this  time,  the  liver  and  muscles  are  found  totally 
free  from  glycogen.  We  might  thus  come  to  the  conclusion 
that  the  sugar  was  derived  from  the  glycogen  stored  up  in  the 
body,  and  from  that  alone,  as  is  the  case  after  the  diabetic 
puncture.  But  if,  when  the  glycosuria  has  ceased  and  the 
body  is  quite  free  from  stored-up  glycogen,  a  second  dose  of 
phloridzin  be  given,  a  still  larger  amount  of  sugar  is  excreted. 
As  the  dogs  were  starved  during  the  experiment,  this  sugar 
must  have  been  derived  from  protein.  Thus  we  see  that 
sugar  as  well  as  glycogen  may  be  manufactured  in  the  body 
directly  from  protein. 

Although  both  after  administration  of  phloridzin  as  well 
as  after  diabetic  puncture,  there  is  disappearance  of  glycogen 
from  the  liver,  the  causation  of  the  glycosuria  in  the  two 
cases  is  quite  different. 

The  blood  normally  contains  about  0"05  to  0"15  per  cent, 
of  dextrose.  This  does  not  escape  into  the  urine,  since  normal 
urine  contains  only  minimal  traces  of  dextrose.  If  however 
the  percentage  of  sugar  in  the  blood  rise  above  0*2  per  cent,, 
the  excess  is  at  once  excreted  by  the  kidneys.  Thus  a  flush- 
ing of  blood  with  sugar  (either  by  excessive  absorption  from 
the  alimentary  canal,  by  artificial  injection  of  dextrose  oi 
abnormal  conversion  of  the  liver  glycogen)  will  bring  about 
glycosuria. 

After  the  injection  of  phloridzin  however,  the  amount  of 
sugar  in  the  blood  is  found  to  be  rather  diminished  than 
increased,  and  this  diminution  is  most  marked  in  the  blood 
of  the  renal  vein.  If  phloridzin  be  injected  into  one  renal 
artery,  the  corresponding  kidney  at  once  begins  to  secrete 
sugar,  and  the  kidney  of  the  opposite  side  follows  suit  only 
after  some  minutes'  interval.  We  must  conclude  therefore 
that  the  primary  action  of  this  drug  is  on  the  kidney,  exciting 
it  to  secrete  the  sugar  normally  present  in  the  blood,  and 
thus  reducing  the  percentage  of  sugar  below  normal.  The 
matter  however  will  not  rest  here.  The  function  of  the  liver 
is  to  keep  the  amount  of  sugar  in  the  blood  normal.  As  soon 
therefore  as  it  receives  the  blood  which  has  been  impoverished 
in"sugar  by  the  abnormal  action  of  the  kidneys,  it  will  take 
of  its  store  of  glycogen,  turning  out  sugar  into  the  blood  until 


.194  PHYSIOLOGY 

the  normal  composition  of  this  fluid  is  restored.  And  so  these 
contending  processes  will  go  on,  the  liver  pouring  sugar  into 
the  blood,  and  the  kidneys  at  once  excreting  it  out  of  the  body. 
This  struggle  will  continue  until  the  hepatic  store  of  glycogen 
is  exhausted,  and  then,  if  the  kidneys  are  still  under  the 
influence  of  phloridzin,  the  liver  will  attack  the  proteins  in 
order  to  keep  up  the  sugar  supply,  and  there  will  be  a  rise  of 
urea  excretion  in  the  urine  which  will  run  exactly  parallel  to 
the  amount  of  sugar  in  this  fluid.  It  has  been  suggested 
that  the  phloretin,  the  nitrogenous  constituent  of  phloridzin, 
may  act  as  a  carrier  of  sugar  across  the  renal  epithelium, 
combining  on  one  side  with  the  blood-sugar  to  form  the 
glucoside  phloridzin,  which  is  then  broken  down  again  by  the 
cells  into  sugar,  which  is  excreted  on  the  other  side,  and 
phloretin,  which  is  free  to  repeat  the  carrying  process. 

In  the  disease  known  as  diabetes,  in  which  the  patient 
passes  large  quantities  of  sugar  with  the  urine,  the  immediate 
cause  of  the  glycosuria  is  an  increased  amount  of  sugar  in  the 
blood,  amounting  in  some  instances  to  as  much  as  0*4  to  0*6 
per  cent.     Some  of  these  cases  may  be  due  to  defective  power 
of  the  liver  to  store  up  glycogen,  so  that  the  excess  of  sugar 
taken  in  with  the  food  passes  at  once  into  the  general  circula- 
tion, and  is  excreted  by  the  kidneys.     Such  cases  maybe  suc- 
cessfully treated  by  limiting  the  amount  of  carbohydrates  taken 
in  with  the  food.     This  however  cannot  be  the  explanation  of 
the  ordinary  cases  of  severe  diabetes.     In  these  the  patient 
passes  sugar  even  during  starvation  or  on  a  pure  protein  diet, 
just  as  in  the  case  of  a  dog  poisoned  with  phloridzin.     In 
these  cases  glycogen  may  be  found  in  the  liver  after  death,  so 
that  this  function  of  the  liver  is  certainly  not  wanting.     The 
passage   of   sugar   into   the  urine  probably  depends  on  the 
inability  of  the  organism  to  utilise  the  sugar  taken  in  with 
the  food  or  split  off  from  the  proteins  of  the  body.     It  is  found 
therefore  that  the  excretion   of   urea  is  also  increased.     In 
fact,  the  man  is  in  the  condition  of  an  animal  fed  on  proteins 
alone.      Since    the    carbohydrates   cannot   be   utilised,    the 
organism  has  recourse  to   the   proteins  to  cover  the  whole 
expenditure  of  energy,  and  hence  the  increased  excretion  of 
urea. 

Why  tlie  power  of  utilisation  of  the  sugar  is  defective  we 
do  not  know.     It  may  be  that  the  sugar  has  to  be  further 


FATE   OF   FOODSTUFFS   IN   THE   OEGANISM  495 

elaborated  in  certain  organs  before  it  can  be  nsed  up  by  the 
general  tissues  of  the  body.  Such  a  function  of  elaboration 
may  possibly  belong  to  the  pancreas,  since  this  organ  has 
been  found  diseased  in  about  one-fifth  of  the  fatal  cases  of 
diabetes.  Whether  or  not  the  pancreas  is  at  fault  in  all  cases 
of  diabetes  is  not  _yet  known.  There  is  no  doubt  however 
that  this  organ  is  in  some  way  connected  with  the  sugar 
metabolism  of  the  body.  If  the  pancreas  be  extirpated  in  a 
dog,  the  animal  acquires  severe  diabetes,  which  proves  fatal 
in  a  few  weeks.  The  glycosuria  continues  even  on  a  pure 
protein  diet  or  during  starvation,  and  under  these  circum- 
stances a  constant  relation  is  found  between  the  urea  and  the 
dextrose,  showing  that  the  latter  has  been  derived  from  the 
disintegration  of  protein.  The  same  conclusion  may  be  drawn 
from  the  severe  wasting  of  the  muscular  tissues  which  occurs. 
In  a  little  time  other  abnormal  products  of  metabolism 
appear  in  the  urine  oxybutyric  and  aceto-acetic  acids,  as  well 
as  aceton  derived  from  a  decomposition  of  the  latter  acid. 
In  this  respect  the  pancreatic  diabetes  resembles  the  severe 
diabetes  of  man.  The  abnormal  formation  of  these  acids  leads 
in  time  to  an  acid  intoxication,  the  animal  dying  in  a  state  of 
coma,  the  so-called  '  diabetic  coma.' 

The  exact  causation  of  this  form  of  diabetes  is  unknown, 
although  many  theories  have  been  proposed,  all  equally  un- 
satisfactory. It  is  necessary  to  extirpate  the  whole  organ, 
since  partial  excision,  ligature  of  the  duct,  or  division  of  all 
the  nerves  to  the  gland  does  not  produce  diabetes.  On  this 
account  it  has  been  suggested  that  the  epithelial  cell-nests 
(islets  of  Langerhans)  of  the  pancreas  are  the  important 
structures,  whose  excision  involves  the  production  of  diabetes. 
Pathologists  who  have  attempted  to  test  this  hypothesis  by 
an  adequate  examination  of  the  pancreas  in  fatal  cases  of 
diabetes  have  arrived  at  divergent  results.  We  have  already 
seen  however  (p.  329)  that  these  islets  in  all  probability 
represent  phases  in  the  activity  of  the  secreting  tissue  of 
the  gland,  and  do  not  need  therefore  to  be  endowed  with  a 
special  function  apart  from  that  of  the  rest  of  the  gland. 
Moreover  we  do  not  get  any  nearer  to  a  solution  of  the  ques- 
tion by  saying  that  the  pancreas  has  an  internal  secretion, 
since  we  have  no  conception  how  such  a  secretion  may  work. 
It  has  been  stated  that  in  pancreatic  diabetes  the  sugar  in 


190  PHYSIOLOPtY 

the  urine  disappears  after  excision  of  the  liver,  showing  that 
in  this,  as  in  the  other  forms  of  glycosuria,  the  liver  is  the 
great  sugar  factory  of  the  body. 

The  whole  question  of  diabetes  is  however  too  difficult  to 
be  dealt  with  at  greater  length  here.  Although  the  researches 
into  its  causation  have  thrown  some  light  on  the  normal 
carbohydrate  metabolism  of  the  body,  the  light  is  at  present 
only  sufficient  to  intensify  the  black  shadows  of  ignorance  that 
obscure  almost  every  part  of  our  knowledge  of  the  subject. 


FATE   OF   FOODSTUFFS   IN   THE   OEGANISM  497 

Section  4 
THE    SOUECE   OF   MUSCULAE   ENEEGY 

It  was  always  maintained  by  Liebig  that  the  foodstuffs 
might  be  divided  into  two  main  groups — the  carbohydrates 
and  fats  that  gave  rise  to  the  heat  produced  in  the  body, 
and  the  proteins  which  were  the  source  of  muscular  energy. 
This  view  was  however  refuted  once  and  for  all  by  the 
classical  experiment  of  Fick  and  Wislicenus  (1865).  These 
observers  ascended  to  the  summit  of  the  Faulhorn,  which  is 
1,956  metres  high.  The  urine  they  secreted  during  the  six 
hours'  ascent  and  during  the  succeeding  six  hours  was  col- 
lected, and  from  the  amount  of  nitrogen  contained  in  this 
was  calculated  the  amount  of  protein  that  was  used  up  during 
this  time.  It  was  found  that  in  the  case  of  Wislicenus  37 
grms.  protein  had  undergone  oxidation.  It  was  calculated 
that  the  oxidation  of  this  quantity  would  produce  250  heat 
units  (the  unit  of  heat  here  employed  is  that  amount  of  heat 
necessary  to  raise  the  temperature  of  one  kilogramme  of 
water  one  degree  Centigrade).  This  heat  is  equivalent  to 
about  100,000  kilogrammetres  of  work.  But  Wislicenus 
weighed  76  kilos,  so  that  in  raising  his  body  to  the  height 
of  1,956  metres  he  had  performed  76x1,956  =  148,656  kilo- 
grammetres of  work.  There  was  moreover  a  large  expenditure 
of  energy  in  the  movements  of  the  heart  and  respiratory 
muscles,  which  is  not  taken  into  account  here  ;  so  that  the 
amount  of  work  done  was  far  larger  than  could  be  accounted 
for  by  the  oxidation  of  proteins. 

If  the  income  and  output  of  a  man  be  compared  for  several 
days,  on  some  of  which  work  is  done,  while  on  others  no 
work  is  done,  it  is  found  that  the  consumption  of  oxygen  and 
the  production  of  CO^  are  much  larger  on  the  working  days 
than  on  the  resting  days,  and  that  in  fact  the  increased 
oxidation  of  carbon  which  takes  place  is  sufficient  to  account 
for  the  energy  expended.  The  nitrogen  excretion  on  the 
other  hand  is  only  slightly  increased.  This  slight  increase 
may  be  due  to  the  increased  wear  and  tear  of  the  protoplasmic 
mechanism  of  the  muscle,  and  is  not  nearly  sufficient  to 
account  for  the  energy  expended. 

32 


498  PHYSIOLOGY 

If  a  clog  be  starved  for  six  successive  days,  and  on  the 
last  three  be  made  to  do  hard  work,  it  is  found  that  the 
increase  in  the  excretion  of  urea  on  the  last  two  days  is  still 
quite  small.  As  we  have  already  seen,  one  day's  starvation 
and  hard  work  are  sufficient  to  get  rid  of  all  glycogen  from  the 
body.  Hence  it  is  evident  that  in  the  last  two  days  the 
greater  part  of  the  energy  for  the  performance  of  muscular 
work  must  have  been  derived  from  fats.  We  must  conclude 
that  normally  the  fats  and  carbohydrates  are  the  chief  sources 
of  muscular  energy.' 

If  however  an  animal  be  fed  on  a  pure  protein  diet,  work 
increases  the  excretion  of  urea,  since  the  energy  in  this  case 
has  to  be  derived  from  the  disintegration  and  oxidation  of 
proteins.  Muscle  draws  its  energy  from  all  three  classes  of 
foodstuffs.  So  long  as  carbonaceous  food  is  supplied  in  suffi- 
cient quantity,  or  is  present  in  the  body  in  the  form  of  fat  or 
glycogen,  the  muscle  chiefly  makes  use  of  this  for  the  energy 
required  in  the  contraction.  If  this  is  not  given,  or  if  the 
carbonaceous  stores  of  fat  and  glycogen  are  used  up,  muscular 
work  must  be  maintained  by  the  disintegration  and  oxidation 
of  protein. 

In  '  training '  for  severe  muscular  exertions  it  has  long 
been  customary  to  give  an  excess  of  protein  diet.  Since  in  most 
cases  the  process  of  training  involves  the  growth  of  muscular 
tissue,  and  therefore  an  addition  to  the  pro']ein  structures  of 
the  body,  a  certain  excess  is  justifiable.  Bat  there  seems  to 
be  no  object  in  increasing  the  protein  beyond  what  is  necessary 
to  supply  the  material  for  this  hypertrophy.  A  well-trained 
man  certainly  works  more  economically  (i.e.  with  less  wasteful 
heat  formation)  than  an  untrained  man,  as  is  shown  l)y  the 
respiratory  exchanges  of  the  two  individuals.  But  for  the 
production  of  this  trained  condition  of  '  fitness  '  it  is  the  con- 
stant exercise  and  the  healthy  life,  together  with  the  absence 
of  superfluous  fat  round  the  muscle  fibres,  that  are  the  chief 

'  But  not  on  a  mixed  diet,  the  sole  source.  The  absence  of  influence  of 
moderate  exercise  on  the  respiratory  quotient  shows  that  in  work  an  animal 
utilises  the  same  foodstuffs  as  at  rest,  i.e.  that  the  muscle  draws  its  energy  from 
all  three  classes  of  food.  This  increased  katabolism  would  be  followed  by  a 
greater  appetite  and  therefore  increased  intake  of  all  three  classes.  Only  when 
the  income  of  protein  is  restricted  do  we  find  a  sparing  of  the  precious  protein, 
so  that  the  energy  of  the  contracting  muscles  appears  to  come  from  the  combus- 
tion of  carbonaceous  material  alonp. 


FATE   OF   FOODSTUFFS   IN   THE   ORGANISM  499 

factors,  and  there  is  no  sufficient  proof  that  a  muscle  works 
more  economically  at  the  expense  of  protein  than  at  the 
expense  of  carbohydrate  or  fat.  In  fact  the  ultimate  and 
chief  source  of  energy  in  protein,  as  in  carbohydrates  and  fats, 
is  the  oxidation  of  the  carbon  of  the  molecules,  and  there 
seems  to  be  no  advantage  to  the  body  in  complicating  this 
process  with  the  splitting  off  of  nitrogen,  or  in  loading  the 
tissues  and  circulating  iluids  with  the  numerous  interme- 
diate products  which  occur  in  the  oxidation  breakdown  of  the 
protein  molecule. 


500  PHYSIOLOGY 


Section  5 
ANIMAL   HEAT 


All  the  energy  that  is  set  free  by  the  decomposition  and 
oxidation  of  the  foodstuffs  appears  as  work  or  heat.  It  has 
been  reckoned  that  about  nine-tenths  of  the  energ}''  set  free 
in  the  body  appears  in  the  form  of  heat,  and  one-tenth  is 
represented  by  the  work  done.  In  the  chapter  on  Muscle 
we  saw  that  its  efficiency  as  a  heat-engine  varied  within  very 
wide  limits,  the  proportion  of  heat  evolved  to  work  done  in 
muscular  contraction  being  from  5  :  1  to  25  :  1.  Even  if  we 
take  the  lowest  of  these  estimates,  we  see  that  a  very  large 
proportion  of  the  heat  produced  in  the  body  must  be  evolved 
by  the  muscles.  The  increased  production  of  heat  attendant 
on  bodily  exercise  is  familar  to  every  one.  If  the  spinal 
cord  of  an  animal  be.  excited  by  faradic  currents,  so  as  to 
cause  tetanic  contraction  of  all  the  limbs,  the  production  of 
heat  in  the  muscles  is  so  large  that  the  temperature  of  the 
animal  may  rise  to  110"  or  112''  Fahr. — a  rise  which  is  fatal. 

Next  to  the  muscles  in  importance  as  a  source  of  heat  to 
the  body  is  the  liver.  In  fact,  under  normal  circumstances 
the  temperature  of  the  blood  in  the  hepatic  vein  is  higher 
than  in  any  other  part  of  the  body.  Wherever  active  pro- 
cesses of  katabolism  are  going  on  there  is  an  evolution  of 
heat.  Probably  there  is  a  similar  evolution  of  heat  accom- 
panying the  activity  of  every  gland  in  the  body. 

We  must  also  look  upon  the  brain  as  a  source  of  heat, 
since  thermometers  inserted  in  it  may  register  a  higher 
temperature  than  that  of  the  blood  which  is  supplied  to 
the  brain.  The  amount  of  heat  produced  here  is  however 
insignificant  compared  with  that  formed  in  the  muscles  and 
liver. 

On  the  processes  of  metabolism — the  decomposition  and 
oxidation  of  foodstuffs — depend  the  maintenance  of  life. 
Hence  all  living  animals  are  continually  producing  heat  and 
imparting  it  to  the  surrounding  bodies  ;  and  unless  this  heat 
production  is  more  than  counterbalanced  by  loss  of  heat  in 
surface  evaporation,  they  must   have  a  higher  temperature 


FATE   OF   FOODSTUFFS   IN   THE   ORGANISM  501 

than  the  surrounding  medium,  although  the  difference  may 
not  amount  to  more  than  two  or  three  degrees  in  cases  where 
metabolic  processes  are  going  on  sluggishly. 

With  respect  to  their  internal  temperature,  animals  may 
be  ranged  into  two  main  classes  :  (1)  those  which  have  a 
fixed  temperature,  which  is  within  certain  limits  independent 
of  the  surrounding  medium — liomoeothermic  animals ;  (2)  those 
in  which  the  temperature  varies  with  that  of  the  surrounding 
medium — poikilothermic  animals.  To  the  first  class  belong 
birds  and  mammals,  including  man  ;  they  are  often  spoken 
of  as  warm-blooded  animals.  The  lower  Vertebrata,  including 
reptiles,  amphibians,  and  fishes,  and  the  Invertebrata,  belong 
to  the  second  class  of  poikilothermic  animals. 

We  must  now  inquire  into  the  means  by  which  this 
regulation  of  the  internal  temperature  in  warm-blooded 
animals  is  effected.  The  temperature  of  an  animal  is  the 
algebraic  sum  of  two  factors — the  amount  of  heat  produced 
and  the  amount  of  heat  lost  in  a  given  time.  If,  while  the 
heat  production  remains  constant,  the  amount  of  heat 
imparted  to  the  surrounding  medium  be  increased,  the  tem- 
perature will  fall.  If,  on  the  other  hand,  heat  loss  remaining 
constant,  heat  production  be  raised,  the  temperature  will  rise 
in  the  same  proportion.  So  the  temperature  may  be  regulated 
by  alterations  in  the  heat  production  or  in  the  heat  loss  ; 
and  if  the  temperature  is  to  remain  constant,  there  must  be 
an  accurate  correlation  between  the  two  processes. 

Begulation  of  Heat  Prochiction 

The  heat  production  in  an  animal  may  be  measured  in 
two  ways,  either  directly  or  indirectly. 

The  direct  measurement  of  the  heat  production  in  an 
animal  is  fraught  with  many  difficulties,  and  it  is  doubtful 
if  any  satisfactory  form  of  calorimeter  has  yet  been  devised 
for  the  purpose.  In  order  to  determine  the  heat  of  com- 
bustion of  any  substance,  the  ordinary  ice  or  water  calori- 
meter is  perfectly  satisfactory.  In  the  case  of  a  mammal 
however  we  have  to  maintain  a  constant  circulation  of  air 
through  the  apparatus  and  otherwise  to  keep  the  animal 
under  as  normal  conditions  as  possible.  One  of  the  best 
calorimeters  for  this  purpose  is  that  devised  by  Haldane  and 


502 


PHYSIOLOGY 


Hale  White,  in  whicli  the  heat  production  of  an  animal  is 
balanced  against  the  heat  produced  by  burning  hj^drogen. 
The  number  of  units  of  heat  given  off  b}'  the  animal  to  the 
apparatus  can  be  directly  calculated  from  tlie  number  of 
grammes  of  water  produced  by  the  l)urning  hydrogen  in  the 
same  time. 

The  calorimeter  consists  essentially  of  two  precisely  similar  chambers, 
A  and  B,  each  of  which  has  double  walls  of  sheet  copper  with  an  intervening  air- 
space. Both  air-spaces  communicate  with  the  two  limbs  of  the  manometer  m, 
which  therefore  serves  as  a  differential  thermometer,  the  slightest  difference 
of  temperature  in  one  chamber  causing  an  expansion  of  the  air  and  consequent 
displacement  of  the  fluid  in  the  tube.  Air  is  led  through  the  chambers  by  the 
tubes  T,  leaving  by  the  tubes  t',  care  being  taken  that  the  ventilation  in  both 
chambers  is  the  same.     The  air  is  dried  before  entering  by  passing  over  pumice 

Fig.  223. 


Differential  calorimeter  of  Ilaldane,  Hale  White,  and  Washbourn. 

and  sulphuric  acid.  The  air  which  leaves  also  passes  over  sulphuric  acid.  The 
animal  is  placed  in  the  cage  in  r,  and  the  hydrogen  flame  in  a  lighted,  and  the 
height  of  the  flame  is  adjusted  by  altering  the  current  of  hydrogen  until  the 
fluid  in  the  manometer  m  remains  absolutely  stationary.  The  heat  production 
in  the  two  chambers  must  now  be  the  same,  and  can  be  determined  by  weighing 
the  sulphuric  acid  bottle  on  the  exit  tube  of  a  before  and  after  the  experiment, 
the  amount  of  water  formed  giving  the  quantity  of  hydrogen  burnt  and  there- 
fore the  total  heat  produced. 

The  practical  carrying  out  of  these  experiments  involves 
BO  many  difiticulties,  that  in  most  cases  it  is  more  convenient 
to  have  recourse  to  the  indirect  method,  i.e.  to  measure  the 
output  of  the  animal  in  CO^  and  the  income  of  oxygen,  and, 
judging  from  the  respiratory  quotient  the  character   of   the 


FATE    OF   FOODSTUFFS   IX   THE   OEGANISM  50S 

foodstuffs  destroyed,  determine  from  tlie  known  heat  of 
combustion  of  these  foodstuff's  the  total  heat  production  of 
the  animal  during  the  experiment.  The  data  will  of  course 
be  more  accurate  if  the  urea  excretion  be  also  measured  and 
taken  as  an  index  to  the  protein  used  up. 

It  has  already  been  mentioned  that,  if  a  frog  or  other 
cold-blooded  animal  be  exposed  to  a  higher  temperature,  its 
internal  temperature  ^Yill  also  rise.  If,  at  the  same  time, 
we  measure  the  respiratory  interchanges  of  the  frog,  we  find 
that  at  the  higher  temperature  more  carbon  dioxide  is 
evolved  and  more  oxygen  taken  up,  showmg  that  in  this 
case  a  rise  of  temperature  in  the  surrounding  medium  causes 
a  rise  in  the  temperature  of  the  frog,  and  at  the  same  time 
increases  the  activity  of  its  metabolic  changes.  Cooling  has 
the  reverse  effect.  If  a  frog  be  cooled  to  0°  C,  the  chemical 
changes  in  its  tissues  are  so  reduced  that  it  may  be  kept 
alive  for  some  days  in  an  atmosphere  devoid  of  oxygen. 
The  case  is  quite  otherwise  with  warm-blooded  animals. 
Exposure  of  one  of  them  to  a  cold  medium  raises  the  amount 
of  carbon  dioxide  given  off'  and  oxygen  taken  in,  while  the 
temperature  of  the  animal  remains  unaltered.  This  power 
of  the  animal  to  react  to  changes  in  the  temperature  of  the 
surrounding  medium  is  dependent  on  the  integrity  of  the 
nervous  system  and  its  connection  with  the  muscles.  If  a 
dog  or  rabbit  be  poisoned  with  curare  (which  paralyses  the 
muscle  end-plates),  or  if  its  spinal  cord  be  divided  just 
below  the  medulla,  its  temperature  sinks  continuousl3^  It 
is  then  found  that  the  animal  reacts  to  changes  in  the 
temperature  of  the  surroundmg  medium  precisely  like  a 
cold-blooded  animal — rise  of  the  external  temperature 
causing  rise  of  the  internal  temperature  and  increased 
elimination  of  CO^,  while  a  fall  of  the  external  temperature 
has  the  reverse  effect. 

It  is  still  a  subject  of  debate,  what  is  the  exact  nature 
of  the  action  of  the  central  nervous  system  on  the  heat 
production  of  the  body.  It  is  evidently  effected  through 
the  muscles,  and,  partly  at  all  events,  by  means  of  muscular 
contractions.  The  increase  of  the  tone  in  the  muscles,  and 
the  general  stringing  up  of  the  body  after  a  cold  bath,  are 
an  example  of  this.  If  the  cold  be  more  severe,  the  reflex 
contractions    are   more   pronounced,   and   take   on   a   clonic 


504  PHYSIOLOGY 

character,  as  shivering  and  chattering.  A  man  too  has 
recourse  instinctively  to  muscular  exercise  to  ward  off  the 
effects  of  extreme  external  cold.  It  has  been  thought  how- 
ever that  the  nervous  system  has  a  distinct  influence  on  the 
thermogenic  properties  of  muscles  apart  from  its  action  in 
producing  muscular  contraction ;  and  that  nervous  impulses 
may  call  forth  chemical  changes  in  the  muscle  which  give 
rise  to  heat  and  heat  only.  This  view  is  supported  by 
several  phenomena,  such  as  the  increased  production  of  heat 
in  fevers,  accompanied  by  a  rapid  wasting  of  the  muscles, 
although  the  muscular  tone  in  these  cases  is  depressed 
rather  than  heightened.  If  the  anterior  part  of  the  corpus 
striatum  be  pricked  or  stimulated,  the  animal  suffers  from 
pyrexia  or  rise  of  internal  temperature  for  a  day  or  two ; 
and  this  rise  is  accompanied  by  increased  elimination  of  CO2 
and  production  of  heat.  No  special  motor  pheromena  are 
observed  in  these  experiments,  and  it  has  therefore  been 
concluded  that  the  increased  heat  production  is  due  to  a 
direct  thermogenic  action  of  the  injury. 

Begulation  of  Heat  Loss 

Far  more  important  however  than  the  regulation  of  the 
temperature  by  the  production,  is  the  regulation  by  the  loss  of 
heat.    The  channels  of  loss  of  heat  may  be  classified  as  follows  : 

1.  By  the  urine  and  faeces.  In  the  warming  of  the  food 
and  drink  taken  into  the  body,  there  must  be  a  certain 
abstraction  of  heat  from  the  tissue  surrounding  the  alimen- 
tary canal,  though  this  heat  is  not  lost  to  the  body  till  the 
warmed  urine  and  faeces  leave  it.  The  amount  lost  in  this 
way  has  been  calculated  to  be  about  3  per  cent,  of  the  total 
heat  loss. 

2.  By  the  inspired  air.  The  inspired  air  is  taken  in  at 
the  temperature  of  the  surrounding  atmosphere,  and  contains 
only  a  small  amount  of  aqueous  vapour.  The  expired  air 
has  a  temperature  of  about  one  degree  lower  than  the  body 
temperature,  and  is  saturated  with  waterj^  vapour.  Heat  is 
therefore  lost  in  respiration  in  two  ways :  1st,  in  warming 
the  inspired  air  ;  and  2nd,  in  the  evaporation  of  large  quan- 
tities of  water.  These  two  sources  of  loss  constitute  about 
20  per  cent,  of  the  total  heat  loss. 


FATE    OF   FOODSTUFFS   JN   THE   ORGANISM  505 

3.  By  the  skin.  Here  again  the  loss  of  heat  is  effected 
in  two  ways.  1st.  By  radiation  and  convection.  By  these 
means  an  interchange  of  heat  takes  place  between  the  sur- 
face of  the  body  and  surrounding  objects,  tending  to  cool  the 
body  under  ordinary  circumstances  when  the  external  tem- 
perature is  below  98*4°  F.,  or  37°  C,  or  to  warm  the  body 
when  the  external  temperature  is  higher  than  this,  as  during 
the  hot  season  in  the  tropics  or  in  a  Turkish  bath.  The 
amount  of  interchange  of  heat  between  two  bodies  is  directly 
proportionate  to  the  difference  of  temperature  between  them. 
Thus  the  warmer  the  surface  of  the  body  in  comparison  with 
that  of  surrounding  objects,  the  greater  will  be  the  amount 
of  heat  interchange,  which  in  this  case  implies  a  loss  of  heat 
to  the  body.  Since  very  little  heat  is  generated  in  the  skin 
itself,  its  temperature  is  intimately  dependent  on  the  amount 
of  blood  flowing  through  it,  and  this  in  its  turn  on  the  con- 
dition of  the  blood-vessels  of  the  skin.  When  these  are 
dilated,  there  is  a  constant  supply  of  warm  blood  from  the 
deeper  parts  of  the  body  to  the  skin,  which  therefore  is  kept 
warm  and  feels  warm,  both  subjectively  and  objectively. 
Hence  dilatation  of  the  blood-vessels  of  the  skin,  under  normal 
circumstances,  brings  about  increased  loss  of  heat.  If,  on  the 
other  hand,  the  vessels  are  constricted,  the  small  amount  of 
blood  supplied  to  the  skin  rapidly  becomes  cooled,  and  the 
skin  is  also  cool,  and  the  loss  of  heat  small. 

2nd.  By  the  evaporation  of  the  sweat.  In  the  conversion 
of  water  into  watery  vapour  a  large  amount  of  heat  becomes 
latent.  This  principle  is  made  use  of  in  making  ice,  or  in 
cooling  a  bottle  of  water  by  surrounding  it  with  damp  cloths 
which  are  exposed  to  a  draught  of  air  to  facilitate  evapora- 
tion. If  the  secretion  of  sweat  is  small  it  evaporates  as  it 
is  secreted,  and  the  skin  remains  dry.  This  is  spoken  of  as 
insensible  perspiration.  If  the  secretion  be  very  copious  it 
may  be  formed  faster  than  it  can  evaporate,  and  appears  on 
the  skin  as  drops  of  sensible  perspiration.  The  formation  of 
sensible  perspiration  depends  then  on  two  factors — the 
amount  of  sweat  secreted,  and  the  rapidity  of  evaporation, 
which  latter  again  is  dependent  on  the  amount  of  saturation 
of  the  surrounding  atmosphere  with  watery  vapour. 

The  loss  of  heat  by  the  skin  amounts  to  about  77  per  cent, 
of  the  total  heat  loss,  and  is  therefore  the  most  important  of 


506  PHYSIOLOGY 

all  the  channels  for  the  discharge  of  heat.  The  regulation 
of  the  total  heat  loss  is  also  effected  chiefly  by  changes  in 
the  loss  through  the  skin.  The  nervous  channels  by  which 
this  is  carried  out  are  the  vaso-motor  and  the  sweat  nerves. 
If  the  external  temperature  be  below  that  of  the  body,  the 
loss  by  radiation  and  convection  may  be  sufiicient  to  get  rid 
of  the  excess  of  heat  produced.  If  however  the  external 
temperature  be  higher  than  that  of  the  body,  radiation  and 
convection  will  serve  only  to  warm  the  body  still  further, 
and  the  sole  loss  of  heat  that  can  be  effected  is  by  the 
evaporation  of  sweat,  which  is  accordingly  under  such  cir- 
cumstances secreted  in  large  quantities. 

Often,  especially  after  severe  muscular  exercise,  radiation 
and  convection  are  not  sufficient  to  carry  oft"  the  excess  of  heat 
produced,  and  hence  there  is  a  copious  secretion  of  sweat 
as  well,  even  though  the  external  temperature  may  be  cool. 

The  evolution  of  heat  is  aided  mider  such  circumstances 
by  two  factors,  viz.  quickened  and  deepened  respiration,  by 
which  a  greater  volume  of  air  is  warmed  up  to  the  body 
temperature  at  the  expense  of  the  body  heat ;  and  quickened 
heart-beat,  by  which  more  blood  is  driven  through  the  dilated 
cutaneous  vessels,  and  so  a  rapid  loss  of  heat  at  the  surface 
provided  for. 

In  the  dog,  where  there  are  no  sweat-glands  on  the 
general  skin,  and  the  loss  of  heat  by  conduction  and  radia- 
tion is  checked  by  the  thick  hairy  coat,  the  quickening  of 
respiration  is  the  most  important  means  for  getting  rid  of 
the  surplus  heat  produced  in  the  body,  and  hence  the  pant- 
ing and  apparent  distress  of  these  animals  in  hot  weather. 

The  extreme  range  of  temperatures  in  a  healthy  man  at 
rest  is  between  36-1°  C.  and  37-8°  C. 

So  perfect  is  the  adaptation  of  the  heat  loss  to  the  heat 
production,  that  a  man  may  travel  from  the  poles  to  the 
equator,  may  eat  or  fast,  take  exercise  or  rest,  without  causing 
any  lasting  alteration  in  his  temperature  of  1°  C,  though 
violent  exercise  may  induce  in  many  individuals  a  temporary 
rise  of  temperature  of  2°  C. 

The  temperature  of  man,  which  varies  from  97*8°  F.  to 
98*4°  F.,  undergoes  certain  diurnal  variations,  which  are 
important,  since  they  are  reproduced  in  an  exaggerated  form 
in  many  fevers.  The  temperature  is  lowest  between  2  and 
4  a.m.,  and  highest  in  the  afternoon  between  4  and  6  p.m. 


FATE    0¥   FOODSTUFFS   IN   THE   ORGANISM  507 


Section  G 
THE   NOEMAL   DIET   OF   MAN 

To  maintain  a  man  in  perfect  health  it  is  necessary  that 
his  food  shall  contain  examples  of  the  five  different  sorts  of 
foodstuffs — proteins,  carbohydrates,  fats,  salts,  and  water. 
The  first  three  classes  serve  as  sources  of  energy  to  the  body. 
Salts  and  water  are  equally  necessary,  although  they  cannot 
serve  as  sources  of  energy. 

Water  forms  an  integral  part  of  all  living  protoplasm ; 
and  the  phenomena  of  life,  even  in  the  lowest  organisms,  are 
dependent  on  an  adequate  supply  of  this  substance.  Apart 
from  its  function  as  a  constituent  of  protoplasm,  it  is  also 
essential  as  a  medium  for  carrying  the  foodstuff's  to  the 
tissues,  and  the  waste  products  from  the  tissues  and  out  of 
the  body.  We  have  seen  that  water,  in  being  discharged 
from  the  body,  has  two  functions — as  a  solvent  of  the  effete 
nitrogenous  and  other  material  contained  in  the  urine,  and 
as  a  powerful  means  by  which  the  excess  of  heat  produced 
in  the  body  is  dissipated.  To  supply  this  loss  water  must  be 
a  constituent  of  the  foodstuff's. 

The  exact  part  played  by  salts  in  the  body  we  do  not 
know,  although  it  has  already  been  shown  that  the  presence 
of  calcium  salts  is  a  necessary  condition  for  two  physiological 
phenomena — the  clotting  of  milk  and  of  blood.  It  has  been 
shown  moreover  that  a  frog's  heart  will  go  on  beating  for 
many  hours  if  fed  with  a  solution  containing  phosphates 
and  chlorides  of  potassium,  sodium,  and  calcium,  although, 
if  any  one  of  these  salts  be  absent,  the  heart  soon  comes  to 
a  standstill.  Of  the  salts  present  in  the  body  and  taken  in 
with  the  food,  sodium  chloride  forms  the  largest  quantity. 
The  presence  of  potassium  and  phosphates  however  seems  to 
be  of  more  importance  in  the  active  phenomena  of  proto- 
plasm, since  these  are  found  in  largest  proportions  in  organs 
consisting  chiefly  of  cells  with  very  little  interstitial  sub- 
stance. It  will  be  remembered  too  that  potassium  and  phos- 
phoric acid  are  the  leading  base  and  acid  present  in  muscle 


508  PHYSIOLOGY 

and  in  blood-corpuscles.  An  animal,  if  fed  on  a  diet  free 
from  salts,  dies  almost  as  quickly  as  an  animal  that  is  starved. 
At  an  early  period  of  life  the  human  animal,  as  all  mam- 
malia, is  fed  exclusively  on  milk,  and  the  composition  of  milk 
agrees  almost  entirely  with  the  ideal  composition  of  the 
normal  human  diet,  that  has  been  worked  out  by  numerous 
authorities  as  the  result  of  many  laborious  experiments. 
Thus  it  has  been  found  that  a  man  may  maintain  himself  in 
perfect  health,  neither  gaining  nor  losing  weight,  on  a  diet 
consisting  of — 

Proteins 100  grms. 

Fats 100     „ 

Carbohydrates 240     „ 

Salts  and  water. 

In  cow's  milk  we  find  that  for  every  100  grms.  of  protein 
we  take  in  107  grms.  fat  and  140  grms.  carbohydrates. 

In  human  milk,  for  every  100  grms.  protein  there  are 
170  grms.  fat  and  270  grms.  carbohydrates. 

The  following  table  represents  the  average  compositions  of 
human  and  cow's  milk  : 


Human 

Cow's 

Gaseinogen  and  lactalbumen 

2 

.       4 

Fats 

.     2-75    .. 

.       4 

Lactose  (sugar  of  milk) 

.     5 

.       4-4 

Salts 

.     0-25    .. 

.       0-6 

Total  solids  .... 

.  10 

.     13 

Water 

.  90 

.     87 

Milk  when  fresh  is  slightly  alkaline  or  neutral.  On 
standing  exposed  to  the  air  the  lactose  is  converted  by  the 
agency  of  micro-organisms  into  lactic  acid.  The  milk  hence 
becomes  sour,  and  the  caseinogen  is  precipitated. 

Human  milk  has  a  specific  gravity  of  1025  to  1035. 

The  2Jroteins  of  milk  consist  of  caseinogen  and  lactalbu- 
men. Caseinogen,  which  forms  by  far  the  greater  quantity^ 
is  a  complex  protein  belonging  to  the  group  of  phospho-pro- 
teins.  From  milk  it  may  be  precipitated  by  the  addition  of 
acetic  acid  or  weak  mineral  acid.  When  purified  it  forms 
a  snow-white  powder,  insoluble  in  water,  but  easily  soluble  in 
dilute  alkaline  solutions,  such  as  soda,  ammonia,  lime,  or 
baryta  water.  From  these  solutions  it  may  be  reprecipitated 
by  neutralisation.  The  purified  caseinogen  when  moist  has 
the   power   of   expelling   CO^   from   certain  carbonates,  and 


FATE   OF  FOODSTUFFS   IN   THE   ORGANISM  509 

is  therefore  looked  upon  as  a  weak  acid.  In  the  milk, 
caseinogen  occurs  in  combination  with  calcium.  Its  power 
of  clotting  with  rennet  ferment  has  been  already  described 
(p.  323). 

Lactalbumen,  which  resembles  very  closely  serum  albumen, 
is  present  only  in  traces  in  cow's  milk,  but  in  much  more 
considerable  quantities  in  human  milk.  The  relatively  smaller 
amount  of  caseinogen  in  human  milk  probably  accounts  for 
the  fact  that  the  clot  produced  by  rennet  in  the  latter  is 
flocculent,  and  does  not  form  a  firm  compact  mass  as  in  cow's 
milk. 

Fats  occur  in  milk  in  the  form  of  minute  droplets  of 
various  sizes.  It  is  the  presence  of  these  which  gives  to 
milk  its  brilliant  white  appearance.  If  milk  be  allowed  to 
stand  they  rise  to  the  surface,  forming  the  layer  of  cream. 
The  droplets  are  probably  prevented  from  running  together 
by  being  surrounded  with  a  protein  envelope,  or  perhaps 
this  is  effected  simply  by  the  physical  nature  of  the  solution 
of  caseinogen  in  which  they  are  suspended.  If  cream  be 
beaten  or  churned,  this  physical  condition  is  overcome,  and 
the  fat  droplets  run  together  to  form  a  mass  known  as  butter. 
The  fats  of  milk  or  butter  consist  of  the  glycerides  of  stearic, 
palmitic,  and  oleic  acids,  with  traces  of  the  glycerides  of 
capric,  caprylic,  capronic,  and  butyric  acids. 

The  carbohydrates  are  represented  by  lactose  (Ci^H.jjO,,) 
or  milk-sugar.  This  is  much  less  soluble  in  water  than  cane- 
sugar,  and  is  only  faintly  sweet.  It  reduces  Fehling's  solu- 
tion, but  does  not  reduce  Barfoed's  solution.  It  forms  with 
phenylhydrazine  an  osazone  which  is  fairly  soluble  in  water. 
On  boiling  with  dilute  acids  it  takes  up  water  and  is  con- 
verted into  equal  parts  of  dextrose  and  galactose : 

C.,H,,0„  +  H,0  =  C,H,A  +  CeH.A. 

Dextrose  Galactose 

Under  the  influence  of  the  lactic  acid  organism  it  is  con- 
verted into  lactic  acid  :  CioH.^Oi,  +  H^O  =  4C.,H,;03.  This 
lactic  acid  is,  like  the  acid  of  muscle,  an  ethylidene  lactic  acid, 
CH3 

CH(OH),  but,  unlike  this,  is  optically  inactive.     The  souring 

COOH 

of  milk  is  due  to  this  conversion  of  lactose  into  lactic  acid. 


510  PHYSIOLOGY 

The  salts  consist  chiefly  of  the  phosphates  and  chlorides 
of  sodium,  potassium,  calcium,  and  magnesium.  Of  these 
calcium  is  present  in  the  largest  quantities  to  supply  the 
material  needed  for  the  rapidly  growing  bones  of  the  young 
animal.  The  potassium  occurs  in  far  larger  amount  than  the 
sodium,  as  would  be  expected  from  what  has  already  been 
said  concerning  the  part  played  by  potassium  in  the  functions 
of  living  protoplasm. 

Milk  also  contains  small  traces  of  iron  in  combination  with 
some  protein  body. 

The  food  of  the  adult  consists  chielly  of  meat,  eggs,  cereals, 
and  green  vegetables. 

The  following  may  be  taken  as  an  example  of  a  complete 
diet  (Waller) : 


Carijou 

Nitrogen 

[  1  pound  bread     . 

.     117  grms.     .. 

5-5  grms 

Foundation  -  h  pound  meat 

.       34     „ 

7-5     ., 

1  ^  pound  fat 

.       84     „ 

— 

/ 1  pound  potatoes 

.       45     „ 

1-3     „ 

i  pint  milk 
Accessories    ,            ^ 

,  i  pound  CKgs 

.       20     ., 
.       ].5     „ 

1-7     „ 
2 

[^  pound  cheese    . 

.       20     „ 

3 

333  21 

This  diet  is  considerably  more  liberal  than  that  given  on 
p.  508. 

Meat  consists  of  several  animal  tissues.  Muscular  tissue 
forms  the  greater  part  of  it,  though  it  also  contains  white 
fibrous  tissue  in  the  connective  tissue  and  aponeuroses,  and 
some  interstitial  fat.  There  is  also  a  greater  or  less  quantity 
of  fat  surrounding  the  muscles.  Meat  in  its  most  general 
sense  therefore  comprises  proteins  (myosin  and  albumen), 
fats,  collagen  or  (in  cooked  meat)  gelatin,  and  minute  traces 
of  carbohydrate,  as  glycogen  or  sugar. ^ 

Eggs  consist  of  two  parts,  the  white  and  the  yolk.  The 
white  is  simply  a  solution  of  egg  albumen,  with  a  small 
amount   of   globulin    and    ovo-mucin,    enclosed    in    delicate 

'  Lean  beef  contains  in  every  100  parts  — 

Proteins         ......  18-36 

Gelatiniferous  substances      .         .         .  1'64 

Fat 0-90 

Extractives 0-00 

Ash 1-30 

Water 7(v80 


FATE   OF   FOODSTUFFS   IN   THE   OEGANISM  511 

membranes.  The  yolk  contains  a  peculiar  phosphorised 
protein,  vitellin,  a  large  amount  of  fats,  salts,  and  traces  of 
sugar  and  iron.  The  latter,  as  in  the  case  of  milk,  is  present 
in  a  complex  organic  compound  allied  to  the  nucleo-proteins. 
A  hen's  egg  weighing  53  grms.  (average  weight)  contains 
31  grms.  of  white  of  egg  (albumen  and  water  with  a  small 
amount  of  globulin),  16  grms.  of  j^olk,  and  0  grms.  of  egg- 
shell. A  man  would  have  to  eat  twenty  eggs  a  day  in  order 
to  obtain  the  amount  of  protein  given  in  the  standard  diet  on 
p.  508. 

The  vegetable  articles  of  diet  are  distinguished  from  the 
animal  in  containing  a  much  larger  proportion  of  indigestible 
material,  chiefly  consisting  of  cellulose.  This  also  encloses 
much  of  the  digestible  portions  of  the  vegetable,  so  that  tliese 
also  pass  out  in  the  faeces  undigested.  On  this  account  a 
certain  amount  of  vegetable  food  is  of  importance  in  the 
normal  dietary,  since  the  indigestible  residue  increases  the 
bulk  of  the  faeces,  and  aids  the  normal  action  of  the  bowels. 

The  cereals  are  the  most  important  class  of  vegetable 
foodstuffs.  They  include  wheat,  barley,  rye,  oats,  maize,  and 
rice.  Wheat-flour,  out  of  which  bread  is  made,  contains  pro- 
teins, carbohydrates,  and  a  small  amount  of  fat  (less  than 
2  per  cent.).  The  proteins,  forming  about  12  per  cent.,  are  two 
in  number — a  protein  belonging  to  the  class  of  globulins, 
soluble  in  10  per  cent.  NaCl,  and  an  albumose.  On  treatment 
of  flour  with  water  a  change  takes  place  in  these  proteins,  and 
the  flour  becomes  sticky  and  '  doughy.'  In  the  dough  two 
proteins  are  found — gluten,  or  vegetable  fibrin,  and  a  sticky 
body,  gleiadin,  which  is  soluble  in  alcohol,  and  gives  the 
reactions  of  an  albumose.  The  carbohydrates  consist  almost 
entirely  of  starch,  which  forms  about  70  per  cent,  of  wheat- 
flour. 

Bread  is  made  by  moistening  flour  with  water,  so  as  to 
form  dough.  This  is  mixed  with  some  dough  which  has  been 
made  previously,  so  that  the  diastase  contained  in  the  flour 
has  had  time  to  act  on  the  starch,  converting  this  into  dextrin 
and  maltose.  The  mixed  doughs  are  then  kneaded  witli  a 
little  yeast,  and  set  in  a  warm  place  to  '  rise.'  By  the  action 
of  the  maltase  of  the  yeast,  the  maltose  is  converted  into 
dextrose,  and  this  is  decomposed  by  the  yeast  with  the  forma- 
tion of  alcohol  and  carbon  dioxide.     This  latter  forms  little 


512  PHYSIOLOGY 

bubbles  in  the  dough,  causing  it  to  swell  up.  The  raised  dough 
is  then  baked.  In  the  latter  process  the  starch,  exposed  to 
a  temperature  of  200°  C.  to  270^  C,  becomes  partly  converted 
into  dextrin,  and  is  therefore  rendered  soluble  in  water. 

Green  vegetables  are  chiefly  valuable  in  the  human  dietary 
owing  to  the  large  proportion  of  salts  and  cellulose  in  them. 
Potatoes  consist  nearly  entirely  of  starch,  and  contain  very 
little  protein.  Hence  to  support  life  by  this  means  alone 
very  large  amounts  must  be  taken.  It  must  be  remembered 
that  starch-grains  are  enclosed  in  a  series  of  cellulose  enve- 
lopes, and  are  therefore  indigestible  when  raw.  On  boiling 
the  starch-grains  swell,  rupturing  these  envelopes,  and  the 
opalescent  semi-solution  of  starch  thus  formed  is  easily  acted 
on  by  the  digestive  juices. 


518 


CHAPTER   XII 

THE    DUCTLESS    GLANDS 

Under  this  title  have  been  grouped  a  number  of  organs,  the 
sole  resemblance  between  which  lies  in  the  fact  that  we  know 
very  little  about  them.  Since  however  they  seem  to  exert 
important  though  obscure  influences  on  the  nutrition  of  the 
body,  they  may  be  fitly  discussed  after  the  chapter  on 
metabolism.  In  many  instances  their  functions  seem  to  be 
to  furnish  an  '  internal  secretion '  to  the  lymph  or  blood 
which  leaves  them.  This  is  almost  certainly  the  case  with 
the  thyroid  gland,  but  the  exact  nature  of  the  secretion  or  its 
action  is  in  all  cases  very  difficult  to  determine.  It  may  act 
as  an  antitoxin  to  certain  poisonous  products  of  the  normal 
activities  of  the  tissues,  or  as  a  necessary  physiological 
stimulus  to  the  growth  or  the  noi-inal  functions  of  one  or 
a  group  of  tissues. 

The  chief  ductless  glands  are  the  spleen,  thyroid,  thymus, 
and  suprarenal  capsules,  but  besides  these,  other  glands,  such 
as  the  pancreas,  ovaries,  testes,  have  been  thought  to  influence 
the  body  by  means  of  internal  secretions.  We  have  also  to 
mention  the  small  bodies  of  unknown  function  such  as  the 
pituitary  body,  the  pineal  body,  the  parathyroids,  the  carotid 
and  coccygeal  glands. 

The  Spleen 

This  organ  is  similar  in  many  respects  to  a  lymphatic 
gland.  It  is  formed  of  a  framework  of  connective  tissue  and 
unstriated  muscular  fibres,  in  the  interstices  of  which  is  con- 
tained the  sjylenic  j)?f //;.  This  consists  of  a  fine  fibrillar  net- 
work, on  the  fibrils  of  which  lie  endothelial  cells.  The  meshes 
contain  the  cells  of  the  splenic  pulp,  which  are  fairl}^  large 
polygonal  cells,  and  leucocytes.  Just  as  in  a  lymphatic  gland 
the  cellular  elements  of  the  tissues  are  bathed  by  the  lymph 

33 


514  PHYSIOLOGY 

which  HowH  through  the  gland,  so  m  the  spleen  the  walls  of 
the  capillaries  become  discontinuous,  and  the  blood  is  poured 
out  into  the  interstices  of  the  tissue.  The  spleen  is  therefore 
the  only  tissue  in  the  body  where  the  blood  comes  in  actual 
contact  with  the  tissue-elements  themselves.  The  blood  from 
the  splenic  pulp  is  collected  into  large  venous  sinuses,  which 
run  along  the  trabeculge  to  the  hilum,  where  they  unite  to 
form  the  splenic  vein.  The  arteries  to  the  spleen  are  beset 
in  their  course  along  the  trabecule  with  small  nodules  of 
lymphoid  tissue,  which  are  known  as  the  Malpighian  follicles. 
It  is  evident  that  the  blood  must  meet  with  considerable 
resistance  in  passing  through  the  close  meshwork  of  the 
splenic  pulp.     To  ensure  a  constant  circulation  through  the 

Fig.  224. 


Carotid 

dog.8'5kllo  all  connections  with  spllen 

severed  except  one  artery  &  vein  * 

0  pressure 


Plethysmogiapliic  tracing  of  spleen  (upper  curve)  from  dog,  showing 
the  spontaneous  contractions  of  this  organ  (reduced  from  a  tracing 
by  Schafer). 

gland,  we  find  that  the  muscular  tissue  of  the  capsule  and 
trabeculse  has  the  property  of  rhythmic  contraction.  If  the 
spleen  be  enclosed  in  a  plethysmograph,  or  splenic  oncometer, 
and  its  volume  be  recorded  by  connecting  this  with  the 
oncograpli,  it  will  be  seen  that  it  is  subject  to  a  series  of 
large,  slow  variations,  each  contraction  and  expansion  last- 
ing about  a  minute,  and  recurruig  with  great  regularity 
(Fig.  224).  Superposed  on  these  large  waves  are  seen  the 
smaller  undulations  due  to  the  respiratory  variations  of  the 
blood -pressure,  and  on  these  again  the  little  excursions 
corresponding  to  each  heart-beat.  The  contractile  power  of 
the  si)leen  is  under  the  control  of  the  nervous  system,  and 


THE   DUCTLESS   GLANDS 


515 


a  rapid  contraction  may  be  induced  by  stimulation   of  the 
splanchnic  nerves. 

Functiuns  of  the  Spleen 

The  structure  of  this  organ  suggests  that  the  splenic 
cells  must  exercise  a  constant  influence  on  the  blood  which 
surrounds  them,  and  that  this  influence  is  not  purely  of  ii 
chemical  nature.  In  the  liver  and  kidneys,  which  exercise 
so  powerful  an  effect  on  the  composition  of  the  blood 
passing   through  them,  the  proper  cells   of   the   organs   are 

Fiu.  225. 


Cells  fi'oiii  a  bciaping  ol  the  spleen  (Kollikei|.  A,  splenic  pulp-cell 
containing  red  blood-corpuscles,  b  (k  =  nucleus) ;  B,  leucocyte 
with  polymorphous  nucleus;  C,  pulp-cell  containing  disinte- 
grated red  corpuscles ;  D,  lymphocyte ;  E,  giant-cell ;  F, 
nucleated  red  corpuscles  ;  G,  normal  red  corpuscle ;  H,  multi- 
nuclear  leucocyte  ;  J,  eosinophile  cell. 


separated  from  the  blood-stream  by  the  capillary  wall. 
Microscopic  examination  of  the  cells  of  the  splenic  pulp 
shows  us  that  these  are  full  of  particles  of  brown  pigments 
or  fragmeiits  of  red  corpuscles  (Fig.  225).  In  many  cases 
of  infectious  disease,  such  as  recurrent  fever,  the  splenic 
cells  are  observed  towards  the  end  of  the  attack  to  be  full  of 
the  organism— spirillum — which  is  the  cause  of  the  disease. 
In  fact,  these  cells  are  so  arranged  that  they  can  take  up 
solid  particles  held  in  suspension  in  the  blood -plasma.  We 
must  indeed  look  upon  the  spleen  as  the  great  blood-filter, 
purifying  the  blood  in  its  passage  by  taking  up  particles  of 
foreign    matter    and    effete   red    corpuscles.     The    process  ot 


516  PHYSIOLOGY 

phagocytosis,  which  was  described  under  Inflammation,  is  in 
the  spleen  a  normal  occurrence. 

A  function  has  also  been  assigned  to  the  spleen  m  the 
formation  of  red  blood-corpuscles,  but  the  evidence  is  not 
sufficient  to  determine  whether  such  a  process  occurs 
normally. 

Chemical  analysis  of  the  spleen  reveals  the  presence  of  a 
large  number  of  what  are  called  extractives,  such  as  succinic, 
formic,  butyric,  and  lactic  acids,  inosit,  leucine,  xanthine, 
hypoxanthine,  and  uric  acid.  There  is  also  a  protein  allied 
to  alkali-albumen,  combined  with  iron,  as  well  as  several 
pigments  probably  derived  from  the  ha>moglobin  of  the  red 
corpuscles  destroyed  by  the  cells  of  the  splenic  pulp.  The 
fact  that,  in  cases  where  the  spleen  is  pathologically  enlarged 
as  in  leucocythccmia,  the  uric  acid  in  the  urine  is  largely 
increased,  pouits  to  a  connection  between  the  spleen  and  the 
formation  of  uric  acid  in  the  body.  The  numerous  extrac- 
tives which  are  formed  probably  owe  their  origin  to  the 
destructive  changes  effected  on  the  effete  constituents  of  the 
blood  by  the  agency  of  the  splenic  pulp-cells. 

The  Thymus  Gland 

The  thi/Diiis  is  a  body  situated  in  the  anterior  medias- 
tinum; it  is  richly  supplied  with  blood-vessels,  and  is  com- 
posed of  modified  lymphoid  tissue,  which  is  peculiar  in  con- 
taining epithelial  remnants  known  as  Hassall's  corpuscles, 
and  derived  from  the  epithelium  of  the  branchial  clefts  of 
the  embryo.  It  is  only  of  importance  in  early  life  ;  it  is 
relatively  large  in  the  fa-tus,  and  increases  in  size  during  the 
first  two  years  after  birth.  It  afterwards  atrophies,  and  in 
adult  life  is  represented  by  a  small  collection  of  adipose 
tissue.  Of  its  function  we  know  nothing.  It  is  supposed 
to  be  of  importance  in  the  formation  of  blood  in  the  young 
animal.  From  the  thymus  a  body  can  be  extracted  (tissue- 
fibrinogen)  which,  injected  into  the  veins  of  an  animal,  causes 
intravascular  clotting.  This  fact  however  does  not  throw  any 
light  on  the  normal  functions  of  the  gland,  since  a  similar 
body  may  be  extracted  from  almost  any  organ  that  is  rich 
in  cells. 


THE    DUCTLESS   GLANDS 


517 


The  Thykoid  Gland 

The  thyroid  was  probably  at  one  time  in  the  history  of 
the  race  a  secreting  gland  in  connection  with  the  alimentary 
canal.  In  the  developing  animal  it  is  formed,  like  the 
pancreas  and  liver,  as  an  outgrowth  from  the  fore-part  of  the 
alimentary  canal.  Long  before  the  end  of  fcetal  life  how- 
ever, its  duct  becomes  obliterated,  and  each  acinus  becomes 
closed.  In  the  adult  it  consists  of  two  oval  lateral  lobes 
lying  on  each  side  of  the  upper  part  of  the  trachea  and 
united  across  the  front  of  this  tube  I)}''  a  middle  lobe  or 
isthmus.  Each  lobe  is  made  up  of  a  number  of  vesicles  of 
various  sizes  imbedded   in   connective    tissue,   which  carries 

Fig.  22G. 


Section  of  thj'ioifl  ^laml  of  dofj',     (Swale  Vincent.) 


the  blood-vessels,  nerves,  and  lymphatic  vessels  of  the  gland. 
The  vesicles  are  lined  by  a  single  layer  of  cubical  epithelium 
and  filled  with  a  peculiar  hyaline  material  known  as  the 
colloid  substance  (Fig.  226).  In  some  cases  this  colloid 
material  can  be  seen  to  extend  between  the  lining  cells  into 
a  neighbouring  lymphatic  vessel.  The  same  path  is  often 
taken  by  artificial  injections,  so  that  it  seems  probable  that  the 
lymphatics  may  l)e  regarded  as  the  efferent  ducts  of  the  gland. 

The  colloid  substance  contains  a  nucleo-albumen,  and  a 
substance  known  as  thyroiodin,  owing  to  the  fact  that  it 
contains  iodine  in  organic  combination  with  protein. 

Although  its  primitive  function  of  secretion  is  lost,  it  is 


518  PHYSIOLOGY' 

still  of  the  utmost  importance  in  the  metaholism  of  the  body. 
If  the  thyroid  gland  be  totally  extirpated  in  young  animals, 
the  operation  is  followed  after  a  short  time  by  severe  sym- 
ptoms, consisting  of  fibrillar  twitchings  and  spasms  of  the 
muscles,  attended  with  weakness  and  stupor.  These  effects 
usually  terminate  in  the  death  of  the  animal.  In  man  it 
has  been  shown  that  a  disease  (myxcedema)  occurring  in  adult 
life  is  dependent  on  the  atrophy  of  this  gland.  The  main 
symptoms  of  this  disease  are  generally  stupidity  and  slowness 
of  speech  of  the  individual,  slow  pulse,  subnormal  tempera- 
ture, and  a  thickening  of  the  subcutaneous  tissues,  so  that 
the  patient  at  first  sight  looks  dropsical.  All  the  processes 
of  the  body  are  slowed.  The  nitrogenous  metabolism  is 
diminished,  as  shown  by  the  lessened  outflow  of  urea  ;  men- 
struation in  women  ceases,  the  hair  falls  out,  and  the  skin 
becomes  dry  and  harsh. 

Cretinism  is  also  associated  with  absence  of  the  thyroid 
gland.  A  cretin  remains  a  childish  idiot  all  his  life,  and 
preserves  his  childish  appearance.  In  these  cases  small 
fatty  tumours  are  often  found  on  each  side  of  the  neck  just 
above  the  clavicle.  We  are  absolutely  unable  to  explain 
the  connection  of  these  manifold  symptoms  with  the  absence 
of  the  thyroid  gland.  There  seems  to  be  no  doubt  however  that 
they  are  due  to  the  absence  in  the  body  of  some  substance 
which  is  normally  supplied  by  the  activity  of  the  thyroid 
gland,  i.e.  an  internal  secretion,  since  the  symptoms  pro- 
duced by  the  extirpation  or  atrophy  of  the  gland  may  be 
relieved  by  the  administration  of  the  fresh  or  dried  glands  of 
sheep.  It  is  interesting  to  note  that  administration  by  the 
mouth  is  as  effective  as  subcutaneous  injection,  showing  that 
the  active  substance,  whatever  it  may  be,  is  easily  absorbed 
by  the  stomach  and  is  not  destroyed  by  digestion. 

In  healthy  individuals  thyroid  extract  is  found  to  in- 
crease the  output  of  urea,  by  causing  increased  tissue  waste. 
Hence  it  has  been  used  as  a  remedy  for  obesity.  Large 
doses  cause  increased  frequency  and  force  of  the  heart-beat, 
and  it  has  been  thought  that  the  symptoms  of  exophthalmic 
goitre,  in  which  an  enlarged  thyroid  is  associated  with  pro- 
trusion of  the  eyeballs  and  great  acceleration  of  the  heart, 
may  be  due  to  an  excessive  production  of  the  specific  secre- 
tion of  the  gland. 


THE   DUCTLESS   GLANDS  519 

The  Parathyroids 

These  are  small  oval  or  rounded  bodies  about  5  to  12  mm. 
long,  situated  near  the  hinder  border  of  each  lobe  of  the 
thyroid,  and  in  some  cases  imbedded  in  the  thyroid  itself. 
They  consist  of  elongated  groups  of  polygonal  cells,  ap- 
parently epithelial,  bound  together  by  connective  tissue, 
which  also  forms  a  capsule  to  the  gland.  They  are  well 
supplied  with  blood-vessels.  Their  mode  of  development  as 
well  as  their  functions  is  unknown.  According  to  some 
authors  they  are  at  least  as  important  for  the  normal  life  of 
the  organism  as  the  thyroid  itself,  and  it  has  been  suggested 
that  the  nervous  symptoms  (muscular  twitchings  and  con- 
vulsions), which  follow  total  extirpation  of  the  thyroid  in 
animals,  are  really  due  to  the  simultaneous  removal  of  the 
parathyroids.  According  to  this  view,  if  the  parathyroids 
were  left  intact,  removal  of  the  thyroid  in  animals  would 
always  give  rise  to  a  condition  analogous  to  that  of  myx- 
cedema  in  man.  It  is  probable,  however,  that  the  parathyroids 
represent  merely  immature  thyroid  tissue,  and  have  therefore 
a  similar  function. 

The  Pituitary  Body 

This  is  a  small  body  filling  up  the  sella  turcica  of  the 
sphenoid  bone.  It  consists  of  two  lobes,  a  posterior  of 
nervous  tissue,  derived  from  the  floor  of  the  third  ventricle 
with  which  it  is  connected  by  the  infundibulum,  and  a  larger 
anterior  lobe  formed  by  an  ingrowth  of  the  epiblast  of  the 
buccal  cavity.  This  lobe  consists  of  alveoli  lined  with  epi- 
thelium and  sometimes  containing  a  material  resembling  the 
colloid  of  the  thyroid,  as  well  as  solid  masses  of  epithelial 
cells,  all  enclosed  and  bound  together  by  a  vascular  connec- 
tive tissue.  The  functions  of  this  body  are  unknown.  Its 
deep  protected  position  has  so  far  militated  against  its  suc- 
cessful extirpation.  It  has  been  found  however  that  in  a 
large  proportion  of  the  cases  of  acromegaly  (a  rare  disease 
characterised  by  overgrowth  of  the  bony  tissues  of  the  face  and 
extremities)  the  pituitary  has  been  the  seat  of  sarcomatous 
enlargement.  It  is  suggested  therefore  that  this  body  secretes 
a  substance  which  is  essential  in  some  way  or  other  for  the 


520  PHYSIOLOGY 

]n'oper  nutrition  and  growth  of  the  bones.  The  posterior  lobe 
has  been  shown  to  yield  a  sii])stanee  having  a  marked  diuretic 
efi'ect. 

The  pineal  gland  represents  the  remains  of  a  ],)rimitive 
dorsal  invertebrate  eye.  So  far  as  we  know,  it  is  at  present 
only  of  historical  interest. 

The  Surr.AP.ENAL  Capsules 

These  are  small  triangular  bodies  situated  one  above 
each  kidney.  They  consist  of  cortex  and  medulla,  the  cortex 
composed  of  groups  and  columns  of  closely  packed  polygonal 
cells,  while  the  cells  of  the  medulla  are  more  irregular  in 
shape  with  a  reticular  arrangement.  The  medulla  presents 
the  openings  of  a  number  of  large  veins.  Its  cells  contain  a 
substance  which  stains  brown  with  chromic  acid  and  its  salts. 

Here  again  pathology  has  taught  us  more  than  physio- 
logical research.  The  disorder  known  as  Addison's  disease, 
and  distinguished  by  the  three  cardinal  symptoms  of  extreme 
weakness,  vomiting,  and  pigmentation  of  the  skin  (which 
acquires  a  bronzed  colour),  was  recognised  by  its  discoverer  to 
be  due  to  atrophy  of  the  suprarenal  capsules.  The  explana- 
tions, which  were  formerly  brought  forward  of  the  connec- 
tion of  these  phenomena  with  the  extirpation  or  destruction 
of  the  capsules,  may  be  classified  into  chemical  and  nervous. 
The  suprarenals  have  a  twofold  origin,  their  medullary  part 
being  derived  from  the  sympathetic  nervous  system,  and 
their  cortical  part  from  the  surrounding  mesoblast.  The 
supporters  of  the  chemical  theoiy  based  their  views  on  the 
fact  that  derivatives  of  lia3moglobin  are  to  l)e  found  in  these 
organs,  and  they  therefore  looked  upon  them  as  chemical 
depots  for  the  removal  or  destruction  of  the  waste  pigmentary 
products.  Retention  of  these  in  the  blood  gave  rise  to 
poisoning  symptoms,  and  to  deposition  of  pigment  in  the  skin. 
As  upholding  the  function  of  these  bodies  in  influencing 
nutrition  through  the  nervous  system,  widespread  degenera- 
tions in  the  central  nervous  system  were  described  after  their 
extirpation  in  animals.  Subsequent  investigations  however 
have  failed  to  confirm  these  results. 

It  seems  more  probable  that  the  suprarenals,  in  part 
at  anv  rate,  owe  their  importance  to  the  production  of    an 


THE   DUCTLESS   GLANDS  521 

internal  secretion,  \Yhich  they  pour  into  the  blood  bathing 
their  cells.  From  the  medulla  of  the  suprarenal  glands  a 
substance,  adrenalin,  can  be  extracted  which  in  minimal  doses 
produces  most  marked  effects.  The  chief  results  of  the 
injection  of  this  substance  are  marked  rise  of  blood-pressure, 
due  to  constriction  of  all  the  arterioles  of  the  body,  and 
increased  strength  of  the  heart-beats.  The  active  principle 
is  not  a  protein,  but  is  diffusible.  It  is  a  derivative  of  pyro- 
cateehin,  from  which  body  it  may  be  prepared  synthetically 
(Dakin).  It  acts  on  nearly  all  muscular  tissues,  producing 
a  lengthening  of  the  contraction  of  skeletal  muscle  as  well  as 
the  effects  on  heart  and  blood-vessels  just  described. 

As  a  rule  its  injection  causes  great  slowing  of  the  heart- 
beats. If  however  the  vagi  be  cut,  this  effect  does  not 
appear  and  the  arterial  pressure  rises  to  two  or  three  times 
its  previous  height.  This  effect  is  not  abolished  by  previous 
section  of  the  cord  or  the  splanchnic  nerves.  This  slowing 
of  the  heart  can  be  probably  regarded  as  directly  occasioned 
by  the  great  rise  of  blood-pressure,  and  only  indirectly  by 
the  adrenalin.  It  is  interesting  to  note  that  not  all  muscular 
tissues  are  thrown  into  action  by  adrenalin.  In  some  cases 
the  muscle  may  be  inhibited  and  relaxed.  In  fact,  in  every 
case  that  has  yet  been  investigated,  the  action  of  adrenalin 
on  muscle  is  identical  with  that  produced  by  stimulation  of 
the  sijmjmthetic  nerve  supply  to  the  tissue.  Thus  we  may 
get,  besides  the  effects  already  mentioned  (viz.  vasocon- 
striction and  augmentation  of  heart-beat),  dilatation  of  the 
pupil,  erection  of  hairs,  relaxation  of  intestinal  wall  and  of 
bladder,  flow  of  saliva,  etc.  This  similarity  of  the  effects  of 
injection  of  adrenalin  to  those  produced  by  general  stimu- 
lation of  the  sympathetic  is  probably  connected  in  some  way 
with  the  development  of  the  medulla  of  the  suprarenals  as 
an  outgrowth  of  the  sympathetic  system  itself. 

It  is  supposed  that  this  substance  is  being  constantly 
produced  and  distributed  over  the  body,  its  absence  being 
responsible  for  the  extreme  prostration  and  muscular  weak- 
ness which  are  so  marked  in  Addison's  disease.  The  pigmenta- 
tion and  vomiting  of  this  disorder  still  remain  to  be  accounted 
for,  nor  have  we  yet  been  able  to  assign  any  function  to 
the  cortex  of  the  suprarenals,  forming  the  greater  bulk  of 
these  glands. 


522  PHYSIOLOGY 


CHAPTER   XIII 

SPECIAL    SENSES 

Section  1 
ON    SENSATION   IN   GENERAL 

We  must  now  consider  the  means  by  which  the  individual 
is  made  aware  of  the  events  occurring  in  the  outside  world, 
the  means  by  which  his  environment  acts  upon  him.  This 
subject  is  comprised  in  the  physiology  of  the  special  senses. 

All  the  organs  of  special  sense  contain  specially  modified 
epithelial  cells,  derived  from  the  epiblastic  layer  of  the 
embryo,  or  processes  of  these  cells.  From  the  deeper  side 
of  these  cells,  processes  or  nerve-fibres  grow  into  the  central 
nervous  system  ;  these  l)reak  up  into  fine  arborisations  of 
fibres  which  come  into  close  contact  with  other  cells  or  fibres, 
and  so  make  functional  connection  with  the  nerve-tracts 
which  serve  as  paths  to  the  higher  centres,  or  to  the  cells 
which  preside  over  the  movements  of  certain  muscles. 

In  some  cases,  such  as  the  olfactory  mucous  membrane 
(Fig.  227,  a),  the  sensory  cell  lies  close  to  the  periphery,  and 
is  the  immediate  recipient  of  the  physical  stimulus  which  it 
has  to  transmute  into  a  phj'siological  nerve  impulse. 

In  the  auditory  organ  the  special  sense-cell  seems  to  be 
represented  by  the  bipolar  cells  of  the  spiral  ganglion. 
These  (Fig.  227,  b)  send  one  process  towards  the  organ  of 
Corti,  where  it  terminates  in  fine  filaments  among  the  hair- 
cells,  and  one  running  in  the  auditory  nerve  towards  the 
medulla. 

In  other  cases  the  sensory  cell  may  lie  still  further  away 
from  the  sensory  surface.  Thus,  in  the  skin,  the  sensory 
filaments  ramifying  in  the  epidermis  represent  the  terminal 
arborisation  of  the  process  of  a  cell  in  a  posterior  root  gan- 
glion.     This    process  joins    by    a    T-shaped    junction    with 


SPECIAL   SENSES 


523 


another  process  whieli  runs  centralwards  and  terminates  in 
fine  filaments  in  the  grey  matter  of  the  spinal  cord  and 
medulla  (Fig.  228,  a). 


Fm.  227. 


:  i\^ 


A.  Connections  of  olfactory  cells  with  olfactory  lobe. 
B.  Auditory  sense-organs. 


The  peripheral  terminal  filaments  of  these  nerve-cells  may 
either  end  freely  among  ordinary  nndift'erentiated  epithelial 
cells  (as  in  Fig.  228,  15),  or  may  he  closely  applied  to  specially 


Fi(i.  228. 


:^?}y>-^-- 


A.  Connections  of  gustatory  fibres.     (Taste-bud.) 
B.  Nerve-ending  in  skin  or  corneal  epithelium  (probably  pain-fibres), 

modified  epithelial  cells.  Such  special  sensory  epithelium 
is  found  in  the  taste-huds  (Fig.  228,  a)  and  in  many  parts  of 
the  skin  (tactile  corpuscles). 


524  PHYSIOLOGY 

In  every  case  a  sensation,  whether  of  heat,  light,  sound, 
or  touch,  is  caused  l)y  some  movement  of  molecules  or  masses 
occurring  in  the  outside  world,  and  the  function  of  the  special 
sense-organs  is  to  be  acted  on  by  these  movements  and  to 
convert  them  into  a  nerve  impulse,  which  ascends  an  afferent 
nerve  towards  the  spinal  cord  or  brain.  Arrived  here,  it 
gives  rise  to  some  form  of  reflex  action,  which  may  be  un- 
conscious or  conscious.  In  the  latter  case  we  become  aware 
of  a  sensation  of  light,  heat,  or  sound,  etc.  Now  it  is  found 
that  if  the  nerve-fibres  coming  from  a  special  sense-organ  be 
stimulated  artificially  by  electric  shocks  or  in  any  other  way, 
we  get  a  sensation  similar  in  kind  to  that  wdiich  would  occur 
if  the  sense-organ  itself  were  stimulated  in  the  normal  way. 
Thus  stimulation  of  the  optic  nerve  gives  rise  to  the  sensa- 
tion of  light ;  of  the  auditory  nerve,  to  one  of  sound ;  of 
the  nerves  of  smell  or  taste,  to  these  respective  sensations. 
This  fact,  which  has  not  been  proved  experimentally  for  all 
sensory  nerves,  is  yet  so  general  that  it  has  been  formulated 
as  a  law,  known  as  Miiller's  law  of  specific  irritability.  This 
law  merely  states  that  every  sensory  nerve  reacts  to  one  form 
of  stimulus  and  gives  rise  to  one  form  of  sensation  only ;  that 
every  sensory  nerve,  in  fact,  minds  its  own  business. 

Most  important  in  connection  with  the  physiology  of  the 
senses  is  the  fact  that  in  some  cases  we  are  able  to  'project 
the  stimulus,  and  recognise  it  as  coming  from  an  object  at 
some  distance  from  us.  We  can  also  localise  the  stimulus, 
and  recognise  what  part  of  the  body  is  being  stimulated,  or 
the  position  of  the  body  in  space  from  which  the  stimulus 
arises. 

There  is  a  certain  proportionality  between  the  strength 
of  the  stimulus  and  the  intensity  of  the  sensation  produced ; 
that  is  to  say,  a  stronger  stimulus  will  produce  a  stronger 
sensation.  As  the  stimulus  is  increased,  the  amount  of 
additional  stimulus  required  to  produce  any  appreciable  in- 
crease of  sensation  is  also  increased.  Thus  we  can  dis- 
tinguish the  heavier  of  two  weights — -one  of  B9  oz.,  the  other 
of  40  oz. ;  we  cannot  however  between  39  lbs.  and  39  lbs. 
1  oz.,  but  must  add  a  whole  pound  to  the  39  lbs.  in  order  to 
appreciate  a  distinct  difference.  This  fact  is  known  as 
"Weber's  law,  which  runs  thus :  The  increase  of  stimulus 
which  is  required  to  produce  distinct  increase  of  sensation 


SPECIAL   SENSES  525 

always  hears  the  same  ratio  to  the  whole  stimulus.  In  the 
example  above  given  this  ratio  is  1  to  40  (muscular  sense). 
In  the  case  of  the  pressure  or  tactile  sense  the  ratio  is  1  to  30. 
This  law  holds  good  only  within  certain  limits,  and  fails 
when  the  stimuli  are  very  strong. 

Besides  the  five  senses  that  are  commonly  recognised — 
of  sight,  hearing,  touch,  taste,  and  smell  -physiologists 
reckon  the  senses  of  heat  and  cold,  pain,  the  muscular  sense, 
and  the  sense  of  equilibrium  or  static  sense.  The  indefinite 
sensations  of  hunger,  thirst,  weariness,  etc.,  defy  accurate 
physiological  analysis. 


526  PHYSIOLOGY 


Section  2 
CUTANEOUS   SENSATIONS 

The  whole  surface  of  the  body  is  susceptible  to  stimuli, 
which  may  give  rise  to  sensations  of  touch,  heat,  cold,  or 
pain  ;  and  it  seems  probable  that  these  different  kinds  of 
sensation  are  served  by  different  sets  of  nerve-fibres.  It  is 
very  difficult  however  to  assign  to  each  form  of  end-organ 
that  has  been  distinguished  by  anatomists  its  proper  function. 
Thus  we  find  in  the  subcutaneous  tissues  the  Pacinian 
corpuscles  as  well  as  the  various  kinds  of  end-bulbs,  and  the 
rich  end-arborisations  on  the  fibrous  tissue  bundles  known 
as  Ruffini's  endings.  In  the  papillary  layer  of  the  corium 
we  find  Meissner's  corpuscles  (the  tactile  corpuscles,  Fig.  229). 
In  the  root-sheaths  of  the  hairs  there  is  a  rich  meshwork 
of  nerve-fibrils,  especially  well  marked  in  the  tactile  hairs. 
Finally  in  the  epidermis  there  is  the  intra-epithelial  plexus 
of  non-meduUated  nerve-fibrils,  as  well  as  the  fibres  which 
end  in  connection  with  specially  modified  epithelial  cells,  the 
so-called  tactile  cells.  Although  we  regard  Meissner's  cor- 
puscles as  essentially  tactile,  there  is  no  doubt  that  the  same 
functions  must  be  served  by  the  brushlike  nerve-endings 
surrounding  the  hair  follicles.  On  the  other  hand,  the  intra- 
epithelial plexus  in  the  cornea  must  set  up  pain  impulses,  since 
these  are  the  only  sensations  which  result  from  stimulation 
of  the  cornea. 

Tactile  Sensations 

By  means  of  these  sensations  we  are  able  to  judge  of  the 
shape,  size,  and  consistence  of  bodies  in  contact  with  the 
skin.  Thus  by  feeling  we  can  tell  that  a  body  is  hard  or  soft, 
rough  or  smooth,  single  or  multiple,  etc.  These  conclusions 
however  are  not  determined  by  the  tactile  sense  alone,  but 
are  dependent  on  a  numl)er  of  other  centripetal  impulses, 
especially  those  concerned  in  the  muscular  sense  or  sense  of 
movement. 

On  careful  testing  of  the  tactile  sense,  in  minute  spots 
over  a  given  area  of  skin,  it  is  found  that  the  sensibility  is 


SPECIAL  SENSES 


527 


not  evenly  distributed  over  the  whole  surface,  but  that  the 
special  tactile  nerve-endings  occur  at  definite  spots,  between 
which  a  strictly  localised  stimulus  (as  by  the  point  of  a  hair) 
evokes  no  tactile  sensation.  These  spots  can  also  be  mapped 
out  by  means  of  localised  electrical  stimulation,  and  it  has 
been  found  that,  using  discontinuous  stimuli,  the  sensation 
produced  is  also  discontinuous  so  long  as  the  rate  of  interrup- 
tion of  the  current  does  not  exceed  130  per  second. 

The  tactile  spots  are  especially  marked  round  each  hair 
follicle,  but  a  few  spots  also  occur  between  the  hair  follicles, 


Tactile  corpuscle  from  a  papilla  of  the  skin  (Kanviei).  n,  two  nerve- 
fibres  passing  to  the  corpuscle  ;  a,  a,  varicose  ramifications  of  the 
axis-cylinders  within  the  corpuscle. 


and  of  course  are  thickly  distributed  over  the  palmar  side  of 
the  hand  and  fingers. 

Every  tactile  sensation  has  what  is  termed  its  local  sign, 
i.e.  we  are  able  to  localise  the  exact  point  at  which  the 
stimulus  is  applied  to  the  skin.  This  power  of  localisation 
is  intimately  connected  with  the  sense  of  movement  and  may 
be  impaired  by  lesions  affecting  apparently  only  the  efferent 
side  of  motor  reactions.  Intimately  connected  with  the  localisa- 
tion is  the  power  of  discrimination  of  stimuli  as  single  or 
multiple.  Thus  if  two  points  tipped  with  cork,  a  quarter  of 
an  inch  apart,  be  applied  to  the  tongue,  they  are  perceived 
as  two  points  ;  applied  to  the  skin  of  the  back  they  give  rise 


528  PHYSIOLOGY 

only  to  one  sensation.     The  following  tal>le  shows  the  dis-. 
tances  which  two  points  must  be  apart  in  order  to  cause  two 
distinct  sensations. 

Tip  of  tongue  ........  1  mm. 

Palmar  surface  of  haml,  teraiiii-il  phalanx        .         .  2  „ 

Lip 9  „ 

Front  of  forearm       .         .         .         .         .         .         .  15  „ 

Forehead 23  „ 

Back  of  hand 30  „ 

Neck,  back,  arm,  and  thigh       ....        50-70  ,, 

As  we  shall  see  later,  tactile  sensations  are  of  immense 
importance  in  the  retlex  maintenance  of  equilibrium  and  the 
performance  of  co-ordinated  movements. 


Temperatitre  Sense 

Our  subjective  feeling  of  warmth  or  cold  depends  not  on 
the  temperature  of  the  body  itself,  but  on  the  temperature  (jf 
the  skin,  where  the  special  sense-organs  are  situated.  It  has 
been  shown  that  in  the  skin  there  are  two  kinds  of  nerve- 
endings  for  temperature,  which  are  excited  respectively  by 
heat  or  cold.  Thus  if  a  small  metallic  pencil,  kept  at  body 
temperature  by  a  stream  of  warm  water  through  it,  be  moved 
gently  over  the  skin,  and  the  attention  be  directed  on  the 
sensations  evoked,  it  will  be  found  that  while  at  some  points  the 
sensations  are  indifferent  or  merely  tactile,  at  certain  points 
the  pencil  may  feel  uncomfortably  hot.  The  points  where 
this  is  found  to  be  the  case  are  mapped  out  as  heat  spots. 
By  using  a  cool  pencil  a  series  of  cold  spots  may  be  mapped 
out  in  the  same  way,  and  it  is  found  that  this  series  does  not 
coincide  with  the  first.  Fig.  230  shows  the  distribution  of 
the  heat  and  cold  spots. 

It  is  worthy  of  note  that  the  cold  spots  are  much  more 
widely  distributed  than  the  warm  spots,  and  this  difference 
is  especially  marked  in  those  portions  of  the  body  which 
are  normally  covered  with  clothes.  The  hand  is  relatively 
insensitive  to  changes  of  temperature,  and  it  may  be  observed 
as  a  general  rule  that  in  those  regions  where  the  tactile  sense 
is  best  developed,  the  temperature  sense  is  least  developed. 
It  is  evidently  of  more  importance  to  the  organism  that  it 
should  feel  discomfort  l)y  a  fall  of  temperature  than  that  it 


SPECIAL   SENSES 


529 


should  by  a  rise  of  temperature,  as  shown  by  the  prepon- 
derance of  social  contrivances,  sartorial  and  otherwise,  for 
maintaming  the  body  temperature  over  those  for  cooling  the 
body.  At  a  certain  temperature  varying  in  different  parts  of 
the  body,  the  sensation  of  '  warm  '  undergoes  a  qualitative 
change  and  becomes  that  of  '  hot,'  and  this  change  occurs 
some  degrees  lower  than  the  temperature  at  which  the 
threshold  of  pain  is  reached.  It  has  been  shown  that,  if  the 
pencil  used  for  testing  the  temperature  sense  is  heated  to 
45"  C.  or  over,  it  excites  also  the  cold  spots.  To  this  excita- 
tion of  cold  spots  is  probably  due  the  shudder  which  frequently 

Fig.  230. 


Cold  spots.  Heat  spots. 

Heat  and  cold  spots  on  part  of  palm  of  light  hand.  The  sensitive 
points  are  shaded,  the  black  being  more  sensitive  than  the  lined 
and  these  than  the  dotted  parts.  The  unshaded  areas  correspond  to 
those  parts  where  no  special  sensation  was  evoked.     (Goldscheider.) 


occurs  on  entering  a  hot  bath,  and  it  seems  that  the  feeling  of 
'  hot '  is  really  due  to  the  simultaneous  stimulation  of  both 
warm  and  cold  spots.  Thus  if  two  adjacent  spots  of  the  skm 
be  excited  by  a  double  pencil,  half  of  which  is  warm  and  the 
other  half  cold,  a  sensation  of  heat  is  produced,  whereas  if 
the  cold  pencil  be  removed,  the  remaining  sensation  is  only 
one  of  w'armth. 

Pain 

Over-excitation  of  the  nerves  of  the  skin,  whether  by 
cuttmg,  electrical  stimuli,  excessive  heat  or  cold,  produces  a 
sensation  of  pain.  Hence  it  has  been  thought  that  pain  is 
merely  an  hypertrophied  tactile  or  temperature  sensation  ;  but 

34 


530  PHYSIOLOGY 

there  are  arguments  which  tend  to  show  that  pain  is  a 
distinct  sense,  and  subserved  by  a  distinct  set  of  nerve-tibres. 
Many  cases  of  disease  occur  in  which  the  patient  can  feel 
the  slightest  touch,  but  is  quite  insensitive  to  pain.  In  other 
cases,  in  which  the  tactile  sense  is  deficient,  the  pain  sense 
may  be  exalted.  We  can  moreover  map  out  on  the  surface  of 
the  skin  pain  spots  similar  to  the  heat  and  cold  spots  men- 
tioned above.  In  certain  regions  of  the  body  only  one  or 
more  of  these  sensations  may  be  present.  Thus  the  surface 
of  the  cornea  is  sensitive  only  to  painful  impressions,  that 
of  the  glans  penis  only  to  sensations  of  pain  and  cold, 
ordinary  tactile  and  heat  sensations  being  almost  entirely 
absent. 

Direct  stimulation  of  the  trunk  of  a  nerve  going  to  the 
skin  gives  rise  only  to  a  sensation  of  pain,  whatever  may 
be  the  nature  of  the  stimulus.  Thus  plunging  the  elbow 
into  a  freezing  mixture  excites  the  ulnar  nerve,  and  causes  a 
sensation  of  pain  which  is  refei'red  to  the  ring  and  little 
iineers. 


SPECIAL   SENSES  531 

Section    3 
SENSATIONS   OP   MOVEMENT  AND   POSITION 

Under  the  above  heading  we  may  group  a  number  of  sen- 
sations which  arise  in  special  sense-organs  in  various  parts 
of  our  body,  in  consequence  of  changes  in  these  parts  them- 
selves, and  are  not  directly  occasioned  by  stimuli,  arriving 
at  the  surface  of  the  body  from  without.  As  examples  we 
may  mention  those  sensations  by  which  we  are  able  to 
tell  the  position  in  space  of  our  body,  and  the  relative 
position  of  its  various  parts,  besides  the  extent,  force,  and 
direction  of  movements  of  any  part,  whether  passive  or 
active.  Although  in  tliemselves  indistinct  and  dilhcult  of 
analysis,  these  sensations  are  of  the  utmost  importance  in 
qualifymg  other  sensations,  especially  those  of  touch  and 
sight,  and  in  imparting  to  these  sensations  the  properties 
which  lead  us  to  infer  the  position  and  extension  in  space  of 
the  object  from  which  the  stimuli  arise. 

These  sensations  may  be  divided  into  two  main  groups — 
the  muscular  sense  and  the  static  sense. 

Muscular  Sensatio)if> 

This  term  is  applied  to  those  sensations  by  which  we 
know  the  position  of  our  limbs,  the  extent  to  and  the  force 
with  which  they  have  been  moved.  Many  authors  have 
ascribed  an  important  part  in  this  knowledge  to  the  so-called 
sense  of  innervation,  i.e.  a  sense  of  the  actual  energy  which  is 
being  discharged  from  the  motor  cells  of  the  central  nervous 
system  to  the  muscles,  and  have  thought  that  when  we  raise  a 
weight  we  judge  of  its  amount,  not  by  the  degree  of  stretching 
of  the  muscle  or  pressure  on  sensory  nerves  in  the  muscle, 
but  by  the  amount  of  force  we  voluntarily  put  out  to  raise 
the  weight.  The  fact  however  that  we  can  judge  of  weights, 
when  the  muscles  are  made  to  contract  by  electrical  stimuli 
and  not  by  voluntary  impulses,  shows  that  this  sense  is  in 
large  part,  if  not  entirely,  peripheral.  It  is  liowever  very 
complex  in  nature  and  is  served  by  a  whole  array  of  different 
end-organs  in  the  skin,  joints,  tendons,  and  muscles.     The 


532 


PHYSIOLOaY 


muscles  themselves  are  known  to  be  well  supplied  with 
aft'erent  nerves.  Stimulation  of  the  central  end  of  a  mus- 
cular nerve  may  reflexly  excite  or  inhibit  movements  of  other 
muscles.  Sherrington  has  shown  that,  after  section  of  the 
motor  roots,  over  one-third  of  the  fibres  in  a  muscular  nerve 
remain  undegenerated,  proving  their  connection  with  the 
posterior  root  ganglia.  The  sensory  nerve-endings  in  the 
muscle  are  represented  partly  by  the  tendon  nerve-endings, 
and  partly  by  the  muscle-spindles. 

The  former  are  richly  branched  end-arborisations  of 
nerve-fibres  on  the  surface  of  the  tendon  bundles.  The 
muscle-spindles  consist  of  one  or  more  muscle-fibres,  often 
continuous  with  normal  fibres,  enclosed  in  a  sheath  composed 
of  several  layers  of  fibrous  tissue  with  intervening  lymph- 
spaces.     One   or   more   nerve-fibres  pierce  this  sheath  and. 

Fig.  231. 


A  neuro-muscular  spindle  of  the  cat  (llutlini).  c,  capsule ;  pr.c, 
piimaiy  ending ;  a.e,  secondary  ending  ;  pl.e,  plate  ending  (all  these 
are  probably  sensory  in  function). 


after  making  many  spiral  turns  round  the  muscle-fibres, 
branch  freely  and  terminate  in  little  knobs  on  the  surface  of 
the  fibres.  The  cross-striation  of  the  muscle-fibres  within  the 
spindle  is  but  faintly  marked.  It  is  evident  that  the  con- 
tinuity of  these  sense-organs  with  the  contracting  muscle 
ensures  in  the  best  possible  way  that  the  organs  should  be 
atiected  by  the  slightest  change  of  tension  of  the  contracting 
muscle,  and  should  transmit  information  of  the  state  of 
tension  to  the  central  nervous  system. 

These  organs,  together  with  the  special  nerve-endings 
found  on  the  tendons,  are  of  importance  in  judging  and  con- 
trolling the  force  and  extent  of  active  movements.  On  the 
other  hand  our  judgment  of   passive  movements  is  largely 


SPECIAL   SENSES  533 

determined  liy  afferent  impulses  wliich  arise  in  the  sensory 
organs  of  the  joints.  It  is  evident  that  our  sensations  of 
resistance,  and  therefore  our  whole  conception  of  force,  power, 
and  effort,  are  dependent  on  the  tension  of  contracting  muscles 
and  on  the  muscular  sensations  thereby  evoked. 

Our  knowledge  of  the  position  of  our  limbs  depends  chiefly 
on  the  complex  of  sensations  which  arise  in  the  end-organs 
of  the  skin  and  su])cutaneous  tissue  and  the  joints.  ]\[uscular 
sensations  appear  here  to  play  but  a  subordinate  part. 

We  shall  later  on  have  to  refer  to  the  importance  of  these 
afferent  impulses  for  the  carrying  out  of  normal  movements. 
We  need  here  only  mention  that,  in  the  absence  of  guiding 
afferent  impulses,  no  co-ordinated  muscular  contractions  are 
possible.  Thus  disease  of  the  sensory  nerves  of  the  muscles 
of  the  legs  or  of  the  afferent  tracts  of  the  spinal  roots  and 
cord,  such  as  occurs  in  tabes  dorsalis,  gives  rise  to  the  well- 
known  ataxj'',  in  which  there  is  an  entire  absence  of  co- 
ordination between  the  contractions  of  the  different  muscles 
of  the  lower  extremities.  In  walking,  the  leg  is  first  raised 
too  high  and  is  then  brought  down  again  on  the  ground  with 
a  jerk.  In  the  later  stages  of  the  disease,  inco-ordination  is 
so  marked  that  with  the  greatest  effort  the  patient  is  unable 
to  move  himself  along,  the  muscles  contracting  violently  and 
in  no  regular  sequence  when  he  attempts  to  walk.  If  the 
posterior  roots  of  the  hind  limbs  of  a  dog  be  divided,  he  is  at 
first  unable  to  stand  on  his  forelegs,  and  in  trying  to  move 
forward  simply  drags  the  apparently  paralysed  limbs  after 
him.  Later  on  the  dog  learns  to  walk  normally,  but  in  this 
case  he  has  simply  replaced  the  normal  afferent  impressions 
from  skin,  joints,  and  muscles  by  visual  sensations.  If  he  be 
painted  with  luminous  pamt,  and  placed  in  a  dark  room  where 
his  visual  sensations  are  absent,  the  paralysis  in  his  hind  limbs 
will  be  seen  to  l)e  as  marked  as  on  the  day  succeeding  the 
operation.  In  the  same  way  division  of  the  posterior  roots  of  a 
fore-limb  of  a  monkey  causes  paralysis  of  the  liml)  which,  for 
the  finer  movements  of  the  hand  and  fingers,  is  permanent. 

In  the  judgment  of  weights,  cutaneous  impressions  take 
little  part.  It  has  been  found  that  the  smallest  appreciable 
difference  is  about  ~._^  of  the  whole  weight — a  proportion 
which  holds  good  in  the  case  of  the  arm  for  all  weights 
between  0  and  6,000  grams. 


5B4  PiiYsroLodY 

The  Static  Sense  or  Sense  of  Posit  km 

Deeply  buried  in  the  bones  of  the  skull  and  in  close  rela- 
tion to  the  auditory  end-orf^an  is  a  special  sense-organ  whose 
function  it  is  to  guide  by  its  impulses  the  position  of  the 
head  and  therefore  of  the  rest  of  the  body,  and  l)y  these 
impulses  to  arouse  sensations  by  which  we  are  informed  of 
the  position  of  the  head  as  well  as  of  the  direction  and  extent 
of  movements,  active  or  passive,  which  the  head  may  undergo. 
This  sense-organ  is  formed  by  the  endings  of  the  vestibular 
nerve  in  the  three  semicircular  canals  and  in  the  utricle  and 
saccule  of  the  internal  ear.  This,  which  is  formed  in  the 
embr3^o  by  an  involution  of  the  epiblast  and  at  first  forms  a 
simple  sac  lined  with  epithelium  and  lying  under  the  surface 
of  the  skin,  later  becomes  modified  hj  outgrowths  in  various 
directions  to  form  a  complex  membranous  tube  lined  })y 
epithelium  and  containing  a  fluid — the  endolymph.  This 
membranous  labyrinth  is  contained  within  a  l)ony  tulje — the 
osseous  labyrintli,  which  it  does  not  wholly  fill,  being  separated 
from  the  bony  wall  over  the  greater  part  of  its  extent  by 
a  fluid — the  perilymph.  The  bony  labyrinth  consists  of  a 
cavity — the  vestibule,  communicating  by  the  fenestra  ovalis 
with  the  tympanic  cavity  externally,  and  with  the  cochlea 
and  auditory  sense-organ  anteriorly.  The  vestibule  contains 
two  sacs,  the  utricle  and  saccule,  the  latter  being  in  com- 
munication with  the  spiral  prolongation,  which  forms  the 
canalis  media  of  the  cochlea  and  is  connected  with  hearing. 
Behind  the  vestil)ule  is  in  communication  with  the  semi- 
circular canals,  three  bony  tubes  lying  in  planes  at  right 
angles  to  one  another  and  opening  into  the  vestibule  by  five 
apertures,  one  aperture  being  common  to  two  tubes.  These 
canals  are  named  respectively  the  external  or  horizontal,  the 
anterior  or  superior  vertical,  and  the  posterior  vertical 
canals.  Within  these  bony  tubes,  and  separated  from  them 
by  the  perilymph,  are  three  membranous  canals,  each  of 
which  has  a  dilatation  or  ampulla  at  one  end,  and  all  com- 
municate with  the  utricle  ;  all  these  parts  being  developed 
from  the  primitive  auditory  vesicle.  In  the  ampulla  are 
situated  the  ultimate  terminations  of  the  vestibular  division 
of  the  auditory  nerve  (Fig.  2B2\ 

When  these  canals    are  injured    definite  disturbances  of 


SPECIAL   SENSES 


535 


equilibrium  are  produced.  Thus  if  in  a  pigeon  the  hovizontal 
canal  be  divided,  the  head  is  thrown  into  a  series  of  oscil- 
lations in  a  horizontal  plane,  which  are  intensified  by  section 
of  the  corresponding  canal  on  the  opposite  side,  so  that  the 
animal  may  fall  down. 

After  section  of  the  posterior  vertical  canals  the  forced 
movements  are  in  a  vertical  plane,  and  the  animal  tends  to 
turn    somersaults    head    over   heels.      After    section    of   the 

Fig.  232. 


End-organ  of  vestibular  nerve  in  ampulla  of  semicircular  canal 
('  crista  aconstica '). 


superior  vertical  canals  the  movements  are  still  in  a  vertical 
plane,  and  the  animal  tends  to  turn  somersaults  l)ackwards. 
After  destruction  of  all  the  canals  on  both  sides  the  disturb- 
ances of  equilibrium  are  most  pronounced  and  complicated. 
The  animal  can  neither  stand  nor  tly,  nor  maintain  any 
fixed  attitude,  but  is  constantly  executing  somersaults,  and 
moving  about  so  violently  and  incoherently  that  it  is  necessary 
to  pad  its  cage  to  prevent  it  killmg  itself. 


536  PHYSIOLOGY 

After  some  months  these  disorders  graduall}^  dis.appear, 
and  the  animal  learns  to  guide  its  movements  by  sensations 
of  touch  and  sight  alone.  But  they  are  instantly  brought 
back  in  all  their  severity  if  the  eyes  be  bandaged  so  as 
to  deprive  the  co-ordinating  centres  of  the  guiding  visual 
sensations. 

It  will  be  noticed  that  the  semicircular  canals  on  each 
side  are  in  three  planes  at  right  angles  to  one  another,  and 
it  is  probable  that  we  learn  the  positive  movements  of  our 
body  with  regard  to  the  three  dimensions  of  space  by  means 
of  impressions  from  the  ampullary  endings  of  the  vestibular 
nerve.  These  impressions  are  caused  by  the  varying  pres- 
sure of  the  endolymph  on  the  ampullary  dilatations  of  the 
semicircular  canals. 

Thus  a  sudden  turning  of  the  head  from  left  to  right 
will  cause  movement  of  endolymph  towards,  and  therefore 
increased  pressure  on,  the  ampullary  nerve-endings  of  the 
left  horizontal  canal,  and  movement  of  endolymph  away 
from,  and  therefore  diminished  pressure  on,  the  correspond- 
ing ampulla  of  the  right  side.  In  this  way,  for  movement  in 
any  given  plane,  the  two  corresponding  semicircular  canals 
of  the  two  sides  are  synergic,  and  unite  in  sending  impulses 
which  guide  the  equilibrating  centres,  and  inform  us  of  the 
position  of  our  head  in  space.  '  One  canal  can  be  affected 
by  and  transmit  the  sensation  of  rotation  about  one  axis  in 
one  direction  only  ;  and  for  complete  perception  of  rotation 
in  any  direction  about  any  axis  six  semicircular  canals  are 
required  in  three  pairs,  each  pair  having  its  two  canals 
parallel  (in  the  same  plane),  and  with  their  ampulla?  turned 
opposite  ways.  Each  pair  would  thus  be  sensitive  to  any 
rotation  about  a  line  at  right  angles  to  its  plane  or  planes, 
the  one  canal  being  influenced  by  rotation  in  the  one  direc- 
tion, the  other  by  rotation  in  the  opposite  direction.'  '  The 
two  horizontal  canals  are  in  the  same  plane,  and  the  posterior 
vertical  canal  of  one  side  is  in  the  same  plane  as  the  superior 
vertical  of  the  other  (Fig.  233). 

In  the  pigeon  it  is  possible  to  cause  a  movement  of  fluid 
in  any  desired  direction  in  one  of  the  canals.  For  this  pur- 
pose one  bony  canal  is  exposed  and  plugged  in  the  middle  of 
its  course  as  a  dentist  stops  a  tooth.     The  bony^  wall  is  then 

Cinm-Brown. 


SPECIAL   SENSES 


537 


removed  immediately  in  front  of  the  stopping  and  a  small  tube 
fixed  on  the  head  l)y  means  of  plaster  of  Paris,  in  such  a 
position  that  a  blast  of  air  from  a  rubber-ball  connected  by  a 
flexible  tubing  with  the  first  tube  can  be  directed  on  to  the  open  - 
ing  in  the  canal,  so  as  to  set  up  a  current  of  endolymph  in  the 
membranous  canal.  It  is  found  invariably  that  the  animal 
responds  to  each  mechanical  stimulus  produced  in  this  way, 
with  a  movement  in  the  head  and  eyes  in  the  same  direction 
as  the  current  of  endolymph,  and  in  the  same  plane  as  the 
semicircular  canal  involved. 

These  reflex  movements  of  head  and  eyes  are  the  invari- 
able result  of  movements  set  up  in  the  endolymph,  and  occur 
equally  well  in  the  absence  of  the  cerebral  hemispheres.     If 

Fm.  238. 


Diagram  of  semieirculav  canals  to  show  their  position  in  three  planes 
at  right  angles  to  one  another.  It  will  be  seen  that  the  two  hori- 
zontal canals  lie  in  the  same  plane  (e),  and  that  the  superior 
vertical  of  one  side  (a)  is  in  the  same  plane  as  the  posterior  vertical 
(p)  of  the  other  side  (from  Esvald). 


an  animal  or  man  be  placed  on  a  turntable  and  rotated,  his 
first  tendency  will  be  to  turn  his  head  and  e^^es  in  the  opposite 
direction  to  that  of  rotation.  If  the  rotation  be  continued,  the 
endolymph  gradually  takes  up  the  movement  of  the  surround- 
ing parts  of  the  head,  and  if  the  eyes  be  closed,  no  movement 
of  head  or  eyes  is  observed.  If  now  the  rotation  be  stopped, 
the  endolymph  will  tend  to  go  on  moving,  and  the  effect 
will  be  the  same  as  if  a  movement  of  rotation  were  suddenly 
begun  in  the  opposite  direction.  Head  and  eyes  will  now  be 
turned,  without  any  voluntary  impulse,  in  the  direction  of  the 
previous  rotation  and  in  consciousness  there  will  be  an  actual 
sensation  of  rotation  in  the  opposite  direction.  This  sensation 
is  in  opposition  to  the  sensations  derived  from  other  parts. 


538  PHYSIOLOGY 

and  hence  the  feelmg  of  giddiness  and  the  aotual  disorders  of 
equilibrium  whicdi  are  its  concomitants. 

That  this  feeling  of  giddiness  on  rotation  is  due  to  impulses 
started  in  the  semicircular  canals  is  shown  by  the  fact  that, 
in  a  large  number  of  deaf-mutes  where  these  organs  are  im- 
perfectly developed,  it  is  impossible  to  produce  giddiness  and 
the  associated  eye-movements  by  passive  rotation. 

It  was  mentioned  above  that,  after  destruction  of  the  semi- 
circular canals  on  both  sides,  the  animal  learns  to  guide  its 
movements  by  sensations  of  touch  and  sight  alone.  This 
compensation  depends  however  on  the  integrity  of  that  part 
of  the  central  nervous  system  which  is  emmently  teachable, 
viz.  the  cerebral  hemispheres.  If  these  be  removed  in  a 
pigeon  and  later  on  the  membranous  labyrinth  on  each  side 
destroyed,  the  disorders  of  equilibrium  which  are  produced 
are  permanent. 

Tlie  Function  of  the  OtolitJifi 

Both  utricle  and  saccule  possess  special  nerve-endings 
known  as  the  manihr  acousticfp.  These  resemble  the  crista 
acoustica  of  the  semicircular  canals  in  structure,  being 
formed  by  an  elevation  in  the  tunica  propria  of  the  lining 
membrane,  which  is  covered  by  columnar  cells  bearing  long 
stiff  tapering  hairs  and  supported  by  thin  bipolar  sustentacular 
or  fibre-cells.  The  hairs  project  into  a  mass  of  a  soft  mucus- 
like substance  which  has  a  numl)er  of  calcareous  particles 
(otoliths)  imbedded  in  it. 

It  is  evident  tliat  the  incidence  of  the  weight  of  the 
otoliths  on  the  hairs  of  the  macula  will  vary  according  to  the 
position  of  the  head.  Since  similar  structures  have  been 
shown  (in  invertebrata)  to  be  essential  for  the  proper  equili- 
bration of  the  animal,  it  is  thought  that  the  '  otolith  organ  ' 
is  specially  adapted  for  conveying  a  sense  of  position  (static 
sense),  while  the  semicircular  canals  are  chiefly  engaged  in 
the  initiation  of  impulses  which  inform  the  central  nervous 
system  of  the  extent  and  direction  of  rotatory  movements  in- 
volving the  head. 


SPECIAL   SENSES  539 


Section  4 
TASTE   AND   SMELL 

By  means  of  our  sense  of  taste  we  distinguish  certain 
qualities  of  soluble  substances  which  are  introduced  into  the 
mouth.  By  means  of  smell  we  determine  the  qualities  of  the 
air  which  passes  through  the  upper  air-passages.  In  many 
cases,  as  in  the  appreciation  of  flavours,  both  senses  are 
employed,  and  the  sensations  are  referred  to  qualities  of  the 
food  or  other  substance  in  the  mouth.  When  the  sense  of 
smell  alone  is  involved,  the  sensations  are  projected  as  a  rule 
outside  the  body,  and  in  many  lower  animals  must  form  a 
considerable  part  of  the  substratum  on  which  the  representa- 
tion in  consciousness  of  the  external  world  is  built  up. 

Together  these  senses  represent  the  chemical  sense,  which 
is  present  to  a  greater  or  less  degree  in  all  living  organisms, 
and  which  can  generally  be  divided,  as  in  the  higher  animals, 
into  a  sense  regulating  the  intake  of  food,  and  a  sense  (corre- 
sponding to  our  projected  sensations)  which  guides  the  relations 
of  the  organism  to  its  environment.  Thus  the  attraction  of 
the  antherozoids  of  ferns  by  the  secretion  of  the  female  organ, 
the  attraction  of  the  plasmodium  of  myxomycetes  l)y  dead  leaves 
and  its  repulsion  by  quinine,  phenomena  described  under 
the  name  of  positive  and  negative  chemiotaxia,  are  exactly 
analogous  to  the  attraction  of  a  dog  by  a  bitch  in  heat,  or  to 
the  avoidance  by  ourselves  of  evil -smelling  thoroughfares. 

Taste 

The  end-organs  of  the  taste-nerves  are  represented  by 
the  taste-huds  (Fig.  234),  which  are  oval  bodies  consistmg  of 
medullary  and  cortical  parts,  the  former  being  composed  of 
columnar  cells,  the  latter  of  thin  fusiform  cells,  among  which 
ramify  the  terminal  fibres  of  the  gustatory  nerves.  These  occur 
scattered  over  the  tongue  and  soft  palate,  but  are  especially 
numerous  in  the  trenches  round  the  circumvallate  papillae.  A 
sapid  substance  to  stimulate  these  organs  must  be  in  solution  ; 
hence  qumine  in  powder  is  almost  tasteless,  owing  to  its  slight 
solubility  in  neutral  or  alkaline  fluids.     We  distinguish  four 


540 


PHYSIOLOGY 


primitive  taste-sensations,  sweet,  sour,  l)itter,  and  salt,  and  it 
is  supposed  that  there  are  different  nerve-fibres  for  each  of 
these  tastes.  The  reasons  for  this  assumption  are  as  follows : 
a.  The  tongue  is  not  equally  sensitive  at  all  points  to  all 
four  tastes.  Thus  the  back  of  the  tongue  is  more  sensitive 
to  bitter,  while  the  tip  and  sides  of  the  tongue  react  more  to 
sweet  and  acid  tastes.  Differences  can  be  detected  even 
between  the  circumvallate  papilhp  themselves ;  a  mixture  of 
quinine  and  sugar  applied  to  one  papilla  may  excite  chiefly 
bitter  taste,  while  with  an  adjacent  papilla  the  sweet  taste  may 
predominate. 

Fio.  234. 


Two  taste-buils  from  the  tongue,  e,  stratified  epithelium  ;  p,  opening 
or  pore  of  taste-bud ;  s,  gustatory  cells ;  st,  sustentacular  cells. 
(Kolliker.) 


h.  If  the  leaves  of  Gymncma  fujlvestre  be  chewed,  the 
sensations  of  bitter  and  sweet  are  abolished,  leaving  intact 
the  acid  and  salt  tastes  and  also  the  general  sensibility  of 
the  mucous  membrane.  On  the  other  hand,  cocaine  abolishes 
general  sensibility  before  it  affects  the  sense  of  taste.  Nor- 
mally the  effect  of  an  acid  is  mixed  with  the  biting  sensation 
due  to  stimulation  of  nerves  of  general  sensibility.  After 
cocaine  this  pungent  sensation  is  al)olished  and  acid  produces 
a  very  pure  and  intense  acid  taste. 

When  two  taste-sensations  are  excited  simultaneously,  the 
total  effect  depends  on  the  strength  of  the  exciting  stimuli. 
With  weak  stimuli,  one  taste  may  annul  the  other,  so  that 


SPECIAL   SENSES  541 

the  mixture  used  seems  insipid.  If  the  stimuli  be  then  in- 
creased, as  by  increasing  the  strength  of  the  soUitions  used 
{e.g.  acid,  quinine,  sugar),  a  compound  sensation  results  in 
which  the  component  simple  sensations  are  easily  distinguish- 
able. 

Most  of  our  so-called  tastes  are  dependent  on  the  sense  of 
smell.  Without  this  sense  there  would  be  very  little  difference 
between  an  onion  and  an  apple.  The  epicure  with  a  fine 
palate  has  really  educated  his  sense  of  smell  rather  than  of 
taste. 

The  nerves  of  taste  are  the  glosso-pharyngeal,  which 
supplies  the  back  part  of  the  tongue,  and  the  lingual  branch 
of  the  fifth  nerve  and  the  chorda  tympani,  which  supply  the 
front  part.  It  is  doubtful  however  whether  there  are  really 
two  nerves  of  taste.  According  to  some  authors  the  taste- 
fibres  of  the  fifth  nerve  are  derived  from  the  glosso-pharyngeal 
nerve,  perhaps  reaching  it  through  the  tympanic  plexus  and 
chorda  tympani  nerve.  Gowers  has  recorded  a  case  however 
of  complete  unilateral  loss  of  taste,  in  which  there  was  a  lesion 
destroying  the  roots  of  the  fifth  nerve,  the  glosso-pharyngeal 
being  intact.  It  seems  probable  therefore  that  the  fifth  nerve 
(as  it  rises  from  the  brain)  is  the  only  nerve  of  taste. 

Smell 

The  organ  of  smell  is  situated  at  the  upper  part  of  the 
nasal  cavities.  Here  the  mucous  membrane  covering  the 
superior  and  middle  turbinate  bones  and  the  corresponding 
part  of  the  septum  is  different  from  that  covermg  the  rest  of 
the  nasal  passages.  Over  the  lower  parts  of  the  nasal  cavities 
the  mucous  membrane  is  of  the  ordmary  respiratory  type, 
and  is  composed  of  ciliated  columnar  epithelium  containing 
a  number  of  goblet-cells.  In  the  olfactory  part  however  the 
epithelium  is  much  thicker,  of  a  yellow  colour,  and  apparently 
composed  of  a  layer  of  columnar  cells  resting  on  several  layers 
of  nuclei.  These  nuclei  belong  to  the  olfactory  cells  proper, 
true  spindle-shaped  nerve-cells  with  one  process  extending 
towards  the  mucus  covering  the  free  surface,  while  the  other 
is  continued  along  channels  in  the  bone,  and  through  the 
cribriform  plate  as  one  of  the  non-medullated  olfactory  nerve- 
fibres.     These  nerve-fibres  dip  into  the  olfactory  lobes,  where 


542 


PHYSIOLOGY 


they  terminate  Ijy  a  much-branched  arborisation  or  end-basket 
in  the  so-called  olfactory  glomeruli,  in  close  connection  with 
a  similarly  branched  dendrite  of  the  large '  mitral '  cells  of  the 
olfactory  lobe.  The  axons  from  these  latter  carry  the  olfactory 
impulse  towards  the  rest  of  the  brain  (Fig.  235).  In  the  con- 
nective tissue  basis  (dermis)  of  the  mucous  membrane  are 
a  number  of  small  mucous  orserous  glands  (Bowman's  glands) 
whose  office  it  is  to  keep  the  surface  of  the  membrane  con- 
stantly moist. 

A  substance  to  excite  a  sense  of  smell  must  be  m  a  gaseous 
condition,  although  before  ali'ectmg  the  olfactory  terminations 

liu.  235. 


Schema  of  course  of  olfactory  impulses  (Ramon  y  Cajal^  A,  olfactory 
mucous  memlnaiic  ;  B,  olfactory  ^^'lomeruli ;  C,  mitral  cells;  E, 
granule  cells  ;  D,  olfactory  tract ;  L,  centrifugal  fibres. 


it  must  be  dissolved  by  the  fluid  bathing  the  nerve  termina- 
tions. If  the  nasal  cavities  be  filled  with  rose  water,  not  only 
ig  no  smell  perceived,  but  the  sense  is  paralysed  for  some 
time  afterwards.  This  result  is  i)robably  due  to  the  injurious 
effect  of  the  water  on  the  mucous  membrane.  It  is  said  that 
if  the  nostrils  be  filled  with  normal  salt  solution  containing 
odorous  substances,  a  sensation  of  smell  is  excited,  but  in  such 
experiments  it  is  difficult  to  ensure  that  all  the  chinks  and 
crannies  of  the  olfactory  passages  are  filled  with  the  solution, 
so  as  to  exclude  the  possibility  of  the  sensation  being  excited 
in  the  ordinary  way  by  diffusion  into  the  aii-  of  these  passages. 
The  sensitiveness  of  the  olfactoi-y  is  much  greater  than 
that  of  the  gustatory  organ.     Thus  whereas  we  can   barely 


SPECIAL   SENSES  543 

taste  5  parts  in  10  inillioii  of  quinine,  applied  to  the  most 
sensitive  part,  the  l)ack  of  the  tongue,  we  can  perceive  tlie 
odour  of  niercaptan  when  it  is  so  dikited  that  1  litre  of  air 
contains  only  0*00000004  milligram  of  this  substance. 

No  satisfactory  classification  of  smells  has  yet  been  made. 
The  following  facts  however  tend  to  show  that  there  are 
a  number  of  primitive  sensations  of  smell,  as  of  other 
sensations  : 

(a)  Certain  individuals,  whose  olfactory  sense  is  in  other 
respects  normal,  have  no  power  of  distinguishmg  some  odours. 
Thus  one  may  be  without  the  sense  of  smell  for  vanilla, 
another  for  violets,  another  for  hydrocyanic  acid. 

(6)  The  olfactory  sense  is  easily  fatigued.  If  it  be  fatigued 
so  as  to  be  absolutely  insensitive  for  one  kijid  of  smell,  it  is 
still  normally  excitable  for  other  smells. 

(c)  It  is  possible  by  mixing  odoriferous  substances  in  certain 
proportions  to  annul  absolutely  their  effect  on  the  olfactory 
organ.  Thus  4  grams  of  iodoform  in  'JOO  grams  Peruvian 
balsam  is  almost  odourless,  and  the  same  neutralisatioi:i  of 
odours  is  obtained  if  the  odour  of  each  substance  be  allowed 
to  act  separately  on  each  side  by  tubes  inserted  into  each 
nostril . 


544  PHYSIOLOGY 


Section  5 
HEAKING 

Sound  is  a  sensation  in'oduced  in  our  ears  by  vibrations 
occurring  in  surrounding  bodies,  and  transmitted  to  them  by 
the  atmosphere. 

Sounds  produced  by  a  regular  series  of  vibrations  are  musical  tones ;  if  the 
vibrations  are  quite  irregular  the  effect  is  a  noise.  In  a  musical  tone  we 
can  distinguish  three  qualities,  according  to  the  character  of  the  vibrations  ; 
these  are  pitch,  loudness,  and  timbre  or  quality.  The  pitch  of  a  note  depends 
on  the  number  of  vibrations  per  second.  A  note  of  400  vibrations  is  an 
octave  higher  than  a  note  of  200  vibrations.  The  loudness  of  a  sound  depends 
on  the  amplitude  of  vibration.  The  timbre  or  quality  is  dependent  on  the 
presence  with  the  fundamental  tone  of  certain  overtones  or  harmonics.  Thus  if  we 
strike  a  piano  string,  the  fundamental  note  of  which  is  100  vibrations,  we  get 
superposed  on  this  tone  a  series  of  notes  whose  vibration  frequencies  are  2,  3,  4, 
5,  6,  7  hundred.  It  is  on  the  varying  predominance  of  these  overtones  that  the 
differences  between  the  sounds  of  a  given  note  produced  severally  by  the  organ, 
piano,  trumpet,  and  violin  depend. 

If  we  raise  the  damper  of  a  piano  and  sing  into  it,  it  will  be  noticed  that 
a  large  number  of  the  strings  go  on  vibrating.  This  is  due  to  the  fact  that 
every  note  in  our  voice  is  accompanied  by  overtones,  and  the  piano  strings  pick 
out  those  overtones  which  correspond  to  them  in  vibration  frequency  (pitch) ; 
they  are  said  to  resonate.  Instead  of  piano  strings  we  may  use  cylinders  of 
different  lengths  as  resonators,  and  by  employing  a  battery  of  these  resonators 
it  is  possible  to  analyse  all  manner  of  compound  sounds. 

The  organ  of  hearing  may  be  considered  as  consisting  of 
an  accessory  part  and  an  essential  part.  The  essential  part 
is  formed  by  the  terminal  expansion  of  the  auditory  nerve  ; 
the  accessory  part  is  constructed  so  as  to  bring  the  waves  of 
sound  to  act  on  the  end-organs. 

The  ear  is  divided  anatomically  into  three  parts  ;  the 
external  ear  with  the  auditory  meatus,  the  tympanum,  and 
the  internal  ear.  The  external  ear  in  the  lower  animals  is 
fashioned  so  as  to  collect  sound-waves  from  different  direc- 
tions ;  and  to  this  end  it  is  provided  with  muscles,  and  is  very 
movable.  This  function  in  man  is  rudimentary,  so  that  he 
can  hear  almost  as  well  with  his  ear  cut  off  as  normally. 
The  meatus  is  separated  from  the  tympanum  by  the  drum  of 
the  ear,  or  membrana  tympani.  This  is  formed  by  a  thin 
layer  of  fibrous  tissue,  covered  with  skin  externally,  and  with 


SPECIAL   SENSES  545 

mucous  membrane  of  the  tympanum  internally.  Attached  to 
the  point  of  its  inner  surface,  and  dragging  it  inwards,  is  the 
handle  of  the  malleus.  The  attachment  of  this  to  the  mem- 
brane is  eccentric — an  arrangement  which  is  of  great  import- 
ance, since  the  membrane  in  this  way  is  rendered  aperiodic, 
i.e.  it  will  vibrate  with  equal  facility  to  any  number  of  vibra- 
tions, and  not  pick  out  a  particular  note,  as  a  drum  that  was 
equally  stretched  all  round  would  do. 

The  cavity  of  the  tympanum  is  connected  in  front  with  the 
pharynx  by  means  of  the  Eustachian  tube.  This  is  opened 
by  each  movement  of  swallowing,  so  that  the  pressure  in  the 
tympanum  is  kept  equal  to  that  of  the  outside  air.  If  the 
Eustachian  tube  be  blocked  by  disease,  the  air  in  the 
tympanum   is   absorbed,    and    the   patient  becomes  deaf   on 

Fig.  23C. 


Diagram  of  auditory  meatus,  with  tympanum  and  auditory  ossicles. 

that  side.  The  inner  wall  of  the  tympanum  presents  in 
the  dried  skull  two  openings,  the  fenestra  ovalis  and  the 
fenestra  rotunda.  The  former  opens  into  the  vestibule  of  the 
internal  ear  and  is  normally  closed  by  the  base  of  the  stapes. 
The  fenestra  rotunda  leads  into  the  scala  tympani  of  the 
cochlea,  and  is  closed  in  the  fresh  skull  by  an  elastic  mem- 
brane. Stretching  across  the  tympanum,  from  the  membrana 
tympani  to  the  outer  wall  of  the  internal  ear,  is  a  chain  of 
ossicles,  the  malleus,  incus,  and  stapes.  The  base  of  the 
stapes  is  inserted  into  the  fenestra  ovalis,  being  joined  to  its 
margins  by  a  membrane.  This  chain  of  bones  acts  as  a 
system  of  levers,  by  which  the  vibrations  of  the  tympanic 
membrane  are  transmitted  to  the  fluid  in  the  internal  ear. 

The  excursion  at  the  end  of  the  lever  formed  by  the  stapes 
is  only  two-thirds  of    the   excursion   of   the   handle   of    the 

35 


546  PHYSIOLOGY 

malleus,  so  that,  in  their  transmission  through  the  ossicles, 
the  vibrations  are  diminished  in  extent  but  increased  in 
force. 

The  tensor  tympani  muscle,  which  is  attached  to  the 
handle  of  the  malleus,  serves  by  its  contraction  to  draw  this 
in,  and  to  render  the  membrane  more  tense,  and  therefore 
more  easily  affected  by  high  notes.  The  stapedius  muscle, 
when  it  contracts,  tilts  the  stapes  backwards.  Its  use  is 
unknown. 

The  internal  ear  consists  essentially  of  a  membranous  sac, 
which  is  formed  by  an  involution  of  the  epithelium  covering 
the  surface  of  the  embryo.  In  the  course  of  development  the 
sac,  which  is  filled  with  a  fluid  called  endolymph,  becomes 
much  modified  in  shape  (Fig.  237),  forming  from  before  back- 

FiG.  237. 


The  membranous  labyrinth,     cm.  Canalis  or  scala  media  of  the  cochlea. 
B.  Saccule,     u.  Utricle,     s.c.  Semicircular  canals. 


wards  the  scala  media  of  the  cochlea,  the  saccule,  the  utricle, 
and  the  three  semicircular  canals.  At  certain  parts  of  its  inner 
surface  thickenings  of  the  epithelium  occur,  which  become 
connected  with  the  terminations  of  the  auditory  or  eighth 
nerve.  This  '  membranous  labyrinth '  lies  inside  a  casing 
of  bone,  from  which  it  is  separated  by  a  layer  of  fluid  called 
the  perilymph.  The  osseous  labyrinth  is  formed  from  before 
backwards  by  the  cochlea,  vestibule,  and  semicircular  canals. 
The  cochlea  (Fig.  238)  is  a  spiral  tube  of  bone,  20  to  80  mm. 
long,  divided  by  the  scala  media  into  two  parts,  the  scala 
vestibuli  and  the  scala  tympani,  which  are  continuous  at  the 
apex  of  the  spiral  (helicotrema).  The  scala  media  contains 
the  essential  part  of  the  organ  of  hearing,  which  is  called  the 
organ  of  Corti  (Fig.  289).  This  consists  of  a  double  row  of 
stiff  cells— the  inner  and  outer  rods  of  Corti,  sujjporting  on 


SPECIAL   SENSES 


547 


each  side  uiie  or  three  rows  of  hair-cells,  which  are  connected 
with  the  terminations  of  the  auditory  nerve.  The  organ 
rests  on  the  basilar  membrane,  which  is  composed  of  a 
nmiiber  of  elastic  fibrils  stretched  in  a  radial  direction  from 
the  central  axis  of  the  cochlea  to  the  middle  of  the  wall  of 

Fir..   238. 


Vertical  section  tluongh  the  cochlea. 

the  spiral.  The  length  of  the  fibrils  forming  the  basilar 
membrane  increases  from  0"041  mm.  at  the  base  to  0*495  mm. 
at  the  helicotrema. 

Somid-waves  falling  on    the   ear   are   collected   into   the 
meatus,  and  strike  the  membrana  tympani.     The  vibrations 


Fk;.  239. 


ni.t 


Section  through  the  end-organ  of  the  auditory  nerve  in  the  eoclilea 
(organ  of  Corti).  b.m.  Basilar  membrane,  c.  Canal  of  Corti.  e.g.  Rods 
of  Corti.  IH.  and  oh.  Inner  and  outer  hair-cells,  s.c.  Sustentacular 
cells.     An.  Auditory  nerve,     m.t.  Membrana  tectoria. 


of  the  membrane  thus  produced  are  transmitted  with  dimi- 
nished amplitude  but  increased  force  by  the  chain  of  ossicles 
to  the  fenestra  ovalis,  where  they  are  communicated  to  the 
perilymph.  The  vibrations  travel  in  the  perilymph  from  the 
vestibule  to  the  scala  vestibuli,  up  the  turns  of  the  cochlea  to 
the  helicotrema,  and  then  back  again  along  the  scala  tympani, 


548  PHYSioL()(n- 

a'ld  end  on  the  membrane  closing  in  the  fenestra  rotunda, 
situated  in  the  inner  wall  of  the  tympanum  at  the  base  of  the 
scala  tympani.  Every  movement  inwards  of  the  base  of  the 
stapes  causes  therefore  a  bulging  of  the  membrane  closing 
the  fenestra  rotunda.  In  their  course  the  vibrations  set  the 
basilar  membrane  of  the  scala  media  into  vibration,  and  in 
this  way  aftect  the  hair-cells  and  the  terminations  of  the 
auditory  nerve. 

The  fact  that  in  many  cases  we  are  able  to  resolve  the 
compound  sound  into  its  simpler  components,  that  a  musician 
can  name  the  notes  forming  a  chord  struck  on  the  piano, 
shows  that  there  must  be  some  mechanism  in  the  ear  by 
which  the  sounds  are  analysed.  This  mechanism  is  supposed 
to  be  furnished  by  the  basilar  membrane.  It  is  thought 
that  the  longer  fibres  near  the  apex  of  the  cochlea  vibrate 
only  to  low  notes,  and  that  the  shorter  fibres  near  the  base 
of  the  cochlea  vibrate  only  to  high  notes,  and  that  when 
a  chord  is  struck  it  sets  into  vibration  fibres  of  the  basilar 
membrane  at  different  parts  of  the  cochlea,  each  of  which 
excites  the  hair-cells  and  auditory  nerve-endings  lying  imme- 
diately on  it,  giving  rise  to  a  series  of  simple  sensations. 

The  objection  that  has  been  raised  to  this  theory  of 
Helmholtz  is  that  there  is  not  enough  difference  between  the 
lengths  of  the  fibres  of  the  basilar  membranes  at  different 
points  to  account  for  the  range  of  sounds  which  it  is  possible 
to  perceive  and  analyse.  It  has  therefore  been  suggested  by 
Ewald  that  the  whole  basilar  membrane  vibrates,  but  with 
nodal  points,  so  that  a  vibrating  pattern  is  impressed  on  the 
nerve-endings,  the  position  of  the  nodal  points,  and  therefore 
the  combination  of  fibres  excited,  differing  with  each  tone  or 
combination  of  tones. 

Another  theory  would  avoid  the  difficulty  altogether  by 
assuming  that  the  basilar  membrane  vibrates  as  a  whole  like 
a  telephone  plate,  and  that  the  impulses  ascending  the  whole 
auditory  nerve  differ  in  quality,  reproducing  physiologically 
the  varying  characters  of  the  physical  disturbances  by  which 
they  were  caused.  Under  this  theory  the  whole  process  of 
analysis  is  relegated  to  the  central  nervous  system. 

Difficulties  in  the  physiological  theory  of  audition  are  represented  by  the 
so-called  combination  and  difference  tones.  If  two  tuning-forks,  one  vibrating 
200  and  the  other  250  times  per  second,  be  set  vibrating,  we  hear  not  only  the 


SPECIAL   SENSES  549 

notes  corresponding  to  these  vibration  frequencies  but  also  two  others,  one  of 
50  vibi'ations  per  second  (the  difference  tone)  and  one  of  450  vibrations  (the 
combination  tone).  Since  it  is- said  that  these  tones  do  not  exist  physically,  we 
must  conclude  that  the  sensations  corresponding  to  them  are  in  some  way  excited 
in  the  auditory  apparatus  or  central  nervous  system  by  the  original  two  stimuli 
of  200  and  250  vibrations  per  second. 

The  rato  of  vibration  frequency,  witliin  wliieli  an  audible 
note  is  produced,  may  extend  from  IG  to  10,000  vibrations 
per  second,  although  in  most  people  no  sound  is  produced  by 
vibration  frequencies  below  30  or  above  30,000.  According 
to  Exner,  two  sounds  following  one  another  are  perceived  as 
distinct  if  the  interval  between  them  is  not  less  than  0*002 
second. 


)50  PHYSIOLOGY 


Section  6 
VISION 


In  treating  of  the  functions  of  the  eye,  the  organ  of  vision, 
we  have  to  consider  the  essential  part,  the  termination  of 
the  optic  nerve  or  retina,  and  the  accessory  part,  a  series 
of  dioptric  mechanisms,  arranged  to  form  a  perfect  image  of 
external  objects  on  the  retina. 

Since  the  two  eyes  are  generally  employed  together,  we 
have  also  to  discuss  binocular  vision  ;  and  lastly  the  cerebral 
processes  engaged  in  the  formation  of  visual  sensations  and 
judgments. 

The  Manner  in  which  a  Distinct  Image  of  External 
Objects  is  formed  on  the  Retina 

The  eye  may  be  compared  to  a  photographic  camera,  the 
lens  ))eing  represented  by   several  refracting   surfaces,    the 

Fig.  240. 


cornea,  lens,  and  vitreous  humour,  and  the  sensitive  plate  on 
which  the  image  is  formed  by  the  retina. 

A  ray  of  light  when  passing  obliquely  from  a  medium  of 
low  density  (such  as  the  air)  to  a  medium  of  high  density 
(such  as  water  or  glass)  changes  its  course,  being  bent 
towards  the  perpendicular  drawn  to  the  surface  separating 
the  two  media.  On  leaving  the  dense  for  a  rarer  medium  it 
is  bent  once  more  away  from  the  perpendicular. 


SPECIAL   SENSES 


551 


Figs,  240  and  241  represent  the  course  of  a  ray  of  light 
in  passing  through  a  plate  of  glass  with  parallel  sides,  and 
through  a  prism. 

Fm.  241. 


By  means  of  a  convex  lens  the  rays  of  light  from  any  one 
source  may  all  be  refracted  so  as  to  meet  at  a  point.  The 
point  at  which  parallel  rays  of  light  (such  as  the  sun's  rays) 
meet  is  called  the  principal  focus  of  the  lens  (Fig.  242). 


Fig.  242. 


Diagram  of  the  course  of  parallel  rays  through  a  biconvex  lens,  by 
which  they  are  converged  to  the  principal  focus,  F. 

If  the  origin  of  the  rays  be  a  point  of  light  near  the  lens, 
so  that  the  rays  are  not  parallel,  they  are  converged  by  the 
lens  to  a  point  (secondary  focus)  situated  further  away  from 

Fig.  243. 


The  rays  of  light  from  A  converge  on  passing  through  the  lens  to 
the  secondary  focus,  F.     F  and  A  are  conjugate  foci. 


the  lens  than  the  principal  focus.  The  two  points,  the  point 
whence  the  rays  of  light  diverge  and  the  point  to  which  they 
converge,  are  caMed  conjugate  foci  (Fig.  2481. 


552 


PHYSIOLOaY 


111  the  eye  there  are  several  surfaces  separating  different 
media  where  refraction  takes  place   (Fig.    244).     Since  the 


Fig.  2-i4. 


Section  through  eyeball  to  show  refractive  media.  Sc.  Sclerotic 
coat.  Ch.  Choroid  coat.  V.  Vitreous  humour.  L.  Lens.  S.  Suspen- 
sory ligament  of  lens.  ON.  Optic  nerve.  C.  Cornea.  A.h.  Aqueous 
humour.     Ir.  Iris. 


refractive  index  of  the  aqueous  humour  is  almost  equal  to 
that  of  the  cornea,  we  may  reduce  the  refracting  surfaces  to 
three,  viz. — 

Anterior  surface  of  cornea. 

Anterior  surface  of  lens, 

Posterior  surface  of  lens  ; 
and  the  refracting  media  to  three — 

Aqueous  humom*  (or  cornea), 

Lens, 

Vitreous  humour. 
These  are  so  adapted  in  the  normal  eye  that  parallel  rays 
falling  on  the  cornea  are  converged  to  a  focus  at  the  yellow 
spot  on  the  retina.  This  pomt  therefore  represents  the 
principal  focus  of  the  eye.  A  line  drawn  from  this  point 
through  the  centre  of  the  cornea  is  the  optic  axis  of  the 
eyeball. 

The  Mechanism  of  Accommodation 

But  we  are  able  also  to  form  a  distinct  image  of  near 
objects  on  the  retina,  and  we  notice  that,  when  we  turn  our 
gaze  from  far  to  near  objects,  there  is  a  distinct  feeling  of 


SPECIAL  SENSES  553 

muscular  eftbrt  in  the  eyes.  There  must  tlien  be  some 
means  l)y  wliich  tlie  eye  can  l)e  altered  and  arranged  for 
focussing  near  objects.  In  a  photographic  camera  the  focus 
may  be  aUered  either  by  changing  the  lens,  putting  in  one  of 
greater  or  less  curvature,  or  by  altering  the  distance  of  the 
screen  from  the  lens.  The  last  method  is  obviousl}'  imprac- 
ticable in  the  rigid  e3'eball,  and  we  fnid  that  the  act  of 
focussing  (or  accommodating)  for  near  objects  is  associated 
with  a  change  in  the  curvature  of  the  lens,  which  becomes 
more  convex  on  its  anterior  surface. 

This  may  be  easily  shown  by  means  of  the  phakoscope 
(Fig.  245).  This  is  simply  a  box,  blackened  inside,  with 
holes  at  a,  b,  c,  and  d.  At  (a)  is  the  observer's  eye;  at  (b)  the 
observed  eye.     Across  the  middle  of  (d)  a  wire  is  stretched. 


Diagram  of  phakoscope. 

A  candle  is  placed  at  (c).  The  observer  at  (a)  then  sees 
three  reflections  of  the  candle  from  the  eye  at  (b)  :  a  bright 
erect  image  from  the  anterior  surface  of  the  cornea ;  a 
larger  but  dimmer  erect  image  from  the  anterior  surface  of 
the  lens  ;  and  a  small  very  dim  inverted  image  from  the 
posterior  surface  of  the  lens.  These  images  must  be  observed 
first  when  the  eye  at  (b)  is  accommodated  for  a  distant 
object,  and  then  when  it  is  accommodated  for  the  wire 
stretched  across  the  opening  (d).  It  will  be  noticed  that  the 
change  of  accommodation  from  far  to  near  objects  is  accom- 
panied by  a  change  in  the  second  image  (that  from  the 
anterior  surface  of  the  lens),  which  becomes  smaller.  The 
change  in  this  image  is  more  easily  seen  if  the  candle  be 
made  to  throw  two  images  on  the  eye  by  interposing  a  double 
prism   at    (c).     Then,   as   the  lens  becomes  more  convex  to 


554  PHYSIOLOGY 

accommodate  for  near  ol)jects,  the  two  images  of  the  candle 
reflected  from  its  anterior  sm'face  approach  one  another 
(Fig.  240). 

We  must  now  inquire  how  this  cliange  in  the  shape  of  the 
lens  is  brought  about. 

By  measuring  the  size  of  the  image  of  the  candle  produced 
by  the  anterior  surface  of  the  lens,  and  knowing  the  size  of 
the  candle  itself  and  the  distance  from  the  observed  eye,  it 
is  possible  to  calculate  the  curvature  of  the  lens  in  the  living 
body. 

The   radius   of   curvature   of   a   reflecting  surface  is   given  approximately 

2  a  b 

by  the  following  formula :  r=— ^ — ,  where  r  is  the  radius  of  curvature,  a  the 

c 

distance  of  the  object,  b  the  size  of  the  image,  and  c  the  size  of  the  object. 

Fio.  246. 


Diagram  of  reflected  images  from  cornea  and  lens  surfaces  seen  in 
phakoscope.  a.  From  anterior  surface  of  cornea,  b.  From  anterior 
surface  of  lens.  c.  From  posterior  surface  of  lens.  1.  During 
accommodation  for  distance.  2.  During  accommodation  for  near 
objects. 


Of  a,  b,  and  c  the  only  measurement  which  presents  any  difHculty  is  b,  the 
size  of  the  image.  For  this  purpose  therefore  the  ophthalmometer  was  devised 
by  Helmholtz.  The  principle  of  this  instrument  may  be  gathered  from  the 
diagram  (Fig.  247).  We  may  suppose  that  it  is  necessary  to  measure  the  line 
a  b,  which  may  be  taken  to  rejjresent  an  image  reflected  from  the  anterior 
surface  of  the  cornea  or  lens.  If  we  look  at  this  line  through  a  plate  of  glass 
the  plane  of  which  is  at  right  angles  to  our  line  of  sight,  no  distortion  of  the  line 
a  b  takes  place.  If  however  the  plate  be  placed  obliquely,  as  at  gr,  </,,  there  will 
be  an  apparent  shifting  of  the  line  sideways  to  c  d.  In  the  ophthalmometer 
there  are  two  glass  discs,  ^,  (/,  and  g^  g.,,  one  immediately  over  the  other,  so  placed 
that  the  image  a  b  is  looked  at  through  the  junction  between  the  two  plates. 
The  plates  are  then  turned,  as  in  the  diagram,  until  a  b  appears  as  two  distinct 
lines  e  c  and  c  d  just  touching  one  another  at  c.  At  this  point  each  image  of 
the  line  a  b  has  been  shifted  through  one-half  the  length  of  a  b.  Knowing  the 
thickness  of  the  plates  and  their  refractive  index,  it  is  easy  to  calculate,  from 
the  angle  through  which  the  plates  have  been  turned,  the  apparent  shifting  of 


SPECUL  SENSES 


555^ 


the  line  a  b.     This'lateral  movement  amounts  to  a  c,  i.e.  to         ,  and  we  have 

2 

merely  to  double  this  result  in  order  to  obtain  the  actual  size  of  the  image  on 

the  cornea  or  lens. 

If  the  lens  l)e  now  cut  out  of  the  e^ye,  it  is  found  when 
freed  from  its  supporting  structures  that  the  curvature  of 
its  anterior  surface  is  much  greater  than  it  was  before.  It  is 
evident  that  a  pressure  is  normally  exerted  by  some  struc- 
ture on  the  anterior  surface  of  the  lens,  repressing  its 
natural  tendencj^  to  become  convex.      If  we  examine  sections 

Fig.  247. 


Diagram  to  illustrate  principle  ol  ophthalmometer  (after  Schenck). 


through  the  eye  we  lind  that  this  structure  is  the  suspensory 
ligament  of  the  lens. 

The  membrana  hj^aloidea  of  the  vitreous  is  thickened  in 
front  and  closely  adherent  to  the  ciliary  processes.  At  the 
margin  of  the  lens  it  divides,  sending  a  thick  tough  expan- 
sion forwards  to  cover  the  anterior  surface  of  the  lens,  and 
a  thin  expansion  behind  which  separates  the  lens  from  the 
vitreous  humour.  The  part  of  the  membrane  extending 
from  the  edge  of  the  lens  to  the  ciliary  processes  is  the 
suspensory  ligament.  This  ligament  is  normally  on  the 
stretch,   and  keeps  the  anterior  surface  of  the  lens  nearl}' 


55()  PHYSIOLOGY 

flat,  so  that  the  eye  is  accominodated  for  intinite  distanee, 
Wlieii  tlie  eye  is  to  be  accommodated  for  near  objects  the 
ciliary  processes  are  pulled  forwards  and  inwards  by  the 
contraction  of  the  ciliary  muscle,  and  so  tlie  suspensory 
ligament  is  relaxed  and  the  front  of  the  lens  allowed  to 
bulge  forwards. 

The  ciliary  muscle  rmis  from  the  corneo-sclerotic  junction, 
to  be  attached  to  the  ciliary  processes  and  front  part  of  the 
choroid. 

Accommodation  is  a  voluntary  action,  although  the  ciliary 
muscle  consists  of  unstriated  fibres.  Contraction  is  brought 
about  through  the  intervention  of  the  short  ciliary  nerves, 
which   are   derived    from   the   third  nerve.     The  nucleus  of 

Fig.  248. 


Diagram  showing  change  in  lens  during  accommodation. 
M.  Ciliary  muscle.  I.  Iris.  L.  Lens.  V.  Vitreous  humour. 
A.  Aqueous  humour.     C.  Cornea. 

the  third  nerve  is  situated  in  the  extreme  hinder  part  of  the 
third  ventricle  and  the  anterior  part  of  the  iter  of  Sylvius. 
The  centre  presiding  over  the  movement  of  accommodation 
occupies  the  most  anterior  part  of  this  nucleus. 

Movements  of  the  Iris 

Accommodation  for  near  objects  is  always  associated  with 
contraction  of  the  iris,  the  function  of  which  we  must  now 
consider.  In  an  ordinary  spherical  biconvex  lens  the  rays 
of  light  passing  through  the  periphery  of  the  lens  come  to  a 
focus  at  a  nearer  point  than  the  rays  passing  through  the 
central  parts.  In  this  way  a  certain  amount  of  blurring  of 
an  image  is  produced,  which  is  spoken  of  as  spherical 
aberration.  This  spherical  aberration  may  be  corrected  in 
three  possible  ways : 


SPECIAL  SENSES 


557 


1.  By  making  the  refractive  index  of  the  lens  higher  at 
its  centre  than  at  its  circumference. 

2.  By  making  the  curvature  of  the  lens  less  near  its 
circumference  than  at  the  centre. 

3.  By  '  stopping  out '  the  peripheral  rays  of  light  by 
means  of  a  diaphragm. 

The  two  latter  methods  are  those  used  in  most  optical 
instruments.  In  the  eye  there  is  an  attempt  at  all  three, 
but  the  most  important  means  is  the  third,  the  diaphragm 
being  formed  by  the  iris.  This  is  a  circular  curtain  with  a 
hole  in  the  middle,  lying  just  on  the  anterior  surface  of  the 

Fig.  249. 


Diagram  to  show  couise  of  tlic  impulses  in  the  light  rcfiex  (marked 
by  single  arrows),  and  of  tliose  which,  starting  from  the  oculomotor 
nucleus,  cause  dilatation  of  the  pupil  (double  aiuows). 


lens.  Pigmented  cells  in  it  effectually  stop  out  peripheral 
rays  of  light,  and  the  size  of  the  opening  in  it,  the  pupil, 
is  controlled  by  the  contraction  or  relaxation  of  a  ring  of 
unstriated  muscular  fibres  situated  near  the  margin  of  the 
pupil. 

The  iris  has  a  twofold  nerve-supply  from  the  third  nerve 
through  the  short  ciliary  nerves,  and  from  the  cervical  sym- 
pathetic through  the  Gasserian  ganglion,  ophthalmic  branch 
of  the  fifth,  and  the  long  ciliary  nerves  (Fig.  249).  Stimula- 
tion of  the  third  nerve  causes  contraction  of  the  pupil.  The 
centre  for  this  movement  is  in  the  anterior  part  of  the  floor 


558  PHYSIOLOGY 

of  the  Sylvian  iter,  just  behind  the  centre  for  accommodation. 
Stimulation  of  the  cervical  sympathetic  produces  dilatation 
of  the  pupil.  The  fibres  serving  this  action  leave  the  spinal 
cord  by  the  second  dorsal  nerve,  and  pass  up  through  the 
stellate  ganglion  in  the  cervical  sympathetic.  In  this  dila- 
tation two  processes  are  involved,  viz.  (1)  the  relaxation  of 
the  circular  muscle-fibres,  the  sphincter  piqyilla',,  and  (2)  the 
contraction  of  the  radiating  fibres  which  form  the  dilatatur 
pupilla'. 

Contraction  of  the  pupil  occurs  under  the  following  con- 
ditions : 

1.  Stimulation  of  the  optic  nerve  by  exposure  of  the 
eye  to  light,  or  by  artificial  means.  In  the  higher  mammals 
this  is  a  crossed  reflex,  exposure  of  one  eye  to  light  causing 
contraction  of  both  pupils. 

2.  Associated  with  movements  of  accommodation  and  con- 
vergence of  the  optic  axes. 

3.  Various  poisons,  especially  opium  and  physostigmin. 
The  latter  drug  exerts  a  local  influence  on  the  iris,  and  can 
cause  contraction  of  the  pupil  when  all  nerves  to  the  eyeball 
are  cut. 

4.  In  sleep. 

The  pupil  is  dilated — 

1.  When  the  eye  is  removed  from  light. 

2.  Eeflexly  by  strong  stimulation  of  any  sensory  surface. 
8.  When  accommodation  is  relaxed. 

4.  Under  the  influence  of  emotion,  such  as  fear. 

5.  In  the  last  stage  of  asphyxia. 

6.  In  deep  chloroform  narcosis,  and  under  the  influence 
of  atropin  and  other  alkaloids  derived  from  the  solanaceous 
family.  Atropin  exerts  a  strong  local  influence  on  the  iris. 
Stimulation  of  the  third  nerve  has  no  power  to  constrict  a 
pupil  that  is  dilated  fully  by^atropin. 

Optical  Defects  of  the  Eye 

Chromatic  aberration. — Since  blue  rays  are  more  re- 
frangible than  red  rays,  they  are  brought  to  a  focus  at  a 
point  nearer  the  lens  than  the  red  rays.  This  is  the  reason 
why  with  an  ordinary  magnifying  glass  we  see  a  coloured 
fringe  round  the  margins  of  the  object.     Chromatic  aberrg,- 


SPECIAL   SENSES 


559 


tion  is  corrected  in  optical  instruments  by  using  two  diHerent 
kinds  of  glass.  In  the  eye  it  is  uncorrected.  Hence  it  is 
that  a  blue  light  and  a  red  light  at  the  same  distance  from 
the  eye  appear  to  be  unequally  distant ;  the  red  light,  re- 
quiring greater  accommodation  than  the  blue,  appears  to  l)e 
the  nearer  of  the  two.  The  error  in  most  cases  is  so  slight 
that  we  do  not  notice  the  chromatic  frmges  under  normal 
circumstances. 

Myopia. — The  normal  or  emmetropic  eye  is  so  constructed 
that,  when    the  ciliary  muscle  is  relaxed,  parallel    rays  are 

Fio.  2-50. 


Diagrams  of  course  taken  by  parallel  rays  in  entering  normal 
(emmetropic)  eye  {A),  hypermetropic  eye  (B),  and  myopic  eye 
(C). 


brought  to  a  focus  on  the  retina.  If  the  eyeball  be  longer 
than  usual,  the  parallel  rays  will  come  to  a  focus  rather  in 
front  of  the  retina,  so  that  it  will  be  impossible  for  a  clear 
image  of  distant  objects  to  be  formed  on  the  retina.  Objects  at 
a  certain  small  distance  from  the  eye  will  be  brought  to  a  focus 
on  the  retina  without  any  effort  of  accommodation.  People 
with  eyes  of  this  description  are  said  to  be  myopic  or  short- 
sighted. Under  these  circumstances  concave  spectacles  are 
necessary,  in  order  to  form  a  distinct  retinal  image  of  distant 
objects. 


560 


PHYSIOLOGY 


Hypcrmetropia. — If,  on  the  other  hand,  the  eyeball  be 
too  short  in  its  antero-posterior  diameter,  the  parallel  rays 
entering  the  eye  will  come  to  a  focus  at  a  point  behind  the 
retina.  In  order  that  a  distinct  image  may  be  formed,  even 
of  distant  objects,  it  will  be  necessary  to  increase  the  curva- 
ture of  the  lens  by  contracting  the  ciliary  muscle.  Such  eyes 
are  hypermetropic  or  long-sighted. 

Fig.  251. 

a  h  e 


Lens  from  human  eye  at  different  periods  of  life  (Allen  Thomson). 
a,  at  birth  ;  b,  adult ;  c,  old  age. 

Presbyopia. — As  old  age  comes  on,  the  lens  becomes  more 
rigid,  and  loses  more  or  less  its  tendency  to  become  convex 
(Fig.  251).  Hence  the  near  limit  of  accommodation  gets 
further  and  further  with  advancing  age.  Such  a  condition 
is  not  to  be  confused  with  long-sightedness  ;  it  is  merely 
a  defect  in  the  power  of  accommodation,  and  not  dependent 

Fig.  252. 


iagrain  showing  course  of  rays  in  an  astigmatic  eye  (Waller).  The 
curvature  of  the  cornea  is  greater  in  the  vertical  meridian  v  v  v  than  in 
the  horizontal  meridian  h  h  h.  Hence  the  rays  of  light  coming  from  the 
point  r  and  passing  through  the  vertical  meridian  come  to  a  focus  at/', 
while  those  through  the  horizontal  meridian  come  to  a  focus  at  f'\ 
There  is  thus  no  point  behind  the  cornea  at  which  all  the  rays  from  P 
will  come  to  a  focus,  and  the  image  of  the  point  must  be  blurred,  being 
elongated  in  a  horizontal  direction  at/',  and  in  a  vertical  direction  at/'''. 

on   a  structural  defect  of  the  eyeball.     It  is  spoken  of  as 
presbyopia. 

Astigmatism. — The  curvature  of  the  vertical  meridian  of 
the  cornea  is  usually  greater  than  that  of  the  horizontal 
meridian.  The  difference  may  be  so  great  as  to  make  it 
impossible   for   a  definite  image  of  a   point  of   light  to   be 


SPECIAL   SENSES 


561 


formed  on  the  retina,  the  rays  diverging  from  the  himinous 
point  in  the  vertical  plane  (greater  curvature)  being  brought 
to  a  focus  sooner  than  those  in  a  horizontal  plane.  To 
correct  this  defect  it  is  necessary  to  use  cylindrical  glasses 
in  order  to  make  up  for  the  lesser  curvature  of  the  cornea  in 
this  direction. 

Eetinal  Changes  involved  in  Vision 

We  have  seen  that,  in  nearly  all  sense-organs,  the  essen- 
tial constituent  is  a  bipolar  nerve-cell  having  a  peripheral 
process  extending  towards  the  surface  and  ending  between 


Fig.  253. 


1     -^1 


6     - 


_----ir'    c.n. 


n.M. 
h 


Schema  of  retina  (from  Bohm  and  Davidolf  after  Cajal). 
1,  nerve-fibre  layer ;  2,  ganglion-cell  layer ;  3,  inner  molecular 
layer ;  4,  inner  nuclear  layer ;  5,  outer  molecular  layer ; 
6,  outer  nuclear  layer;  c,  cone  ;  r,  rod  ;  b,  bipolar  cells  ;  S,  spon- 
gioblast ;  am.,  amacrine  cell ;  c.n.,  centrifugal  nerve-fibre ; 
M,  fibre  of  Miiller ;  ^i.M.,  nucleus  of  fibre  of  Miiller ;  n,  neu- 
roglia ;  0,  outer  limiting  membrane. 


the  epithelial  cells  covering  that  surface,  and  a  central  process 
which  runs  towards  the  central  nervous  system,  where  it 
terminates  in  close  contact  with  other  nerve-cells. 

The  retina  however  represents  genetically  not  a  simple 
sense-organ,  but  a  whole  lobe  of  the  brain,  and  has  therefore 

36 


562 


PHYSIOLOGY 


a  much  more  complicated  structure.     It  is  composed  of  three 
separate  relays  of  nerve-elements  (neurons).     These  are — 

(1)  The  rod  and  cone  cells,  with  their  nuclei  (rod  and 
cone  and  outer  nuclear  layers). 

(2)  Bipolar  cells  (inner  nuclear  layer). 

(3)  Ganglion  cells  (ganglion -cell  layer),  from  which  spring 
the  axis-cylinders  joining  the  nerve-libre  layer,  and  which 
run  along  the  optic  nerves  and  tracts  to  terminate  in  the 
region  of  the  anterior  corpus  quadrigeminum.  The  functional 
connection  between  the  processes  of  these  three  sets  of  nerve- 


Ophthalmoscopic  view  of  fundus  of  eye,  showing  the  optic  disc, 
or  point  of  entry  of  the  optic  nerve,  with  the  retinal  vessels 
branching  from  its  centre. 


cells  takes  place  in  the  outer  and  inner  molecular  layers 
(Fig.  253). 

Of  these  layers  of  the  retina,  the  hindmost,  the  layer  of 
rods  and  cones,  represents  the  end-organ  of  vision  ;  and 
therefore,  for  distinct  vision  to  take  place,  the  image  of 
external  objects  must  be  formed  on  this  layer.  This  is  shown 
by  the  following  facts  : 

a.  The  point  of  entry  of  the  optic  nerve,  where  the  whole 
thickness  of  the  retina  is  composed  of  nerve-fibres,  is  abso- 
lutely insensitive  to  light,  and  constitutes  the  '  blind  spot ' 
(Fig.  254), 


fiPECIAL   SENSES 


563 


h.  At  the  macula  lutea,  where  vision  is  most  distinct,  all 
the  layers  of  the  retina  are  diminished  except  the  layer  of 
rods  and  cones. 

c.  Piirhinjes  figures.  If  a  strong  light  be  focussed  by 
means  of  a  lens  on  the  sclerotic  just  outside  the  cornea,  and 
the  eye  be  made  to  stare  fixedly  at  a  dull  l)ackground,  an 
arborescent  image  of  the  retinal  vessels  will  appear  on  the 
background.  On  moving  the  illumination  the  image  of  the 
vessels  will  move  in  the  same  direction.  Knowing  the 
dimensions  of  the  eyeball  and  the  distance  of  the  background 
from  the  eye,  as  well  as  the  angle  through  which  the  light  is 

Fig.  255. 


•Ji  Xi 


Dia},'ram  of  the  path  of  the  lays  of  light  iu  the  formation  of  Purkinje's 
figures,  (v)  represents  a  retinal  vessel.  When  this  is  illuminated 
from  (A),  a  shadow  is  formed  on  the  hinder  layers  of  the  retina  at 
(a').  This  is  projected  along  a  line  passing  through  the  optic  axis, 
and  appears  to  come  from  a  point  (a")  on  the  wall.  On  moving  the 
light  from  (A)  to  (B),  the  image  of  the  vessel  appears  to  move  from 
(a")  to  (b"). 


moved  and  the  apparent  displacement  of  the  image  of  the 
vessels,  the  distance  of  the  percipient  part  of  the  retina  behind 
the  vessels  may  be  calculated.  This  distance  is  fomid  to 
correspond  with  the  distance  of  the  rod  and  cone  layer  from 
the  retina]  vessels,  and  hence  this  layer  is  taken  to  be  the 
end-organ  of  vision. 

When  light  falls  on  the  retina  certain  chemical  and 
physical  changes  take  place.  These  either  originate  or  accom- 
pany the  transmutation  of  the  ether  vibrations  into  the 
nerve-impulses,  which  ascend  the  optic  nerve.  If  a  frog  that 
has  been  in  the  dark  for  some  time  be  killed,  an  eve  taken 


564 


I'JIYHIOLOGY 


out  bisected,  and  the  retina  removed  and  examined  by  a 
weak  light,  it  will  be  found  that  this  latter  has  a  purplish- 
red  colour.  On  microscopical  examination  this  colour  is  seen 
to  be  confined  to  the  outer  limbs  of  the  rods.  After  a  very 
short  exposure  to  diffuse  daylight  the  colour  disappears.  The 
colouring  matter  {rlio  clop  sin)  may  be  dissolved  out  by  means 
of  a  solution  of  bile  salts.  The  purple-red  solution  thus 
formed  also  bleaches  rapidly  on  exposure  to  light.  By 
means  of  this  rhodopsin,  photographs  or  '  optograms '  of 
external  objects  may  be  taken  on  the  retina.  The  frog's  eye 
which  is  cut  out  is  placed  in  front  of  a  window.  After 
some  time  the  eye  is  bisected  and  plunged  into  a  4  per  cent, 
solution  of  alum,  which  fixes  the  optogram,  and  a  permanent 

Fui.  25(5. 


Sections  of  the  frog's  retina.  A,  kept  in  the  dark.  B,  after 
exposure  to  hght,  showing  retraction  of  the  cones,  and  pro- 
trusion of  the  pigmented  epithelium  between  the  outer  Umbs  of 
the  rods.     (Engelmann.) 


inverted  picture  of  the  window  with  its  cross-bars  is  obtained 
on  the  retina. 

If  a  retina,  which  has  been  bleached  by  exposure  to  light, 
be  replaced  on  the  pigment-layer  lining  the  choroid,  in  a 
short  time  the  colour  will  be  restored.  On  examining  sections 
through  the  retina  it  is  found  that,  in  those  which  have 
been  exposed  to  light,  the  cells  of  the  layer  of  pigmented 
epithelium  send  up  fine  processes  full  of  pigmented  granules 
between  the  outer  linabs  of  the  rods.     In  an  eye  which  has 


SPECIAL    SENSES 


565 


been  kept  in  the  dark,  on  the  other  hand,  the  cells  of  the  pig- 
ment-layer are  quite  flat,  so  that  the  front  part  of  the  retina, 
including  the  rods  and  cones,  can  be  removed  without  any 
difficulty  (Fig.  256).  Thus  tiie  function  of  the  pigmented 
epithelium  is  to  supply  visual  purple  to  the  outer  limbs  of 
the  rods  as  fast  as  the  pigment  already  there  is  bleached  by 
light.  It  might  be  thought  that  this  chemical  change  was 
the  active  agent  in  producing  excitation  of  the  optic  nerve- 
fibres  ;  but  the  facts  that  in  the  fovea  centralis,  the  region  of 

Fig.  257. 


Three  types  of  retinal  variiition  obtained  on  exposure  to  light 
(Waller).  I,  fresh  retina.  11,  same  retina  after  two  hours. 
Ill,  same  retina  after  twenty-four  hours.  The  black  line  at  the 
lower  border  of  each  record  marks  the  period  of  exposure  to 
lioht. 


most  distinct  vision,  we  iind  only  cones  which  contain  no  visual 
purple,  and  that  in  certain  birds  there  are  no  rods  and  no 
visual  purple  in  the  whole  retina,  show  that  this  chemical 
process,  interesting  though  it  may  be,  is  not  essential  for  the 
conversion  of  light-waves  into  a  nervous  impulse. 

When  light  falls  upon  the  retina  the  cones  are  reti-acted, 
and  lie  close  upon  the  external  limiting  membrane  ;  whereas 
in  an  eye  that  has  been  kept  in  the  dark  they  extend  down 
between  the  rods  as  far  as  the  pigmented  layer. 


506  PHYSIOLOGY 

The  falling  of  light  on  the  retina  is  also  accompanied  by 
an  electrical  change,  which  may  be  regarded  as  analogous  to 
the  current  of  action  in  nerve. 


The  conditions  in  the  retina  are  however  rather  more  complicated.  An 
eyeball  of  the  frog  led  off  from  its  anterior  and  posterior  surfaces  shows  a 
current  directed  in  the  eyeball  from  behind  forwards  (the  resting  or  demarcation 
cvirrent).  On  allowing  light  to  fall  into  the  eye  this  current  after  a  delay  of 
several  seconds  is  markedly  increased.  On  shutting  off  the  light  there  is  a 
momentary  further  increase  and  then  diminution  of  the  current  to  its  resting 
value  (Fig.  257,  I).  Waller  interprets  this  change  as  showing  a  twofold  process 
of  disintegration  and  anabolism  during  stimulation  by  light — the  anabolic  effect 
however  predominating  so  far  as  regards  the  changes  affecting  the  galvanometer. 
On  shutting  oft'  light,  the  dissimilative  changes  cease  at  once,  the  assimilative 
more  slowly ;  hence  the  further  positive  variation.  On  keeping  the  eyeball  the 
positive  variation  gradually  diminishes,  so  that,  twenty-four  hours  after  excision, 
exposure  to  light  may  cause  a  pure  negative  variation,  succeeded  by  a  slight 
positive  effect  when  the  light  is  shut  oft"  (Fig.  2-57,  III). 


Adaptation 

The  sensitiveness  of  the  retina  to  light  is  continually 
altering  according  to  the  strength  of  the  illumination.  Thus, 
when  we  go  from- a  brightly  lighted  place  into  a  dark  room, 
at  first  we  are  unable  to  distinguish  objects.  The  pupil 
dilates  widely  from  the  absence  of  stimulation.  In  a  short 
time,  however,  '  adaptation  '  occurs,  and  we  may  be  able  to 
see  quite  clearly.  When  the  eyes  are  removed  from  light  to 
darkness,  within  the  first  ten  minutes  the  sensitiveness  of  the 
retina  is  increased  twenty-five  times,  and  at  the  end  of  two 
hours  is  as  much  as  thirty-five  times  as  great  as  it  was  in 
full  illumination.  This  adaptation  is  retinal  and  not  con- 
ditioned by  changes  in  the  size  of  the  pupil,  which  after  the 
first  reaction  to  the  change  of  illumination  resumes  it's  normal 
size. 

In  the  '  dark-adapted  '  eye  {i.e.  one  that  has  been  removed 
from  all  sources  of  illumination  for  a  considerable  time)  a 
remarkable  difference  is  observed  in  its  sensitiveness  to  light 
of  different  colours  as  compared  with  the  sensitiveness  of  the 
normal  eye.  In  the  latter  the  spectrum  appears  brightest  in 
the  yellow  (between  the  lines  D  and  E). 

Tlie  following  figures  represent  the  relative  ])rightness  of 
the  spectrum  as  it  appears  to  the  '  light-adapted  '  eye  : — 


SPECLIL   SENSES  567 

Red                      (Fraunhofer  line  B)  .  .  32 

Orange  „             ,,    C  .  .  94 

Beddish  yellow  „             „    D  .  .  640 

Yellow,                                   D  to  E .  .  1,000 

Green,                                               E.  .  .  480 

Blue  green,                                     F .  .  .  170 

Blue,                                               G.  .  .  31 

Violet,                                               H.  .  .  (3 

In  the  dark-adapted  eye  the  point  of  maximum  brightness 
is  shifted  towards  the  blue  end  of  the  spectrum,  so  that  the 
})righteBt  part  may  lie  in  the  green,  and  the  red  end  may 
become  quite  invisible.  When  the  light  is  sufficiently  reduced 
in  intensity,  the  whole  spectrum  may  appear  colourless. 
Hence,  in  a  garden  full  of  bright  flowers,  at  night  only 
.differences  of  black  and  white  are  perceived.  As  morning 
dawns,  the  blue  flowers  and  green  leaves  appear  in  their  true 
colours,  while  the  scarlet  geraniums  still  appear  black  ;  while 
a  little  later  with  increasing  illumination  all  the  flowers 
are  seen  in  their  true  colours.  This  colour-blindness  to  weak 
illumination  is  possessed  by  the  extreme  peripheral  parts  of 
the  retina  (where  only  rods  are  found)  under  all  illuminations. 
This  fact,  combined  with  the  fact  that  the  sensitiveness  of  the 
dark-adapted  eye  to  different  parts  of  the  spectrum  agrees 
with  the  bleaching  powers  of  the  different  parts  on  visual 
purple,  suggests  that  we  have  in  the  retina  two  distinct 
apparatus  for  the  appreciation  of  light,  viz. : — 

(a)  The  cones,  appreciating  colour  differences,  especially 
concentrated  at  the  centre  of  the  retina  and  forming  the 
whole  fovea  centralis. 

(h)  The  rods,  sensitised  by  the  visual  purple,  giving 
sensations  only  of  light  and  darkness,  and  more  sensitive 
than  the  cones  to  weak  illumination.  Whereas  the  cones  are 
more  easily  excited  by  light  in  the  yellow  part  of  the  spectrum, 
the  rods  are  more  excited  l)y  the  more  refrangible  rays  of  the 
green  and  blue. 


t) 


Binocular  Vision 

Under  normal  circumstances  we  use  both  eyes  in  seeing. 
Since  however  the  visual  impression  produced  by  the  two 
retinal  images  is  not  double  but  single,  there  must  be  a  series 
of  points  in    each    retina  which,  stimulated  simultaneously, 


568 


PHYSIOLOGY 


give   rise   to  a  single  impression.     These  points  are   called 
'  corresponding '  points.     Thus,  when  we  look  at  a  spot,  the 


Fig.  258. 


dbt.sup. 


oil.  Slip. 


Sl/Jl, 


r.i'nt 


r.ext.    r.suj).  r.int 
r.iftf. 

Diagram  to  show  points  of  attachment  and  lines  of  action  of 
extrinsic  ocular  muscles. 


axes  of  the  eyes  are  so  directed  that  an  image  of  it  falls  on 
the  yellow  spots  of  the  two  retinae.  The  images  of  all  points 
to  the  right  of  this  spot  will  fall  on  the  nasal   side  of  the 


40 


Fio. 

2.59. 

T.Bup. 

/so 

/lO 
30 

3o\ 

20 

10 

30    20  10    1 

10  20   30 
10 

30  / 

20 

50X 
ol9LBnp 

Uo 
Wo 

\50 
r.inf 

T  int 


40 


Diiigram  to  show  direction  in  which  pupil  will  move  under  the 
action  of  the  various  ocular  muscles. 


SPECIAL   SENSES  569 

right  retina  and  on  the  temporal  side  of  the  left  retina,  and 
vice  verm.  So  if  the  riglit  retina  were  cut  out  and  placed  on 
the  left,  the  corresponding  points  in  the  two  retina?  would  be 
exactly  over  one  another.  In  order  that  we  may  have  single 
vision  it  is  necessary  that  the  images  of  external  objects 
should  fall  on  corresponding  points  of  the  two  retinae.  This 
is  effected  by  the  harmonious  co-operation  of  the  muscles  of 
the  eyeball.  These  are  six  in  number  :  superior,  inferior, 
external,  and  internal  recti,  superior  and  inferior  oblique. 
The  action  of  these  muscles  is  as  follows : 

Superior  rectus  moves  the  centre  of  the  cornea  upwards  and  inwards 
Inferior  „  „  „  downwards  and  inwards. 


Internal      „ 
External     „ 
Superior  oblique 
Inferior       „ 


directly  inwards, 
directly  outwards, 
downwards  and  outwards, 
upwards  and  outwards. 


So  the  muscles  required  for  the  following  movements 
will  be — 

Looking  upwards,  superior  recti  and  inferior  oblique  muscles. 
„         downwards,  inferior  recti  and  superior  oblique  muscles. 
,,         inwards  (convergence  of  eyes),  the  two  internal  recti. 
„        to  the  right,  the  right  external  rectus  and  the  left  internal  rectus. 
,,         to  the  left,  the  left  external  rectus  and  the  right  internal  rectus. 

The  movements  of  the  e^^es  to  one  side  or  the  other  are 
spoken  of  as  conjugate  deviation.  The  centres  of  most  of 
these  movements  are  situated  in  the  floor  of  the  iter  of 
Sylvius.  The  movements  which  involve  the  external  recti 
are  carried  out  by  the  nucleus  of  the  sixth  nerve,  which  is 
functionally  connected  with  nuclei  in  the  floor  of  the  iter  by 
the  posterior  longitudinal  bimdle. 

If  from  weakness  of  one  of  the  ocular  muscles  the  optic 
axes  cannot  be  made  to  converge  to  any  pomt  in  the  field  of 
vision,  so  that  the  images  of  external  objects  do  not  fall  upon 
correspondmg  pomts  of  the  two  retinae,  double  vision  results, 
and  the  patient  is  said  to  suffer  from  a  squint.  In  this  case 
the  image  which  is  formed  in  the  sound  eye  is  spoken  of  as 
the  true,  and  the  other  the  false  image.  From  the  relation 
in  space  of  the  false  to  the  true  image,  it  is  possible  to  tell 
which  muscle  is  affected. 


570  PHYSIOLOGY 


Visual  Sensations 

When  a  ray  of  light  from  an  object  to  the  outer  side  of 
the  eye  falls  upon  the  cornea,  an  image  of  it  is  formed  on  the 
nasal  side  of  the  retina.  If  the  source  of  light  be  above  the 
visual  axis,  the  image  is  formed  on  the  lower  half  of  the 
retina.  Hence,  whenever  the  retina  is  excited  at  these  points, 
whether  by  light  falling  on  the  eye  from  without  or  by  direct 
stimulation,  we  refer  the  sensation  produced  to  some  position 
in  the  outside  world  which  the  experience  gained  by  all  our 
other  senses  points  out. 

Thus  if  the  right  eye  be  turned  inwards,  and  pressure  with 
the  finger  made  on  the  outside  of  the  sclerotic  near  the  outer 
angle  of  the  eyelids,  we  have  a  sensation  of  a  ruig  of  light 
produced  by  the  direct  excitation  of  the  outer  part  of  the 
retina,  which  we  refer  or  '  project '  to  a  point  on  the  extreme 
inner  side  of  the  eye.  It  has  often  been  discussed  how  it  is 
that  we  see  external  objects  erect  when  the  retinal  image  is 
inverted.  But  we  do  not  look  at  the  image  on  the  retina. 
The  stimulation  of  the  retina  at  a  point  on  the  nasal  side 
merely  gives  rise  to  sensations  which  experience  has  taught 
us  to  recognise  as  coming  from  an  object  to  the  outer  side  of 
the  visual  axis. 

Atrophy  of  the  nasal  half  of  the  right  retina  therefore 
would  give  rise  to  blindness  to  the  outer  side  of  that  eye, 
which  would  probably  be  recognised  only  when  the  left  eye 
was  closed. 

lufen.'iLty  of  stimulus. — Weber's  law,  that  the  increase  of 
stimulus  necessary  to  cause  an  increase  of  sensation  always 
bears  the  same  ratio  to  the  whole  stimulus,  holds  good  also 
for  visual  sensations. 

This  ratio  in  the  case  of  the  eye  is  about  j-jVtt-  We  can 
thus  distinguish  between  two  lights  of  20  and  20^^  candle- 
power,  or  between  two  of  99  and  100  candle-power.  If  the 
illumination  be  excessive  the  law  no  longer  holds  good ;  and 
we  should  be  unable  to  tell  the  difference  between  two  arc 
lamps  at  a  short  distance,  although  one  might  be  much 
stronger  than  the  other,  and  the  difference  much  greater 
than  y-i-jy  of  the  total  light. 

Duration    of  stimulus.  -We    do   not    knov  how  long   a 


SPECIAL  SEiSrSES  571 

stimulus  of  light  must  act  on  the  retina  in  order  to  produce 
a  definite  sensation.  But  the  duration  is  very  short,  since 
an  electric  spark,  which  is  almost  instantaneous  in  its  ap- 
pearance and  disappearance,  may  excite  a  strong  sensation 
of  light.  This  momentary  stimulus  however,  as  in  the  case 
of  muscle,  excites  a  condition  of  activity  and  change  in  the 
retina  which  lasts  a  measurable  period. 

The  sensation  produced  by  a  momentary  stimulus  rises 
sharply  to  a  maximum  and  then  sinks,  first  quickly  and  then 
more  gradually.  The  first  part  of  the  fall,  after  the  attain- 
ment of  the  maximum  sensation,  is  more  rapid  in  the  case  of 
strong  than  of  weak  stimuli. 

This  duration  of  the  sensation  after  the  stimulus  has 
ceased  may  be  so  pronounced,  when  the  stimulus  is  very 
strong,  as  to  give  rise  to  a  definite  '  after-image.'  After 
looking  at  the  sun  for  some  time  and  then  turning  away, 
we  may  see  an  after-image  that  may  last  several  seconds  or 
minutes. 

If  one  stimulus  follows  another  at  a  very  short  interval 
we  get  a  summation  of  stimuli,  and  the  two  sensations  are 
fused  into  one.  The  interval  which  must  intervene  between 
two  stimuli,  in  order  that  two  distinct  sensations  may  be 
produced,  is  greater  when  the  stimuli  are  small  than  when 
they  are  intense. 

This  interval  may  be  determined  by  causing  a  disc,  on 
which  alternate  sectors  of  black  and  white  are  painted,  to 
revolve  at  known  rates,  and  noticing  the  time  that  a  white 
sector  takes  to  pass  a  given  point  (in  the  visual  axis)  when 
the  sensations  are  just  fused.  If  the  illumination  of  the  disc 
be  feeble,  this  time  will  be  found  to  be  about  jV  second. 
If  now  the  illumination  be  increased,  the  grey  disappears, 
and  we  observe  a  flickering  of  the  disc  due  to  imperfect 
fusion  of  the  separate  visual  sensations  (cf.  imperfect  tetanus 
of  muscle).  In  the  latter  case  the  time  between  two  suc- 
cessive stimuli  may  be  reduced  to  ^  or  -V  second  before 
apparent  fusion  of  the  sectors  takes  place. 

The  production  of  a  circle  of  light  when  a  stick  with  a 
glowing  end  is  rapidly  whirled  round,  and  all  the  effects  of 
pyrotechny,  are  dependent  on  this  persistence  of  retinal 
activity  after  the  stimulus  calling  it  forth  has  ceased. 


572  PHYSIOLOGY 


Colour  Vision 

If  a  ray  of  white  light  be  passed  through  a  prism,  it  is 
unequally  refracted,  so  that  it  is  widened  out  into  a  broad 
band  or  spectrum,  which  is  variously  coloured,  the  red  rays 
at  one  end  being  less  refrangible  than  the  blue  rays  at  the 
other.  We  may  divide  the  colours  of  the  spectrum  into 
seven — red,  orange,  yellow,  green,  blue,  indigo,  violet;  but 
the  division  is  quite  arbitrary,  the  colours  shading  so  gradu- 
ally into  one  another  that  no  two  observers  would  agree 
exactly  on  the  limits  between  them.  This  spectrum  can  be 
recomposed  by  another  prism  in  the  reverse  direction  with 
the  formation  of  white  light,  so  that  we  say  white  light  is 
composed  of  all  these  different  colours.  It  might  at  first  be 
thought  that  the  retina  could  respond  with  a  simple  sensa- 
tion to  stimulation  by  any  part  of  the  spectrum,  a  low  number 
of  ether  vibrations  per  second  producing  a  sensation  of  red, 
a  number  rather  higher  a  sensation  of  orange  ;  so  that  the 
sensation  produced  by  any  part  of  the  spectrum  would  be  a 
simple  colour  sensation,  of  which  there  would  in  this  case 
be  an  infinite  number.  But  a  simple  analysis  of  our  own 
sensations  seems  to  show  that  some  of  the  spectral  colours 
are  mixed  sensations.  Thus  most  people  will  say  at  once 
that  orange  is  a  mixture  of  red  and  yellow,  and  as  a 
matter  of  fact  we  find  that,  on  mixing  rays  from  the  red  with 
others  from  the  yellow  part  of  the  spectrum,  we  do  get  a 
sensation  of  orange.  The  stimulus  obtained  by  mixing  red 
and  yellow  rays  is  not  the  same  as  a  stimulus  caused  by  rays 
from  the  orange  part  of  the  spectrum.  In  the  former  case 
compound  waves  made  up  of  the  two  wave-lengths,  656  X  and 
564  X,  are  falling  on  the  retina ;  in  the  latter  case  a  simple 
wave  with  length  608  A, ;  and  yet  the  sensations  produced  are 
identical. 

In  order  to  recompose  the  white  light  it  is  not  necessary 
to  mix  all  the  spectral  colours.  We  may  take  a  pair  of 
colours,  situated  a  certain  distance  apart  in  the  spectrum, 
and  by  combining  these  form  white  light.  Thus  red  with 
green,  or  blue  with  j^ellow,  will  give  white  light.  Any  pair 
of  colours,  which  together  give  rise  to  a  sensation  of  white,  are 
called  comphmentayy.     By  taking  three  colours,  such  as  red, 


SPECIAL   SENSES  573 

green,  and  violet,  it  is  possible  by  mixing  them  in  various 
proportions  to  form  either  white  light  or  any  colour  of  the 
spectrum.  The  colours  so  formed  differ  from  the  spectral 
colours  in  being  less  saturated;  i.e.  the}^  contain,  besides  the 
pure  colour,  white  light. 

These  experiments  on  mixing  colours  can  be  performed  in  various  ways. 

A.  Sectors  of  the  different  colours  are  painted  on  a  disc,  and  the  colour 
sensations  are  fused  by  rapid  rotation  of  the  disc  (Maxwell's  colour-top). 

B.  Two  small  coloured  discs  are  placed  on  the  table,  with  a  vertical  glass 
plate  between  them.  It  is  possible  so  to  arrange  the  direction  of  vision  that 
the  reflected  image  of  the  disc  from  the  glass  plate  coincides  in  position  with 
the  other  disc  seen  through  the  plate. 

c.  These  methods  with  painted  discs  are  open  to  the  objection  that  no 
pigments  give  perfectly  pure  colour  sensations.  It  is  therefore  better  to  use  the 
pure  colours  of  the  spectrum  itself,  combining  any  two  portions  of  the  spectrum 
by  means  of  reflectors  or  prisms.  A  less  perfect  method  is  to  cause  light  from 
two  sources,  coloured  by  different  coloured  glass,  to  fall  on  the  same  surface. 

These  facts  show  that  in  all  probability  the  primitive 
colour  sensations  are  few  in  number,  and  that  the  various 
colour  sensations  of  a  spectrum  are  not  pure,  but  mixtures  of 
these  primary  sensations.  There  are  two  theories  of  colour 
vision^the  Young-Helmholtz  and  Hering's. 

According  to  the  former,  there  are  three  primary'  colour 
sensations — red,  green,  and  violet — each  of  which  is  repre- 
sented by  a  separate  set  of  nerve-fibrils.  One  set  of  fibres 
is  most  sensitive  to  red  rays,  and  only  slightly  sensitive  to 
the  green  and  blue  parts  of  the  spectrum  ;  the  second  set  is 
most  sensitive  to  the  middle,  and  the  third  set  to  the  blue 
end  of  the  spectrum.  White  light  is  produced  by  an  equal 
stimulation  of  the  three  sets. 

Hermg  distmguishes  four  primary  colour  sensations — red, 
yellow,  green,  and  blue — and  also  considers  the  sensations 
of  white  and  black  as  primary  visual  sensations.  These 
sensations  are  placed  m  three  groups,  red  and  green,  yellow 
and  blue,  white  and  black.  For  each  pair  of  sensations  he 
considers  that  there  is  a  special  substance  in  the  retina,  dis- 
similation or  katabolism  of  which  gives  rise  to  one  colour 
sensation  ;  anabolism  or  assimilation  to  the  other.  Thus  if 
white  light  falls  on  the  retina,  it  causes  a  breaking  down  or 
katabolism  of  the  white-black  substance.  This  breakmg 
down  excites  certain  fibres  of  the  optic  nerve,  and  produces 
m  consciousness  a   sensation  of  white.     If  the  light  be  now 


571 


PHYSIOLOGY 


removed,  this  bretiking  down  gives  place  to  anabolism  or 
building  up  of  the  white-black  substance,  which  excites  the 
same  nerve-fibrils  in  a  different  way,  giving  rise  to  a  sensa- 
tion of  black.  The  white-black  substance  is  affected  not 
only  by  white  light,  l)ut  also  by  the  colours  red,  green, 
yellow,  blue,  and  their  mixtures.  The  other  two  visual  sul)- 
stances  are  affected  only  by  red  and  green  or  by  yellow  and 
blue  respectively.  Hence  even  the  spectral  colours  do  not 
give  rise  to  pure  sensations,  there  being  always  some  mixture 
of  a  sensation  of  white  with  the  proper  colour  sensation. 

The  phenomena  of  colour  vision  that  we  have  mentioned 
above  can  be  equally  well  explained  on  either  theory.  Thus 
the  fact  that  blue  and  yellow  together  give  rise  to  a  sensa- 

Fk;.  260. 


''"'"''"!||ii!'i''"ii'ifcii«^ 


-^  jiailiilil 


_J^. iiijmnTininntl] 


liiJiaiMiiPiiMM^^ 


o 


ni. 


Curves  showing  sensitiveness  of  the  three  varieties  of  nerve- 
fibres  to  different  parts  of  the  spectrum.  1.  Red  fibres. 
2.  Green  fibres.     3.  Violet  fibres. 


tion  of  white  may  be  explained  on  the  Young-Helmholtz 
theory  by  saying  that  the  stimulation  of  all  three  sets  of 
fibrils  is  equal — as  will  be  seen  by  adding  together  the 
ordinates  of  each  curve  in  Fig.  260  at  yellow  and  at  blue. 

Adopting  Bering's  hypothesis,  we  may  say  that,  anabolism 
and  katabolism  being  equally  excited  in  the  yellow-blue 
substance,  no  change  in  it  takes  place,  and  the  sole  sensa- 
tion is  that  produced  by  the  stimulation  of  the  white  black 
substance. 

The  fact  that  any  coloured  light,  if  very  dim  or  if  falling 
on  only  a  minute  part  of  the  retina,  produces  a  sensation  of 
white,  is  more  readily  explicable  on  Bering's  than  on  the 
Young-ITelmholtz  tlieory. 

Cases  a]'e  not  rare  in  which  a  person  is  unable  to  dis- 


SPECIAL   SENSES  575 

tiiiguisli  between  reel  and  green,  so  that  he  can  only  tell  a 
cherry  from  the  leaves  on  the  tree  by  its  shape.  Such  cases 
may  be  explamed  on  either  theory.  Bering's  theory  how- 
ever seems  necessary  to  account  for  the  cases  of  complete 
colour-blindness  which  are  said  to  occur.  In  these  the  only 
sensations  are  of  light  and  shade,  and  we  may  suppose  that 
the  red-green  and  blue-yellow  substances  are  lacking  in  the 
retina. 

Contrast  phenomena. — If  a  grey  disc  be  placed  on  a  piece 
of  red  paper,  and  the  whole  covered  with  tissue-paper,  the 
disc  will  take  on  a  greenish  tinge.  If  the  ground  colour  be 
green,  the  disc  will  appear  red  ;  if  blue,  the  disc  will  appear 
yellow ;  in  fine,  whatever  be  the  ground  colour,  the  colour 
of  the  disc  will  be  complementary  to  it.  These  effects  are 
spoken  of  as  simultaneous  contrast. 

If,  after  gazing  steadily  for  some  time  at  a  red  disc  on  a 
white  surface,  the  eyes  be  turned  towards  a  plain  white 
surface,  a  negative  after-image  of  the  disc  is  seen  on  the 
paper,  coloured  green,  i.e.  the  complementary  colour  of 
the  red  disc.  Surrounding  this  the  paper  appears  red.  If 
we  look  at  the  sun  for  some  time,  and  then  turn  our  eyes 
away,  there  is  at  first  a  positive  after-image,  and  we  see  a 
bright  sun  wherever  we  look.  In  a  short  time  this  dis- 
appears and  gives  way  to  a  black  sun  (a  negative  after- 
image). Thus  we  may  say  that  stimulation  of  any  part  of 
the  retma  with  any  colour  is  followed  by  a  colour  sensation, 
referred  to  the  same  part  of  the  visual  field  and  comple- 
mentary to  the  first. 

It  has  been  much  discussed  whether  these  phenomena  are 
simply  effects  of  judgment,  or  whether  they  are  produced 
by  definite  changes  taking  place  m  the  retina. 

Helmholtz  explains  them  by  the  first  hypothesis,  and  looks 
upon  them  as  cerebral  processes. 

Hering,  on  the  other  hand,  has  extended  his  theory  so  as 
to  embrace  these  phenomena,  and  ascribes  them  to  definite 
changes  in  the  retina,  or  at  any  rate  in  the  peripheral  part 
of  the  visual  mechanism. 

A  corollary  to  his  theory  that  we  mentioned  above  is  that, 
if  dissimilation  of  a  visual  substance  be  excited  at  any  point 
of  the  retina,  assimilation  of  the  same  substance  is  set  up  in 
the  parts  of  the  retina  immediately  adjoining  that  point.     In 


576 


PHYSIOLOGY 


this  way  the  phenomena  of  simultaneous  contrast  may  be 
explained. 

Thus  if  a  ray  of  red  light  falls  on  any  spot,  it  may  be 
supposed  to  excite  dissimilation  of  the  red-green  substance 
at  this  spot.  This  sets  up  assimilation  of  the  same  substance 
in  the  adjoinmg  parts  of  the  retina,  and  the  red  object  is 
therefore  surrounded  with  a  green  halo,  which  at  once 
becomes  evident  if  we  increase  our  appreciation  for  slight 
colour-tones  by  diminishing  the  total  amount  of  light  by 
means  of  tissue-paper. 

The  question  between  the  two  theories  is  whether  the  contrast  phenomena 
depend  upon  psychical  or  retinal  events.  There'  is  no  doubt  that  the  question 
must  be  answered  in  the  latter  sense,  and  that  these  phenomena  are  quite 
independent  of  the  judgment  of  the  individual.     This  is  shown  clearly  by  two 


Fig.  2(51. 


Purple 


Purple       Yellow 


Green        Purple 


Purple 


experiments.  A  box  (Fig.  2(51)  is  divided  into  two  long  compartments,  a  b  and 
c  d.  At  a  the  compartment  is  closed  by  a  red  glass-plate  and  at  c  by  a  blue 
glass-plate.  Apertures  are  provided  at  b  and  d  for  the  observer's  eyes.  At 
-f  and  +  two  small  grey  crosses  are  fixed  about  the  middle  of  the  compartment 
on  sheets  of  transparent  glass.  On  looking  through  the  openings  b  and  d  and 
converging  the  eyeballs  so  as  to  fix  e  line  o,  we  get  a  fusion  more  or  less 
complete  of  the  two  colours,  red  blue,  so  that  the  background  appears 

purple ;  or  there  may  be  a  struggle  between  the  colours,  at  one  time  blue,  at 
another  red  predominating.  To  the  judgment  however  there  is  one  background 
and  not  two,  and  therefore,  according  to  the  theory  of  Helmholtz,  the  grey 
crosses  should  by  contrast  both  acquire  the  same  induced  colour,  which  would 
be  complementary  for  purple.  But  it  is  found  that  the  two  crosses  are  perfectly 
distinct  in  colour,  that  which  is  seen  by  the  eye  against  the  blue  ground 


SPECIAL   SENSES  577 

yellow  while  that  on  the  red  ground  is  green,  showing  that  the  phenomena  of 
simultaneous  contrast  are  peripheral  and  not  cerebral  in  their  causation.  The 
same  fact  is  very  definitely  established  by  the  following  experiment  devised  by 
Sherrington.  The  disc  (Fig.  262)  presents  two  rings,  each  half  blue  and  half 
black.  The  outer  ring  is  intensified  when  at  rest  by  simultaneous  contrast,  the 
black  half  being  seen  against  the  surrounding  yellow,  while  the  luminosity  of 
the  blue  half  is  increased  by  the  effect  of  the  surrounding  black.     In  the  inner 

Fig    262. 


ring  the  blue  half  is  darkened  by  contrast  with  the  surrounding  yellow  while 
the  black  half  is  not  evident  at  all.  If  the  disc  be  rotated,  we  get  two  concentric 
rings  on  an  apparently  homogeneous  field.  It  is  found  however  that  the  outer 
ring  flickers  long  after  complete  fusion  has  taken  place  in  the  inner  ring, 
showing  that  the  stimulation  of  the  retina  by  the  outer  ring  is  increased  under 
the  influence  of  contrast. 

On  this  theory  successive  contrast  phenomena  are  analo- 
gous to  certain  phenomena  we  have  already  studied  in  other 
tissues.  If  extensive  breaking  down  of  the  visual  stuff  has 
been  occurring,  when  the  stimulus  is  removed  there  will  be 
a  swing  back  of  the  condition  of  the  protoplasm  of  the  nerve- 
endings  in  the  opposite  direction,  and  the  katabolic  will  be 
replaced  by  anabohc  changes  ;  just  as,  on  breakmg  a  con- 
stant current  that  has  been  flowing  through  a  nerve,  the 
condition  of  raised  irritability  at  the  kathode  gives  place  to 
a  condition  in  which  the  irritability  is  depressed  below  the 

normal. 

The  improving  effect  on  the  heart  of  stimulation  of  the 

37 


578 


PHYSIOLOGY 


vagus  is  also  exactly  analogous  to  a  successive  contrast 
effect.  During  stimulation  of  the  vagus  the  breaking  down 
of  the  contractile  substance  is  stopped  or  checked,  so  that 
building  up  or  anabolism  can  go  on  without  interruption. 
When  the  excitation  of  the  vagus  ceases  there  is  an  extra 
store  of  contractile  material  in  the  muscle-cells.  This  causes 
the  beat  to  be  more  vigorous,  and  we  may  say  that  the 
increased  anabolism  has  been  followed  by  a  period  of  in- 
creased katabolism,  just  as  strong  stimulation  of  a  part  of 
the  retina  with  green  (anabolism)  gives  rise  to  a  red  after- 
image (katabolism). 

Visual  Judgments 

Size. — The  apparent  size  of  an  object  is  determined  by 
the  magnitude  of  its  image  formed  on  the  retina.  As  will 
be  evident  from  the  diagram  (Fig.  263),  the  apparent  size  in 
any  diameter  of  any  given  object  is  inversely  proportional  to 
the  distance.  Thus  the  size  of  the  image  on  the  retina  of  an 
object  two  inches  long  at  a  distance  of  one  foot  is  equal  to  the 
image  of  an  object  four  inches  long  at  a  distance  of  two  feet. 

Fig.  263. 


An  object  can  be  seen  if  the  visual  angle  subtended  by 
it  (the  angle  A  c  B  in  Fig.  263)  is  not  less  than  sixty 
seconds.  This  is  equivalent  to  an  image  on  the  fovea  cen- 
tralis of  the  retina  about  4  /x  ^  across,  which  corresponds  to  the 
diameter  of  a  cone. 

Estimation  of  distance  depends  partly  on  muscular  sensa- 
tions from  the  degree  of  accommodation  and  of  convergence 
of  the  optic  axes,  partly  on  comparisons  of  the  apparent 
size  of  the  object  with  that  of  a  neighbouring  object  (such 
as  a  man)   the  real  size  of  which  is  known,  and  partly  on 

fx  =  0-001  millimetre. 


SPECIAL   SENSES 


579 


the  amount  of  blurring  of  the  outlines  of  the  object  due  to 
the  haziness  of  the  atmosphere.  The  latter  factor  is  of 
great  importance  when  the  object  is  too  large  and  remote  to 
be  compared  with  others  of  a  known  size.  After  a  storm  of 
rain  distant  mountains  may  seem  to  be  many  miles  nearer 
than  they  did  before. 


Judgment  of  Solidity — Stereoscopic  Vision 

If  we  look  at  a  solid  object,  such  as  a  cube,  with  both 
eyes,  the  images  formed  on  the  corresponding  points  of  the 

Fig.  2G4. 


two  retinse  are  not  identical,  the  one  in  the  right  eye  repre- 
senting more  of  the  right  side  of  the  cube,  and  in  the  left  eye 
more  of  the  left  side  (Fig.  264). 

If  the  two  images  (a)  and  (b)  be  so  arranged  that  they  fall 
on  corresponding  points  of  the  two  retinae,  the  resulting 
impression  is  that  of  a  solid  body,  in  the  form  of  a  cube. 
This  is  the  principle  involved  in  the  stereoscope.  When 
only  one  eye  is  used,  the  external  world  has  a  much  flatter 
appearance,  although  some  idea  of  solidity  is  still  gained 
from  the  fact  that  the  accommodation  has  to  be  altered  in 
order  to  bring  different  parts  of  the  solid  body  into  focus. 
The  effects  of  light  and  shade  also  aid  in  the  judgment  of 
solidity. 

Accessory  Parts  of  the  Eye 

The  eyeball  is  protected  in  front  by  the  eyelids.  These 
are  lined  internally  with  a  delicate  mucous  membrane,  con- 
tinuous with  the  conjunctiva  covering  the  anterior  surface  of 
the  eyeball.  This  membrane  is  kept  constantly  moist  by 
the  secretion  of  the  lachrymal  gland,  a  small  acino-tubular 
gland  built  up  on  the  type  of  a  serous  gland,  situated  at  the 
upper  and  outer  angle  of  the  orbit.     The  excess  of  fluid  is 


580  PHYSIOLOGY 

drained  off  by  the  nasal  duct,  which  leads  from  the  conjunc- 
tival sac  to  the  nasal  cavity  on  the  same  side.  If  the  eyes 
be  kept  open  for  some  minutes,  the  conjunctiva  covering  the 
eyeball  becomes  dry,  and  irritation  is  set  up.  Normally  the 
membrane,  and  especially  that  over  the  cornea,  is  kept  moist 
and  transparent  by  involuntary  movements  of  the  eyelids, 
which  close  or  blink  about  twice  a  minute,  and  so  distribute 
the  lachrymal  secretion  over  the  whole  conjunctival  surface. 

This  blinking  is  a  reflex  act,  the  afferent  channels  being 
fibres  of  the  fifth  nerve,  and  the  efferent  the  fibres  of  the 
facial  nerve  supplying  the  orbicularis  palpebrarum.  It  is 
spoken  of  as  the  '  conjunctival  reflex,'  and  is  one  of  the  last 
reflexes  to  disappear  in  chloroform  or  ether  narcosis. 


The  Nutrition  of  the  Eyeball.     The  Intraocular  Pressure 

The  eyeball  is  formed  of  a  tough  inextensible  capsule,  the 
sclerotic,  filled  with  fluid  or  semi-fluid  contents.  In  order  that 
the  eyeball  may  be  sufficiently  rigid  to  maintain  the  normal 
relations  of  the  various  refractive  media,  and  to  afford  a  fixed 
point  for  the  action  of  the  ciliary  muscle,  this  fluid  must  be 
under  pressure.  On  connecting  a  small  manometer  with  the 
anterior  chamber,  care  being  taken  to  prevent  any  escape  of 
the  intraocular  fluid,  it  is  found  in  the  normal  eye  that  this 
pressure  is  about  25  mm.  Hg.  On  making  an  opening  into 
the  cornea  the  fluid  drains  away,  and  the  eyeball  becomes  soft 
and  collapsed,  the  cornea  becoming  folded,  and  the  eye  being 
naturally  useless  as  an  optical  instrument.  The  fluid  which 
flows  away,  and  which  forms  the  aqueous  humour  and  also 
fills  the  interstices  of  the  gelatinous  tissue  of  the  vitreous, 
contains  only  a  minute  trace  of  proteid,  consisting  in  every  100 
parts  of  98-7  parts  water  and  1-2  to  1*3  total  solids,  of  which 
only  0-08  to  0*12  parts  consist  of  proteid.  If  a  cannula  be  kept 
in  the  anterior  chamber  this  fluid  rapidly  alters  in  character, 
becoming  coagulable,  and  containing  3  to  4  per  cent  of  proteids. 

The  intraocular  fluid  is  continually  being  renewed.  The 
eyeball  receives  a  rich  vascular  supply,  which  forms  a  close 
network  of  vessels  and  capillaries  in  the  choroid  coat,  with  its 
prolongations  the  ciliary  processes  and  iris.  The  chief  seat 
of  formation  of  the  intraocular  fluid  is  the  ciliary^processes. 


SPECIAL  SENSES.  581 

Here  there  is  a  constant  transudation  of  fluid  from  the  blood- 
vessels into  the  anterior  part  of  the  vitreous  cavity,  the  amount 
of  the  transudation  varying  with  the  pressure  in  the  blood 
capillaries,  being  increased  by  any  rise  in  the  capillary  blood 
pressure  or  by  any  fall  in  the  intraocular  pressure.  Of  the  fluid 
poured  out  by  the  ciliary  processes  a  very  small  proportion 
(perhaps  one-fiftieth)  passes  backwards  into  the  vitreous  humour 
and  gradually  drains  out  of  the  eyeball  by  the  lymphatic  spaces 
of  the  optic  nerve.  By  far  the  larger  amount  passes  forward 
through  the  fibres  of  the  suspensory  ligament  into  the  posterior 
chamber  (the  annular  cavity  between  the  iris  in  front  and 
the  lens  and  ciliary  processes  behind),  and  thence  round  the 
margin  of  the  iris  into  the  anterior  chamber.  From  the  anterior 
,  chamber  it  passes  into  the  spaces  of  Fontana  at  the  outer  angle 
of  the  chamber,  whence,  under  pressure,  it  can  filter  slowly 
between  the  endothelial  cells  lining  the  canal  of  Schlemm  into 
this  vessel  and  so  into  the  venous  system. 

A  considerable  resistance  is  offered  to  the  passage  of  fluid 
into  the  canal  of  Schlemm.  Hence  the  constant  transudation 
of  fluid  from  the  ciliary  processes  raises  the  intraocular  pressure 
to  25  mm.  Hg.,  and  a  continuous  production  of  about  6  cubic 
millimetres  of  fluid  per  minute  suffices  to  maintain  the  pres- 
sure at  this  height.  If  the  anterior  angle  of  the  eye  becomes 
blocked,  the  absorption  of  intraocular  fluid  becomes  more 
and  more  difficult.  There  is  therefore  a  rise  of  intraocular 
pressure  to  far  above  normal,  and  in  consequence  there  is 
atrophy  of  the  retina  followed  by  disturbance  of  the  nutrition 
of  the  whole  eyeball.  This  condition  of  raised  intraocular 
tension  occurs  in  the  disease  known  as  glaucoma. 

The  constant  renewal  of  the  intraocular  fluid  is  important, 
not  only  for  the  maintenance  of  the  intraocular  pressure,  but 
also  for  the  nutrition  of  the  structures  such  as  the  lens,  sus- 
pensory ligament  and  vitreous  humour,  which  do  not  receive 
any  vascular  supply. 


582 


CHAPTER   XIV 

THE    SPINAL     CORD 

Section  1 
STKUCTUEE    AND    TKACTS    OF    THE    COED 

The  spinal  cord  and  bulb  may  be  regarded  in  two  lights, 
as  a  centre  presiding  over  reflex  actions  and  as  a  channel  of 
communication  between  the  periphery  and  the  brain.  Its 
structure  corresponds,  roughly  speaking,  to  this  twofold 
action,  consisting  as  it  does  of  a  tube  of  grey  matter 
internally,  which  may  be  looked  upon  as  a  collection  of 
reflex  centres,  surrounded  externally  by  a  layer  of  white 
matter,  composed  of  medullated  nerve-fibres  and  serving  as 
simple  conducting  tissue.  It  lies  in  the  spinal  canal,  pro- 
tected by  its  three  membranes,  dura  mater,  arachnoid,  and 
pia  mater,  and  suspended  by  the  attachment  of  its  thirty-one 
pairs  of  nerves,  as  they  pierce  the  dura  mater.  The  structure 
of  the  cord  is  best  studied  in  cross-sections.  A  cross-section 
through  the  dorsal  region  is  approximately  circular,  and 
consists  of  two  symmetrical  halves,  separated  in  front  by  the 
anterior  fissure,  and  behind  by  the  posterior  fissure.  Each 
half  contains  a  crescentic  or  comma-shaped  area  of  grey 
matter,  surrounded  on  its  front,  lateral,  and  mesial  borders 
by  white  matter,  and  connected  by  an  isthmus  to  the  crescent 
of  the  opposite  half.  In  the  centre  of  this  isthmus  is  the 
central  canal  of  the  cord,  the  grey  matter  ui  front  and  behind 
it  being  known  as  the  anterior  and  posterior  grey  commissures. 
At  the  base  of  the  anterior  fissure  the  white  matter  is  con- 
tinuous between  the  two  halves  of  the  cord,  forming  the 
anterior  white  commissure. 

The   Nerve-roots 

Each  nerve  of  the  thirty-one  pairs  that  arise  from  the 
spinal    cord    has    two    roots,    anterior    and    posterior.      The 


THE   SPINAL  CORD 


683 


anterior  root  arises  by  several  bundles  from  the  antero- 
lateral part  of  the  cord  ;  the  posterior  root  arises  as  a  smgle 
bundle,  emerging  from  the  spinal  cord  opposite  the  posterior 
horn  of  grey  matter.  The  two  roots  join  to  form  the  trunk 
of  the  spinal  nerve.  On  the  posterior  root,  just  before  it 
joins  the  anterior  root,  is  situated  a  ganglion,  the  posterior 
root  ganglion. 

If  we  study  the  development  of  these  roots,  we  find  that 
they  have  different  origins.  Whereas  the  axis-cylinders  of 
the  anterior  roots  are  formed  by  the  outgrowth  of  the  axons 
of  cells  in  the  grey  matter  of  the  cord,  chiefly  in  the  anterior 
cornu    (Fig.  '265),  the   posterior   roots    are   derived    from  a 

Fig.  265. 


Transverse  section  of  spinal  cord  of  chick  to  show  developing 
nerve-roots  (stained  by  Golgi's  method)  (Ramon  y  Cajal).  A, 
anterior  root-fibres  growing  from  the  anterior  cornual  cells,  c  ; 
B,  posterior  root-tibres  passing  from  bipolar  cells  of  ganglion 
into  cord. 


separate  mass  of  cells  formed  from  the  epiblast  close  to  the 
dorsal  surface  of  the  cord.  These  epiblastic  cells  become 
oval  and  send  out  a  process  at  each  extremity,  one  process 
growing  into  the  spinal  cord,  while  the  other  meets  the 
anterior  roots  and  grows  with  these  downwards  towards  the 
periphery.  In  some  animals,  such  as  fishes,  the  cells  of  the 
posterior  root-ganglia  retain  this  bipolar  character  through- 
out life,  and  this  is  also  the  case  with  the  analogous  ganglion 
situated  on  the  course  of  the  auditory  nerve  in  the  cochlea, 
the  so-called  spiral  ganglion.  In  all  higher  vertebrates 
the  two  processes  of  the  cells  of  the  spinal  ganglia 
become  approximated  in   the  course  of  growth   and  finally 


584  PHYSIOLOGY 

arise  from  the  cell   as  one  process  which  divides  into  two  by 
a  T-shaped  junction  at  the  first  node  of  Eanvier. 

Thus  from  the  point  of  view  of  development,  the  nerve- 
fibres  making  up  a  mixed  spmal  nerve  are  of  twofold 
origin,  and  represent  the  arms  or  elongated  processes  of 
two  distinct  sets  of  cells.  If  a  unicellular  animal  be  cut 
into  two  parts,  it  is  found  that  the  half  containmg  the  nucleus 
will  regenerate  the  missing  part  and  will  continue  to  live, 
whereas  the  part  which  contains  no  nucleus,  although  able 
for  a  short  time  to  carry  out  movements  or  even  to  ingest 
food-granules,  is  unable  to  assimilate  its  food  and  grow,  and 
finally  dies.  Exactly  the  same  thing  happens  in  the  case  of 
a  nerve-cell.  If  we  cut  through  that  part  of  the  cell  which 
forms  the  axis-cylinder  of  a  nerve-fibre,  the  part  cut  av^ay, 
although  for  a  time  excitable  and  able  to  conduct  impulses, 
finally  dies ;  whereas  the  part  attached  to  the  cell-body  with 
its  nucleus  continues  to  live  and  may  under  favourable 
circumstances  regenerate  the  part  that  has  been  cut  off. 
These  facts  furnish  the  basis  of  a  method  for  determining 
the  exact  situation  of  the  nerve-cell  from  which  a  given  axis- 
cylinder  is  derived. 

We  have  already  seen  that  section  of  a  peripheral  nerve 
causes,  after  a  small  initial  rise,  a  gradual  fall  of  irritability 
in  the  part  of  the  nerve  below  the  section.  This  goes  on  to 
complete  loss  of  irritability,  and  on  microscopic  investigation 
it  is  found  that  the  physiological  change  is  accompanied  by 
definite  progressive  structural  changes. 

About  four  days  after  the  section  (in  mammals)  the  myelin 
forming  the  medullary  sheath  of  the  nerve-fibres  in  the 
peripheral  part  of  the  nerve  becomes  segmented,  and  breaks 
up  into  drops  of  various  size  (Fig.  266).  A  little  later  the  axis- 
cylinder  is  also  broken  across,  so  that  there  is  no  longer  any 
physiological  continuity  in  the  nerve-fibre.  This  is  followed 
by  enlargement  and  proliferation  of  the  internodal  nuclei ; 
the  protoplasm  within  the  primitive  sheath  increases  in 
quantity,  and  the  drops  of  myelin  are  gradually  absorbed 
and  disappear.  Finally,  about  the  twenty-first  day  or  later, 
the  original  structure  of  the  nerve-fibres  has  entirely  dis- 
appeared, and  they  consist  merely  of  a  tubular  sheath,  contam- 
ing  nuclei  and  structureless  protoplasm.  If  no  regeneration 
can  take  place  these  structures  also  disappear,  giving  place  to 


THE   SPINAL   CORD 


585 


simple  connective  tissue.  If  however,  after  the  section,  the 
two  ends  of  the  nerve  have  been  kept  in  close  apposition  by 
means  of  sutures,  regeneration  of  the  peripheral  part  of  the 


Fig.  266. 


//  \/- 


Degeneration  and  regeneration  of  nerve-fibres  in  the  rabbit 
(SchJifer,  after  Eanvier).  A,  part  of  a  nerve-fibre  in  which 
degeneration  has  commenced  in  consequence  of  the  section, 
fifty  hours  previously,  of  the  trunk  of  the  nerve  higher  up ; 
viy,  medullary  sheath  becoming  broken  up  into  drops  of 
myelin ;  p,  granular  protoplasmic  substance  which  is  re- 
placing the  myelin ;  n,  nucleus ;  g,  neurilemma.  B,  another 
fibre  in  which  degeneration  is  proceeding,  the  nerve  having 
been  cut  four  days  previously ;  p,  as  before  ;  cij,  axis-cylinder 
partly  broken  up,  and  the  pieces  inclosed  in  portions  of  myelin. 
C,  more  advanced  stage  of  degeneration,  the  medullary  sheath 
having  almost  disappeared,  and  being  replaced  by  protoplasm 
in  which,  besides  drops  of  myelin,  are  numerous  nuclei  which 
have  resulted  from  the  division  of  the  single  nucleus  of  the 
internode.  D,  commencing  regeneration  of  a  nerve-fibre. 
Several  small  fibres,  t'  t",  have  sprouted  from  the  somewhat 
bulbous  cut  end,  b,  of  the  original  fibre,  t;  a,  an  axis-cylinder 
■which  has  not  yet  acquired  its  medullary  sheath  ;  s,  s',  primi- 
tive sheath  of  the  original  fibre.  A,  C,  and  D  are  from  osmic 
preparations;  B  from  an  alcohol  and  carmine  preparation. 


580  PHYSIOLOGY 

nerve  takes  place.  New  axis -cylinders  grow  out  from  the 
old  axis-cjdinders  of  the  central  part  of  the  nerve  at  the 
node  of  Ranvier  just  above  the  point  of  division,  and  these 
grow  down  into  the  structureless  protoplasm  filling  the 
sheaths  of  the  peripheral  nerve-fibres,  thus  restoring  func- 
tional continuity.  The  myelin  sheaths  of  the  regenerated 
nerve-fibres  make  their  appearance  rather  later.  Nerve- 
fibres  have  already  been  spoken  of  as  being  enormously 
elongated  cell-processes,  and  it  seems  that  a  fibre  degenerates 
whenever  it  is  separated  from  the  cell  of  which  it  is  an  out- 
growth, and  must  be  regenerated  by  a  renewed  outgrowth 
from  this  cell.  We  may  look  upon  the  nerve-cells  as 
presiding  over  the  nutrition  of  the  fibres  which  spring  from 
them  ;  and  they  are  therefore  called  the  *  trophic  centres  '  of 
these  fibres.  If  a  nerve  be  divided,  only  that  half  which  is 
separated  from  its  trophic  centre  will  degenerate.  This  fact 
was  first  pointed  out  clearly  by  Waller,  and  hence  the  method 
of  diagnosing  the  course  of  tracts  in  the  central  nervous 
system  is  named  the  Wallerian  method. 

A  large  majority  of  the  white  fibres  of  the  spinal  cord  are 
dependent  for  their  nutrition  upon  their  continuity  with  a 
nerve-cell,  and  if  this  be  abolished  the  part  of  the  nerve-fibre 
severed  from  the  cell  degenerates.  If  the  anterior  root  be 
divided,  the  part  attached  to  the  cord  remains  intact,  but  the 
whole  peripheral  part  of  the  fibres  degenerates,  so  that  in  a 
section  of  the  mixed  nerve  the  degenerated  motor-fibres  can 
be  identified  (Fig.  267,  II). 

If  the  posterior  root  be  divided  between  the  ganglion  and 
its  junction  with  the  anterior  root,  all  the  sensory  fibres  in 
the  mixed  nerve  below  the  junction  degenerate.  If  however 
it  be  divided  between  the  ganglion  and  the  cord,  the  sensory 
fibres  in  the  mixed  nerve  remain  intact,  but  the  central  parts 
of  the  fibres  degenerate  right  up  into  the  cord,  and  may  be 
traced  in  the  cord  as  far  up  as  the  medulla. 

It  has  already  been  stated  that  the  anterior  root  is  motor 
or  eft'erent,  and  the  posterior  root  sensory  or  aft'erent.  The 
evidence  for  this  is  as  follows  : — If  the  anterior  root  be 
divided,  the  muscles  supplied  by  the  nerve  are  paralysed. 
Excitation  of  the  peripheral  end  of  the  anterior  root  will 
cause  them  to  contract.  Excitation  of  its  central  end  has 
no  effect. 


THE   SPINAL   COED 


587 


Section  of  the  posterior  root  causes  loss  of  sensation  in 
its  area  of  distribution.  Stimulation  of  its  peripheral  end 
has  no  effect.  Stimulation  of  its  central  end  causes  marked 
signs  of  pain,  such  as  struggling,  crying  out  or,  in  a  curarised 
animal,  rise  of  blood-pressure. 

In  some  cases  we  may  find  that  stimulation  of  the  peri- 
pheral end  of  the  anterior  root  gives  rise  to  evidence  of 
pain.  This  is  spoken  of  as  recurrent  senslhility^  and  is  due 
to  stimulation  of  fibres  which  leave  the  cord  by  the  posterior 


Figures  (from  Yeo)  to  illustrate  the  degree  and  direction  of 
degeneration  as  a  result  of  section  of  the  spinal  roots. 
I,  division  of  whole  nerve  below  ganglion.  II,  division  of 
anterior  root.  Ill,  division  of  posterior  root  above  ganglion. 
IV,  division  of  posterior  root  above  and  below  ganglion. 

roots,  and  after  travelling  some  distance  towards  the  peri- 
phery turn  back  and  run  up  in  the  anterior  root.  Recurrent 
sensibility  is  abolished,  as  would  be  expected,  by  division  of 
the  posterior  root. 

The  Grey  Matter  of  the  Cord 

The  crescentic  mass  of  grey  matter  in  each  half  of  a  cross- 
section  of  a  cord  is  larger  in  front  than  behind,  the  anterior 
and  posterior  halves  being  spoken  of  as  the  anterior  and 
posterior  horns  or  cornua  respectively  (Fig.  272,  p.  593). 
Each  horn  is  again  divided  into  the  caput  or  head,  forming  the 
large  extremity,  and  the  neck,  which  is  the  narrower  part  by 
which  the  caput  is  connected  with  the  central  mass  of  grey 
matter.  When  examined  in  section,  the  grey  matter  at  first 
presents  an  inextricable  confusion  of  nerve  cells  and  fibres  of 
all  descriptions.  But  of  late  years  various  methods  have  come 
to  our  assistance  in  the  unravelling  of  the  tangled  mass.    The 


588  PHYSIOLOGY 

most  important  of  these  methods  are  the  methylene  blue 
method  of  Ehrlich  and  Golgi's  silver  chromate  method  with 
its  various  modifications.  Both  these  methods  stain  nerve- 
cells  with  all  their  processes,  and  since  in  a  given  segment 
of  the  cord  only  a  few  of  the  nerve-cells  are  stained,  it  is 
possible  to  trace  their  processes  through  a  considerable 
thickness  of  the  cord.  The  grey  matter  consists  of  nerve- 
cells  with  their  processes,  of  the  branching  terminations  of 
various  nerve-fibres  derived  from  the  white  matter  of  the  cord 
or  the  posterior  nerve-roots,  and  of  the  supporting  framework 

Fig.  268. 


Nerve-celJ  from  the  spinal  cord,  stained  by  Nisd's  naethod. 
a,  axis-cylinder  process  or  axon ;  fe,  protoplasm  of  cell,  con- 
sisting of  c,  fibrillated  gromid  substance,  and  e,  the  granules 
of  Nissl ;  d,  nucleus.     (Lenhossek.) 

or  neurogha.  All  the  nerve-cells  of  the  cord  are  multipolar 
(Fig.  268).  The  processes  however  are  of  two  kinds.  One 
kind  of  process,  the  neuraxon  or  axon,  of  which  only  one 
is  present,  is  in  most  cases  prolonged  into  a  nerve-fibre,  of 
which  it  becomes  the  axis-cylinder,  generally  acquiring  at 
the  same  time  a  medullary  sheath.  This  process  may  send 
off  a  few  fine  branches,  the  so-called  collaterals,  but  in  most 
cases  does  not  undergo  any  extensive  branching  until 
nearing  its  periphery,  where  it  may  break  up  into  the  rich 
arborisations  with  which  we  are  already  acquainted  as  the 


THE   SPINAL   COED 


589 


motor  and  sensory  nerve-endings.  All  the  other  processes 
of  the  cell  are  generally  thicker  at  their  origin  than  the 
axon,  and  very  rapidly  break  up  into  branches  which  end 
freely  in  the  neighbouring  grey  matter.  In  many  cases 
these  dendrites  have  serrated  margins— an  appearance  espe- 
cially well-marked  in  certain  cells  of  the  cerebrum  and 
cerebellum.  The  body  of  the  cell,  sometimes  called  the  peri- 
karyon, is  granular  and  surrounds  a  large  vesicular  nucleus 
with  well-marked  nuclear  membrane,  which  only  stains 
faintly  with  nuclear  dyes.     By  special  methods  a  fibrillation 

Fig.  269. 


The  point  of  origin  of  the  axon,  the  '  nerve-hillock,'  highly 
magnified,  to  show  absence  of  Nissl's  granules  from  the  origin 
of  the  process.     (Held. ) 


of  the  protoplasm  has  been  demonstrated,  the  fibrillae  sweep- 
ing across  the  cell  from  process  to  process,  and  many  converg- 
ing towards  the  point  of  origin  of  the  axon  (Fig.  269).  While 
some  observers  regard  this  fibrillation  as  an  artefact  due  to 
the  coagulating  reagents  employed,  others  attach  extreme 
importance  to  it,  and  look  upon  the  fibrillae  as  the  essential 
conducting  elements  of  the  central  nervous  system,  continuous 
from  cell  to  cell  and  throughout  the  whole  body. 

According  to  most  observers  however,  such  an  anatomical 
continuity  does  not  exist,  at  any  rate  in  vertebrate  animals. 
All  the  constituents  of  the  central  nervous  system  arise  from 
cell-units,  and  so  far  as  we  can  tell,  the  processes  of  these 


590 


PHYSIOLOGY 


cells  do  not  grow  together  but  simply  remain  in  contact.  Thus 
the  axis-cylinder  of  an  anterior  cornual  cell  ends  on  a  muscle- 
fibre  as  an  arborisation,  the  branches  of  which  end  freely 
on  the  surface  of  the  fibre.  The  cells  of  the  central  nervous 
system  are  influenced  in  the  same  way.  An  axon  from  the 
periphery  or  from  another  part  of  the  central  nervous  system 
ends  in  an  arborisation  in  contact  either  with  the  cell-body  or 
with  the  dendritic  processes  (Fig.  270).     Any  im^mlse  arriving 

Fig.  270. 


Arborisation  of  collaterals  from  the  posterior  root-fibres  round 
the  cells  of  the  posterior  horn.     (Ramon  y  Cajal.) 


at  this  arborisation  sets  up  a  new  impulse  in  the  cell-body, 
just  as  a  motor  impulse  descending  a  nerve  sets  up  a  new 
excitatory  impulse  in  the  muscle  with  an  energy  considerably 
in  excess  of  the  original  nerve-impulse.  Thus  the  whole 
nervous  system  can  be  regarded  as  made  up  of  a  numberless 
array  of  nerve-cells  with  their  processes  (we«?-o??s),  the  activity 
of  each  being  regulated  by  the  connections  of  its  dendrites  and 
the  destiny  of  its  axon.  It  is  possible  that  the  office  of  each 
cell  is  not  merely  to  transmit  the  disturbance  arriving  at  it,  but 


THE   SPINAL   COED 


591 


to  send  it  on  with  increased  energy,  acting  like  a  battery  relay 
in  a  telegraphic  circuit.  In  all  cases  where  there  is  a  distinction 
between  the  dendrites  and  the  axon,  the  direction  of  conduc- 
tion seems  to  be  in  the  dendrites  towards  the  cell,  cellulipetal, 
and  in  the  axon  away  from  the  cell,  celluHfugal.  In  bipolar 
cells,  as  in  the  spinal  ganglia,  it  is  impossible  as  a  rule  to 
draw  any  distinction  between  the  two  processes. 

If  the  nerve-cells  serve  as  relays  of  energy,  nervous 
activity  must  be  associated  with  a  using  up  of  material  in 
the  cell ;  and  many  attempts  have  been  made  to  discover  his- 
tological evidences  of  nerve-cell  activity.     Of  importance  in 

Fig.  271. 


Cells  from  the  oculomotor  nuclei  thirteen  days  after  section  of 
the  nerve  on  one  side,  a,  cell  from  healthy  side  ;  b,  cell  from 
side  on  which  nerve  was  divided  (Flatau). 


this  connection  is  the  presence  ivf  the  nerve-cells  of  certain 
bodies  which  are  known  as  Nissl's  granules.  If  a  section  of 
nervous  tissue  fixed  with  alcohol,  formol,  or  corrosive  sublimate 
be  stamed  with  basic  dyes  such  as  methylene  blue  or  toluidin 
blue,  the  bodies  of  the  cells  are  seen  to  contain  a  number  of 
coarse  angular  granules  or  masses  arranged  more  or  less 
symmetrically  round  the  cell,  and  extending  for  a  considerable 
distance  along  the  dendrites  (Fig.  271,  a).  The  axon  and  that 
part  of  the  cell  from  which  it  arises  (the  nerve-hillock)  are 
quite  free  from  these  granules  (cf.  Fig.  269).  As  the  result  of 
stimulation,  changes  have  been  described  both  in  these  granules 


592  PHYSIOLOGY 

and  in  the  nucleus  of  the  cell.  Of  more  practical  importance 
however  are  the  changes  produced  in  these  granules  by 
section  of  the  axon  itself.  As  a  matter  of  fact,  although 
division  of  an  axon  causes  the  more  profound  changes  in  that 
part  which  is  separated  from  the  cell-body  or  'perikaryon,  the 
temporary  or  permanent  loss  of  function  does  not  leave  the 
cell  unaffected.  It  was  long  ago  shown  that  amputation  of 
a  limb  was  followed  after  many  years  by  a  shrinkage  and 
atrophy  of  all  that  portion  of  the  grey  matter  from  which 
the  nerves  to  the  limb  were  derived.  The  introduction  of 
Nissl's  method  has  shown  that  an  appreciable  change  is 
produced  within  a  few  weeks,  the  granules  losing  their  indi- 
viduality and  becoming  broken  up,  so  that  the  whole  cell 
takes  on  a  diffuse  blue  stain  (Fig.  271,  h).  This  change  is 
followed  after  some  considerable  time  by  a  gradual  atrophy 
of  the  cell  and  all  its  processes.  It  is  sometimes  spoken  of 
as  secondary,  or  better  as  '  retrograde  '  degeneration. 

The  grey  matter  is  thus  made  up  of  nerve-cells  embedded 
in  a  close  felt-work  of  fibres  of  various  descriptions,  fine 
meduUated  nerve-fibres  coming  in  as  collaterals  from  the 
surrounding  white  matter  and  breaking  up  into  naked 
arborisations.  Branching  axis-cylinders,  which  begin  and  end 
in  the  grey  matter  itself,  as  well  as  the  richly  branched 
dendrites  of  the  various  nerve-cells,  all  these  elements  are 
embedded  in  and  supported  by  the  special  connective  tissue 
of  the  nervous  system,  the  neuroglia. 

The  neuroglia  resembles  the  rest  of  the  nervous  system 
in  being  of  epiblastic  origin.  The  cells  forming  the  primitive 
neural  groove  are  of  two  kinds,  the  neuroblasts,  from  which 
the  nerve-cells  are  derived,  and  the  spongioblasts.  These 
latter  at  first  form  a  continuous  lining  of  the  central  canal, 
and  send  out  one  process  to  the  periphery  of  the  cord. 
This  process  branches  freely,  and  in  course  of  time  some  of 
the  cells  wander  out  into  the  substance  of  the  cord  and 
acquire  many  processes,  which  run  out  in  all  directions  and 
cross  those  of  adjacent  cells.  In  the  adult  cord  the  cell- 
bodies  of  these  neuroglia  cells  either  disappear  altogether  or 
are  reduced  to  little  more  than  the  nucleus,  so  that  the 
neuroglia  consists  of  a  rich  felt-work  of  branching  fibres 
crossing  each  other  in  all  directions  and  forming  a  framework 
for  the  nervous  elements  of  the  cord. 


Fig.  272. 


Cervical. 


Dorsal.  \ 


Lumbar 


ANTERIOR 
ROOT    BUNDLCS 

Sections  of  human  spinal  cord  from  the  lower  cervical,  mid-dorsal, 
and  mid-lmnbar  regions,  showing  the  principal  groups  of  nerve- 
cells,  and  on  the  right  side  of  each  section  the  conducting  tracts 
as  they  occur  in  the  several  regions  (magnified  about  7  diameters). 
(E.  A.  Schafer.)  a,  6,  c,  groups  of  cells  of  the  anterior  horn  ;  d,  cells 
of  the  lateral  hour ;  e,  middle  group  of  cells  ;  /,  cells  of  Clarke's 
column  ;  g,  cells  of  posterior  horn  ;  c.c,  central  canal ;  a.c,  anterior 
commissure. 

38 


694  PHYSIOLOGY 

In  accordance  with  its  epiblastic  origin,  the  neuroglia  presents  no  chemical 
resemblances  to  the  group  of  connective  tissues.  Its  main  constituent  is  a 
body  allied  to  keratin,  known  as  neuro-keratin,  giving  all  the  reactions  of 
proteins,  but  distinguished  from  the  ordinary  members  of  this  group  by  its 
insolubility  in  the  digestive  juices  as  well  as  by  the  high  proportion  of  sulphur 
in  its  molecule. 

This  neuroglia,  thoiigli  extending  through  the  whole  cord, 
is  especially  concentrated  at  two  points,  viz.  in  the  immediate 
vicinity  of  the  central  canal  and  at  the  head  of  the  posterior 
horn,  where  mingled  with  small  nerve-cells  it  forms  a  sort 
of  cap  to  the  grey  matter  and  is  known  as  the  substantia 
gelatinosa  of  Rolando. 

The  grey  matter  of  the  cord  may  be  considered  as  a  con- 
tinuous tube  formed  by  the  fusion  of  a  series  of  paired 
ganglia,  each  of  which  innervated  one  body  segment.  This 
regular  arrangement  has  been  modified  by  the  development 
at  a  later  period  of  the  limbs,  and  the  necessary  growth  of 
new  cells  to  supply  the  limbs.  Hence  we  find  a  considerable 
enlargement  of  the  grey  matter  in  two  situations,  the  cervical 
enlargement  correspondmg  to  the  brachial  plexus,  and  the 
lumbo-sacral  enlargement  which  gives  origin  to  the  nerves  of 
the  hind  limb. 

The  cells  of  the  grey  matter  are  arranged  in  definite 
groups  or  columns,  some  of  which  extend  throughout  the 
whole  length  of  the  cord,  whilst  others  are  confined  to  certain 
regions.     These  columns  are  — 

1.  In  the  anterior  cornu,  two  sets  of  cells,  the  anterior  or 
median  and  the  external  groups.  These  cells  are  the  largest 
in  the  cord  ;  they  have  many  processes,  one  process  being 
continued  into  the  medullated  nerve-fibre  of  an  anterior  root, 
the  other  processes  (dendrites)  breaking  up  into  a  fine  mesh- 
work  of  non-medullated  fibrils,  which  become  lost  in  the 
meshwork  of  the  grey  matter.  The  external  group  {b,  Fig.  272) 
is  especially  connected  with  the  motor  nerves  to  the  limb 
muscles. 

2.  The  lateral  column  or  intermedio-lateral  tract,  chiefly 
marked  in  the  dorsal  and  upper  part  of  the  lumbar  spinal 
cord.  These  cells  are  almost  certainly  connected  with  the 
visceral  outflow  which  occurs  in  this  region.  Their  axons 
pass  out  by  the  anterior  roots,  and  then  by  the  white  rami 
communicantes  to  the  sympathetic  system,  in  some  ganglion 
of  which  they  terminate. 

3.  The  cells  of  the  posterior  horn,  small  multipolar  cells. 


THE   SPINAL   CORD 


595 


4.  Clarke's  column  or  posterior  vesicular  column  (Fig. 
2'il%f),  reaching  from  the  seventh  or  eighth  cervical  nerve  to 
the  third  lumbar  nerve,  and  represented  opposite  the  second 
and  third  cervical  nerves  by  a  small  group  of  cells  and  possibly 
also  in  the  sacral  region  by  a  group  known  as  Stilling's  nucleus. 
The  cells  composing  this  column  are  large  and  fusiform,  with 
their  long  axes  parallel  to  that  of  the  cord,  so  that  in  cross- 
section  they  have  the  appearance  of  small  round  cells. 


.pot 


Fig.  273. 
Direct pyram/ri^^^ 


Ant.  lat 

asc.tra£t 


Direct 

Cerebellat 


Z-PosteriorJtoots. 
with  collaterals. 


Spinal  cord  (after  Lenhossek). 

On  left  side  of  figure  are  shown  the  nerve-cells  with  their  axis-cylinder 
processes.  On  the  right  side  the  distribution  of  the  chief  collaterals. 
1.  Motor  cells.  2.  Cells  of  the  columns.  2a.  Cells  of  Clarke's  column, 
sending  processes  across  into  direct  cerebellar  tract.  3,  4,  and  5. 
Commissural  cells. 


A  more  general  classification  of  the  nerve-cells  may  be 
based  on  the  destmation  of  their  nerve-fibre  processes  (Fig. 
273).     In  this  way  we  may  distinguish — 

(1)  Motor  cells.  These  are  the  large  cells  already  de- 
scribed in  the  anterior  cornua.  Their  axis-cylinder  processes 
all  run  out  into  an  anterior  nerve-root,  and  end  for  the  most 
part  in  the  motor  end-plate  on  a  muscular  fibre. 

(2)  Cells  of  the  columns.  The  nerve-processes  of  these 
run  out  into  the  white  matter,  and  the  majority  ascend 
in  one  of  the  columns  of  the  cord.  We  find  these  cells 
in  the  anterior  and  lateral  cornua,  sending  fibres  into  the 
anterior  and  lateral  columns.     There  are  also  a  few  in  the 


596  PHYSIOLOGY 

posterior  corniia  which  send  their  processes  into  the  posterior 
columns.  The  best-marked  group  however  is  that  ah'eady 
described  as  forming  Clarke's  column.  These  send  their  nerve- 
fibre  processes  right  across  the  grey  matter  into  the  lateral 
column  of  the  same  side,  where  they  turn  upwards,  forming  a 
distinct  tract  of  fibres — the  direct  or  posterior  cerebellar  tract. 
(3)  Commissural  cells.  This  class  embraces  a  number  of 
cells  of  different  sizes  and  shapes.  Their  axons  either  end 
in  the  grey  matter  of  the  same  side,  or  traverse  the  cord  to 
form  connections  with  the  grey  matter  of  the  other  side. 
Many  of  these  fibres  pass  through  the  anterior  white  com- 
missure. Among  these  we  find  the  Golgi  type  of  cell,  i.e. 
a  cell  whose  axon  begins  to  divide  almost  immediately  after 
leaving  the  cell,  and  branching  profusely  ends  in  the  near 
neighbourhood  of  the  cell  from  which  it  started. 

The   White  Matter  of  the  Cord 

The  white  matter  consists  almost  exclusively  of  medullated 
nerve-fibres,  which  run  for  the  most  part  longitudinally. 
They  are  of  various  sizes,  some  of  the  smaller  fibres  being 
branches  (collaterals),  which  have  been  given  off  from  the 
larger  ones,  and  which  will  shortly  turn  into  the  grey  matter. 
In  section  they  resemble  closely  the  fibres  of  an  ordinary 
peripheral  nerve,  but  they  differ  from  these  in  that  they  have 
no  primitive  sheath  or  neurilemma.  Each  consists  simply  of 
an  axis-cylinder  surrounded  by  a  thick  medullary  sheath,  the 
whole  embedded  in  a  tube  formed  by  the  neuroglia.  The 
white  matter  is  divided  by  the  anterior  and  posterior  fissures, 
and  the  nerve-roots  into  anterior  or  ventral,  lateral,  and 
posterior  or  dorsal  columns.  A  further  subdivision  of  the 
anterior  column  into  antero-median  and  antero-lateral,  and 
of  the  posterior  column  into  postero-median  and  postero- 
lateral, is  indicated  by  slight  grooves  on  the  surface  of  the 
cord  in  the  cervical  region.  In  order  however  to  arrive  at  a 
knowledge  of  the  origin,  course,  and  destiny  of  the  fibres 
composing  the  white  matter,  we  must  have  recourse  to  indirect 
methods.     The  following  methods  may  be  employed  : 

1.  Wallerian  method.  If  the  cord  be  cut  across  trans- 
versely, some  tracts  of  white  matter  will  degenerate  in  the 
cord  above  the  lesion  (ascending  degeneration),  while  other 
tracts  will  degenerate  in  the  cord  below  the  lesion  (descending 
degeneration),  according  as  the  cells  of  origin  of  the  fibres 


THE   SPINAL  CORD  597 

lie  below  or  above  the  lesion.  In  this  way  the  white  matter 
may  be  divided  mto  ascending  and  descending  tracts.'  This 
method  may  be  applied  in  two  ways.  The  spinal  cord  may 
be  cut  and  the  animal  kept  alive  for  three  to  six  months. 
At  the  end  of  this  time  all  the  degenerated  fibres  have  dis- 
appeared and  their  place  is  taken  by  nem-oglia.  On  staining 
sections  with  carmine,  the  degenerated  part  therefore  looks 
pinker  than  the  rest  of  the  section.  A  better  method  is  to 
treat  the  sections  by  some  process  such  as  Weigert's,  which 
stains  the  normal  medullary  sheaths  black.  By  this  treatment, 
the  degenerated  area  is  at  once  detected  by  its  failure  to  take 
the  stam. 

This  method  will  not  serve  to  display  the  presence  of 
isolated  degenerated  fibres  among  a  mass  of  normal  fibres. 
For  this  purpose  we  may  use  Marchi's  method,  which  is  based 
on  the  fact  that  the  medullary  sheath,  when  it  breaks  up, 
undergoes  chemical  change,  and  acquires  the  composition  of 
ordinary  neutral  fat.  If  a  tissue  be  placed  in  Miiller's  fluid 
for  a  fortnight,  and  then  in  a  mixture  of  osmic  acid  and 
Miiller's  fluid,  no  blackenmg  of  the  normal  medullary  sheath 
is  produced,  although  fat  is  blackened  by  the  osmic  acid  in 
the  ordmary  way.  The  presence  therefore  of  one  or  two 
degenerated  fibres  in  a  section  of  spinal  cord  will  be  at  once 
evinced  by  the  appearance  of  black  dots  in  the  otherwise 
unstained  section.  In  employing  this  method,  the  cord  must 
be  hardened  before  the  broken-up  medullary  sheath  has  begun 
to  be  absorbed,  i.e.  about  two  or  three  weeks  after  the  lesion. 

2.  Method  of  retrograde  degeneration.  This  is  based  on 
the  changes  undergone  by  Nissl's  granules  (chromatolysis) 
as  the  result  of  section  of  the  axon  of  a  cell.  For  example, 
the  hypoglossal  nerve  may  be  divided  and  three  weeks  later 
the  animal  killed  and  sections  of  the  medulla  cut  and  stained 
by  Nissl's  method.  The  distribution  of  the  cells  of  origin  of 
this  nerve  is  at  once  shown  by  the  marked  changes  in  the 
cells  on  the  side  of  the  lesion  as  contrasted  with  the  normal 
cells  on  the  opposite  side  (cf.  Fig.  271,  p.  591). 

3.  Developmental    method    (Flechsig).     This    method    is 

'  A  caution  is  here  necessary.  It  is  often  assumed  that  a  tract  which 
degenerates  upwards  is  necessarily  afferent  in  function,  and  vice  versa.  But 
the  result  of  section  of  a  peripheral  nerve,  after  which  sensory  as  well  as  motor 
fibres  degenerate  below  the  section,  shows  that  the  direction  of  degeneration  is 
not  necessarily  the  same  as  the  direction  of  conduction. 


598  PHYSIOLOGY 

founded  on  the  fact  that,  when  the  nerve-fibres  are  first 
formed  in  the  foetal  cord,  they  are  non-meduUated,  and  the 
different  tracts  of  the  cord  acquire  a  medullary  sheath  at 
different  intervals,  the  pyramidal  tracts  being  latest  of  all  in 
acquiring  their  sheath  (Fig.  274). 

4.  Electrical  method.  The  passage  of  a  nerve-impulse 
along  the  cord,  as  along  a  nerve,  is  accompanied  by  an 
electrical  change  (current  of  action).  It  is  possible  to  find 
out  by  what  path  the  electrical  change  travels,  and  thereby 
to  determine  the  path  of  the  impulse  of  which  the  electrical 
change  is  the  concomitant  (Gotch  and  Horsley). 

Fig.  274. 


r/x.. 


Section  through  the  cervical  spinal  cord  of  a  new-born  child, 
stained  by  Weigert's  method,  to  show  absence  of  niedullation 
in  pyramidal  tract,  ca,  anterior  commissure ;  Fp,  crossed 
pyramidal  tract ;  Fe,  direct  cerebellar  tract ;  Zrp,  posterior 
root  zone;  rp',  posterior  root-fibres  (Bechterew). 

5.  Experimental  method.  Different  parts  of  the  white 
columns  may  be  cut  through,  and  the  effects  observed  that 
are  produced  in  this  way  on  the  conduction  of  motor  or 
sensory  impulses.  Evidence  in  this  direction  is  also  furnished 
by  the  clinical  results  of  lesions  of  various  parts  of  the  cord. 

By  a  combination  of  these  methods  the  following  con- 
clusions have  been  arrived  at.  The  white  matter  of  the  cord 
may  be  divided  into  ascenduig  and  descending  tracts  (Fig.  275 ; 
see  also  Fig.  272). 

A.  Descending  tracts. — If  the  spinal  cord  be  divided  in 
the  cervical  region,  degeneration  of  two  distinct  tracts  in  the 


THE   SPINAL   CORD 


599 


anterior  and  postero-lateral  columns  is  produced.  These  are 
the  anterior  or  direct  and  the  crossed  pyramidal  tracts.  The 
fibres  composing  these  tracts  are  derived  from  the  pyramidal 
cells  in  the  motor  area  of  the  cerebral  cortex,  and  are  there- 
fore found  degenerated  if  the  motor  area  of  the  cortex  is 
destroyed.  The  crossed  pyramidal  tracts  are  derived  from 
the  other  side  of  the  cerebral  cortex  and  have  crossed  the 
middle  line  at  the  lower  part  of  the  medulla,  at  the  pyramidal 
decussation.  The  anterior  tracts  contmue  the  course  of  the 
pyramids  in  the  medulla  for  a  time,  but  cross  gradually  by 
the  anterior  commissure  on  their  way  down  the  cord.     Fmally, 


al.asc. 


Diagram  of  spinal  cord.  a.r.  Anterior  spinal  nerve-roots,  p.r. 
Posterior  root.  a.py.  Anterior  pyi-amidal  tract,  l.py.  Lateral 
pyramidal  tract,  d.cbl.  Direct  cerebellar  tract,  p.m.c.  Posterior 
median  cokimn.  p.e.c.  Posterior  external  column,  al.asc. 
Antero-lateral  ascending  tract. 

therefore,  all  the  fibres  reach  the  crossed  pyramidal  tracts. 
They  end  in  the  spinal  cord  by  turning  into  the  grey  matter, 
where  they  break  up  into  a  fine  bunch  of  fibrils  in  close 
connection  with  the  motor  cells  of  the  anterior  cornua  (or, 
according  to  SchJifer,  with  the  cells  of  the  posterior  cornua). 
On  their  way  down  the  cord  they  give  oii"  fine  side  branches 
or  '  collaterals,'  which  run  into  the  anterior  cornu  and  there 
terminate,  thus  establishing  connections  between  one  cortical 
cell  and  the  anterior  cornual  cells  of  several  diflerent  seg- 
ments of  the  spinal  cord.  It  is  therefore  concluded  that 
they  carry  motor  impulses  from  the  cerebral  cortex  to  the 
ganglion  cells  of  the  cord.  Destruction  of  these  columns  by 
disease  or  otherwise  causes  the  abolition  of  voluntary  control 
over  the  muscles. 


600 


PHYSIOLOGY 


Ventrally  to  the  pyramidal  tracts  there  is  a  fairly  compact  group  of  fibres 
which  degenerate  downwards.  This  is  known  as  the  pre-pyramidal  or  rubro- 
spinal tract,  since  its  fibres  can  be  traced  up  to  the  cells  in  the  red  nucleus, 
a  mass  of  grey  matter  in  the  mid-brain  situated  ventrally  to  the  nucleus  of  the 
third  nerve. 

There  are  also  some  scattered  fibres  in  the  antero-lateral 
column,  which  degenerate  in  the  downward  direction.  These 
were  formerly  supposed  to  be  derived  from  the  cerebellum  of 
the  same  side,  but  it  has  been  shown  that  they  are  in  all 
probability  derived  from  Deiters'  nucleus  in  the  medulla, 
which    stands   as  a  downward  transmitting  station   between 


Fig.  276. 


spL 


Diagram  (from  Schafer)  showing  the  ascending  (right  side)  and  the 
descending  (left  side)  tracts  in  the  spinal  cord. 
1.  Crossed  pyramidal.  2.  Direct  i^yramidal.  3.  Antero-lateral 
descending.  3a.  Spino-olivary  descending  (bundle  of  Helweg). 
4.  Pre-pyramidal  (rubrospinal).  5.  Comma.  6.  Postero-mesial. 
7.  Postero-lateral.  8.  Lissauer's  tract.  9.  Dorsal  (ascending) 
cerebellar.  10.  Antero-lateral  ascending,  s.m.  Septo-marginal. 
s.p.l.  Dorsal  root  zone.  a.  Anterior  horn  cells,  i.  Intermedio- 
lateral  horn.  p.  Cells  of  posterior  horn.  d.  Clarke's  column. 
The  fine  dots  represent  the  situation  of  the  '  internuncial ' 
or   'endogenous  '  fibres  of  the|spinal  cord. 


cerebellum   and    cord.      These  are  sometimes  known  as  the 
vestibulo-spinal  tract. 

Beside  these  tracts  there  is  a  little  collection  of  fibres  in 
the  posterior  columns  which  degenerates  for  a  few  segments 
of  the  cord  heloio  a  transverse  lesion  (the  '  comma  '  tract). 
They  consist  largely  of  fibres  derived  from  the  posterior  roots, 
which  divide  into  ascending  and  descending  branches  as  they 
enter  the  cord,  but  also  contain  probably  certain  fibres  derived 
from  cells  in  the  cord  and  serving  to  connect  one  segment 
of  the  cord  with  another. 


Fig.  277. 

,cere!bral  cortex 


CEREBELLAR 
HEMISPHERE 


Diagram  showing  the  course,  origin,  and  termination  of  the  principal 
tracts  of  the  spinal  cord  (Schiifer). 
'  Descending  tracts.'— la.  A  fibre  of  the  crossed  pyramidal  tract,  lb. 
An  uncrossed  fibre  of  the  pyramidal  tract  passing  to  the  lateral 
column  of  the  same  side.  2.  A  fibre  of  the  direct  pyramidal 
tract.  3.  Antero-lateral  descending  tract.  4.  Pre-pyramidal 
tract.  5.  Comma  tract.  '  Ascencliufj  tracts.'' — 6.  Postero-mesial. 
7.  Postero-lateral  tract.  9.  Dorsal  cerebellar.  10.  Antero-lateral 
ascending,     m.  Motor  nerve  fibre,     s.  Sensory  roots. 


602  PHYSIOLOGY 

B.  Ascending  tracts. — The  tracts  which  degenerate  in  the 
cord  above  a  transverse  section  are  four  in  number : 

1.  The  postero-median,  as  far  up  as  the  nucleus  gracilis 
in  the  medulla. 

2.  The  posterior  root-zone  for  one  or  two  segments  above 
the  lesion. 

3.  The  direct  or  posterior  cerebellar  tract  as  far  as  the 
cerebellum. 

4.  The  antero-lateral  ascending  tract  or  anterior  cerebellar 
tract.  The  fibres  of  this  tract  also  end  in  the  cerebellum 
near  those  of  No.  3,  but  take  a  more  circuitous  path  than 
those  of  the  latter  tract. 

Division  of  all  the  posterior  roots  on  one  side  causes 
degeneration  of  the  posterior  root-zone  and  postero-median 
column  on  the  same  side,  as  well  as  of  some  fibres  in  the 
*  comma '  tract.  Hence  the  fibres  of  these  columns  have 
their  trophic  centres  on  the  ganglia  of  the  posterior  roots. 
The  other  two  ascending  tracts  do  not  degenerate  after  section 
of  the  posterior  roots,  and  must  therefore  have  their  trophic 
centres  in  the  cells  of  the  grey  matter  of  the  cord.  We  have 
already  mentioned  that  the  direct  cerebellar  tract  is  derived 
from  the  cells  of  the  posterior  vesicular  column  of  Clarke, 
and  it  has  been  stated  that  the  antero-lateral  ascending  tract 
also  originates  in  the  same  way.  All  these  tracts,  on  their 
way  up  the  cord,  send  off  branches  or  collaterals  which 
terminate  round  the  cells  of  the  grey  matter  in  the  different 
segments  of  the  cord.  Some  of  these  run  from  the  posterior 
root-fibres  directly  across  to  the  anterior  cornu  of  the  same 
side,  and  thus  subserve  the  simplest  forms  of  reflex  action 
(cf.  Figs.  273  and  276). 

The  deeper  parts  of  the  white  matter,  near  the  grey 
matter,  in  the  anterior  and  lateral  columns,  are  known  as  the 
ventral  and  lateral  basis  bundles.  They  contain  fibres,  some 
ascending,  some  descending,  which  seem  to  connect  different 
levels  of  the  cord,  some  fibres  of  which  can  be  traced  from 
the  cervical  to  the  lumbo-sacral  region.  Besides  these  'inter- 
nuncial '  fibres,  there  is  a  large  number  of  fibres  in  the  white 
matter  which  do  not  degenerate  more  than  one  spinal  segment 
either  above  or  below  a  transverse  section  of  the  cord.  These 
are  supposed  to  be  commissural,  serving  to  connect  one 
segment  of  the  cord  with  the  other. 


THE  SPINAL  COED  608 

Section  2 
PATHS   OF   IMPULSES   IN   THE   COED 

All  impulses  which  pass  through  the  cord  form  parts  of 
reflex  actions,  carried  out  either  by  the  cord  alone,  or  with 
the  intervention  of  the  higher  parts  of  the  central  nervous 
system,  the  bulb,  cerebellum,  or  cerebral  hemispheres.  We 
may  therefore  conveniently  distinguish  the  local  spinal  re- 
flexes from  the  brain  reflexes,  including  the  complex  ones 
involving  conscious  sensation  and  volition.  All  these  reflexes 
are  inaugurated  by  afterent  impulses,  which  reach  the  central 
nervous  system  along  sensory  nerve-roots. 

The  posterior  spinal  (sensory)  roots  at  their  entrance  into 
•  the  cord  divide  into  two  bundles.  The  smaller  of  the  two, 
situated  more  laterally  and  consisting  of  fine  fibres,  enters 
opposite  the  tip  of  the  posterior  horn  and  turns  up  at  once  in 
Lissauer's  tract,  a  bundle  of  fine  longitudinal  fibres  lying  close 
to  the  periphery  of  the  cord.  The  fibres  seem  to  pass  into 
and  end  in  the  substance  of  Rolando.  The  larger  median 
bundle  of  coarse  fibres  passes  mto  the  postero-external  column. 
Here  each  fibre  divides  into  a  descending  and  an  ascending 
branch,  the  former  running  in  the  comma  tract,  the  latter  in 
the  posterior  columns  up  as  far  as  the  gracile  and  cuneate 
nuclei  of  the  medulla.  Both  of  these  branches  give  oft'  col- 
laterals in  the  whole  of  their  course,  most  numerous  near  the 
point  of  entry  of  the  nerve.  These  collaterals  may  be  divided 
into  four  sets  according  to  their  destination. 

1.  Fibres  ending  round  cells  of  anterior  horn  on  same 
side  or  crossing  by  posterior  commissure  to  grey  matter  on 
other  side. 

2.  Fibres  ending  in  grey  matter  of  posterior  horns. 

3.  Fibres  ending  round  cells  of  Clarke's  column. 

4.  Fibres  to  lateral  horn. 

Since  the  motor  nerves  arise  from  the  anterior  horn  cells, 
the  first  set,  the  so-called  sensori-motor  collaterals,  represent 
the  shortest  possible  spinal  reflex  path.  The  second  group  may 
also  represent  a  spinal  reflex  path  with  two  relays  of  cells,  and 
therefore  greater  choice  of  response  and  longer  reaction  time. 

The  third  set  puts  into  action  the  cerebellar  tracts  which 
arise  from  the  cells  of  Clarke's  column,  and  therefore  call  into 
play  a  much  more  complicated  mechanism,  the  limits  of  whose 
action  it  would  be  difficult  to  define.     The  collaterals  to  the 


604  PHYSIOLOGY 

lateral  horn  probably  represent  the  afferent  tracts  of  the  various 
visceral  and  vaso-motor  reflexes  which  we  studied  in  the  earlier 
chapters. 

In  dealing  with  the  reflexes  involving  the  co-operation  of 
the  brain  we  find  no  special  tracts  devoted  to  those  impulses 
which  affect  consciousness  as  sensations.  All  tracts  going 
towards  the  cerebral  hemispheres  are  interrupted  by  cell  re- 
lays in  the  medulla  or  cerebellum,  and  must  serve  as  afferent 
channels  for  unconscious  as  well  as  for  conscious  reactions. 
The  quality  of  an  afferent  impulse  can  only  be  defined  by  its 
origm,  or  by  its  effect  on  consciousness  ;  and  much  discussion 
has  arisen  as  to  the  exact  path  of  the  various  cutaneous  and 
muscular  sensations  in  the  cord. 

It  is  evident  that  an  impulse  may  travel  to  the  cortex  by 
way  of  the  two  cerebellar  tracts  through  the  cerebellum,  or  by 
way  of  the  posterior  columns  through  the  intermediation  of  the 
bulbar  nuclei,  or  by  a  series  of  relays  from  one  segment  of  the 
cord  to  another  through  grey  and  white  matter  alternately. 

It  is  supposed  that  all  of  the  ascending  tracts  may  con- 
vey afferent  impulses  from  the  posterior  roots  to  the  brain, 
although  evidence  as  to  the  part  taken  by  each  tract  is  very 
conflicting.  The  following  account  represents  the  views 
which  may  be  regarded  as  the  most  probable  (Page  May)  : 
Pain  impulses,  on  entering  the  cord  by  the  posterior  roots, 
cross  to  the  other  side  at  once,  and  then  pass  up,  chiefly  in 
the  antero-lateral  ascending  tract  of  Gowers,  as  far  as  the 
optic  thalamus.  Sensations  of  heat  and  cold  take  a  very 
similar  course.  Hence  they  are  generally  affected  by  lesions 
of  the  cord  in  the  same  way  as  pain  sensations.  Impulses  of 
touch  and  pressure,  after  entering  the  cord,  pass  up  in  the 
posterior  column  of  the  same  side  for  four  or  five  segments,  then 
cross  gradually  and  pass  up  in  the  opposite  anterior  column. 
Muscular  sensibility,  including  the  impulses  from  joints  and 
tendons,  take  two  courses.  Those  which  do  not  reach  con- 
sciousness, and  are  involved  in  the  involuntary  guidance  of 
muscular  movements,  run  up  chiefly  in  the  anterior  and 
posterior  cerebellar  tracts  of  the  same  side.  Those  which 
furnish  the  material  for  conscious  sensations  and  give 
information  as  to  the  position  of  the  limbs,  &c.,  are  entirely 
homolateral,  and  travel  up  in  the  posterior  columns  of  the 
same  side  of  the  cord.  All  impulses  which  reach  the  brain 
cross  finally  to  the  optic  thalamus  and  thence  to  the  cortex  of 
the  opposite  side. 


THE    SPINAL   COED  605 

Hemisection  of  the  cord  on  one  side  causes  paralysis  of 
the  same  half  of  the  body  below  the  lesion.  Muscular  sensi- 
bility is  also  destroyed  on  this  side.  On  the  other  side  of 
the  body,  sensation  to  heat,  cold,  pain,  and  partial^  to  touch, 
is  destroyed.  The  sensory  disturbances  however  rapidly  pass 
off,  and  in  animals  the  spinal  cord  can  be  hemisected  alter- 
nately on  right  and  left  side  at  three  dijBferent  levels  without 
causing  a  lasting  anesthesia,  showing  that  in  the  absence  of 
the  direct  paths  the  sensory  impulses  may  zigzag  up  the  cord 
and  finally  reach  their  destination  and  affect  consciousness. 

The  only  direct  unbroken  cortico- spinal  fibres  are  those 
contained  in  the  pyramidal  tracts.  Motor  impulses,  which 
start  from  the  cerebral  cortex  on  one  side,  pass  down  that 
s;de  till  they  reach  the  lower  part  of  the  medulla.  Here  the 
greater  number  of  the  fibres  cross  over  in  the  pyramidal 
decussation  to  run  down  in  the  crossed  pyramidal  tract  on 
the  other  side  of  the  cord.  The  few  fibres  which  do  not  cross 
over  in  the  pyramidal  decussation  are  continued  as  the  direct 
or  anterior  pyramidal  tract.  These  however  also  cross  to  the 
other  side  in  their  passage  down  the  cord  before  becoming 
connected  with  the  anterior  cornual  cells.  Hemisection 
therefore  of  the  spinal  cord  in  the  dorsal  region  will  produce 
paralysis  of  motion  and  loss  of  or  impaired  sensation  in  the 
parts  supplied  by  the  nerves  on  the  same  side  below  the  lesion.^ 

A  great  part  of  the  white  matter  of  the  cord  is  concerned 
then  in  maintaining  connection  between  the  brain  and  higher 
parts  of  the  nervous  system  and  the  periphery,  through  the 
intermediation  of  the  cells  of  the  grey  matter  of  the  cord. 
Corresponding  to  this  function  we  find  a  gradual  increase  in 
the  number  of  fibres  in  the  white  matter  as  we  ascend  from 
the. sacral  part  of  the  cord  to  the  medulla,  the  white  matter 
being  continually  reinforced  as  it  ascends  the  cord  by  fibres 
establishing  connection  witli  the  ganglion  cells  forming  the 
nuclei  of  the  nerve-roots. 

Vaso-motor  impulses  to  the  limbs  travel  down  the  lateral 
columns  of  the  cord  on  the  same  side. 

'  It  has  been  recently  stated  by  Sebafer  that  the  pyramidal  fibres  end,  not 
in  the  anterior  cornua  but  round  the  cells  of  the  posterior  cornua.  If  this  be 
confirmed,  it  would  show  that  the  cortex  affected  the  spinal  motor  apparatus 
by  attacking  its  sensory  rather  than  its  motor  side,  so  as  to  save  a  multiplication 
of  co-ordinating  mechanisms. 


606  PHYSIOLOGY 

Section  3 
THE    COED    AS    EEFLEX    CENTEE 

In  the  lower  animals,  such  as  the  frog,  the  spinal  cord  of 
itself  is  able  to  carry  out  many  complex  reflex  actions.  If 
the  skin  around  the  anus  of  a  decapitated  frog  is  stimulated, 
a  sudden  extension  of  both  legs  is  produced,  so  that  the 
animal  leaps  away  from  the  stimulus.  If  a  small  piece  of 
filter-paper  moistened  with  acetic  acid  be  placed  on  the  inner 
side  of  the  right  thigh,  the  right  foot  will  be  raised  and 
used  to  wipe  away  the  offending  object.  If  the  right  leg  be 
held  or  be  cut  off,  after  various  fruitless  endeavours  to  re- 
move the  irritant  with  this  limb,  the  left  leg  may  be  raised 
and  used  for  this  purpose.  These  and  many  other  similar 
experiments  show  that  the  spinal  cord,  separated  from  the 
upper  part  of  the  nervous  system,  is  capable  in  the  frog  of 
bringing  about  many  highly  complex  co-ordinated  move- 
ments, which  are  apparently  purposive,  i.e.  they  seem  to  have 
a  definite  object  in  view ;  and  arguing  from  experiments, 
such  as  the  second  one  we  have  mentioned,  it  has  been  thought 
that  psychical  phenomena  may  accompany  these  reflex  actions. 
But  it  must  be  remembered  that  summation  of  afferent  im- 
pulses occurs  just  as  summation  of  stimuli  applied  to  a  frog's 
ventricle.  A  single  stimulus,  too  weak  to  evoke  a  reflex 
contraction,  may  do  so  if  repeated  several  times.  In  our 
experiment,  the  right  leg  being  unable  to  remove  the  offend- 
ing object,  stimulation  goes  on,  and  the  effect  is  summated 
until  it  is  strong  enough  to  spread  to  the  other  side  of  the 
cord  and  so  set  the  left  leg  in  motion. 

The  time  taken  up  in  the  transmutation  of  afferent  into 
efferent  impulses  in  the  spinal  cord  may  be  estimated  by 
measuring  the  interval  that  elapses  between  stimulation  of  a 
sensory  nerve  with  a  single  induction-shock  and  the  resulting 
muscular  contraction,  and  subtracting  from  the  amount  so 
determmed  the  time  taken  in  the  passage  of  the  impulse  up 
and  down  the  nerve-fibres  and  the  latent  period  of  the 
muscle,  the  contraction  of  which  is  recorded.  The  reflex 
time  or  '  reduced  reaction  time  '  measured  in  this  way  is 
found  to  be  about  0*01  second. 


THE   SPINAL  COED  607 

A  slight  stimulus  causes  reflex  contraction  only  of  the 
limb  stimulated.  A  stronger  stimulus  causes  contraction  of 
the  corresponding  limb  of  the  other  side  also,  and  the  effect 
of  a  still  stronger  stimulus  may  extend  to  the  other  two 
limbs.  With  this  resistance  to  passage  of  impulses  in  the 
cord  across  the  middle  line  and  longitudinally  from  one 
segment  to  another,  we  find  a  corresponding  increase  in  the 
reflex  time. 

Effect  of  strychnine. — If  strj^chnine  be  injected  into  the 
dorsal  lymph -sac  of  a  frog,  the  spinal  cord  is  so  affected  that 
the  normal  resistance  to  passage  of  impulses  is  abolished. 
The  slightest  stimulus  of  the  skin  now  evokes  a  maximal  reflex 
action,  there  being  no  longer  any  proportionality  between  the 
magnitude  of  the  stimulus  and  that  of  the  reflex  effect  pro- 
duced. The  minimal  reflex  time  is  not  diminished,  but  the 
smallest  stimulus  can  travel  equally  well  in  all  directions  in 
the  cord.  Hence  the  slightest  touch  of  the  skin  sends  all 
the  muscles  mto  prolonged  tetanic  contraction,  and  the  frog 
becomes  stretched  out  with  its  limbs  stiff  and  rigid. 

It  has  been  shown  by  Sherrington  that  the  most  characteristic  point  about 
the  action  of  strychnine  is  that  this  drug  converts  inhibitory  into  motor 
reflexes.  Tlius  when  flexion  of  the  knee  is  excited  by  stimulation  of  the  pad 
of  the  foot,  this  movement  involves  not  only  contraction  of  the  hamstring 
muscles  but  also  relaxation  of  the  extensors  of  the  knee.  After  injection  of 
strychnine  a  similar  stimulus  applied  to  the  foot  causes  simultaneous  con- 
traction both  of  extensors  and  flexors,  all  the  inhibitory  phenomena  of  every 
co-ordinated  muscular  reaction  being  abolished  by  the  drug. 

Inhibition. — The  reflex  action  normally  following  a  slight 
stimulus  of  any  part  of  the  body  may  be  completely  i)re- 
vented  or  inhibited  by  strong  sensory  stimulation  of  some 
other  part.  If  the  optic  lobes  of  a  frog  be  stimulated  by 
putting  a  crystal  of  salt  on  them,  or  the  central  end  of  the 
right  sciatic  nerve  by  means  of  a  farad ic  current,  stimulation 
of  the  skin  of  the  left  leg  with  acid  produces  no  effects 
whatever.  A  striking  parallel  instance  of  this  occurs  in  our 
daily  mental  life.  Concentration  of  the  attention  in  any  one 
direction,  either  by  severe  pain  or  through  psychical  excite- 
ment, causes  similar  stimuli  to  be  quite  unheeded,  so  that  in 
battle  a  man  may  be  unaware  that  he  is  severely  wounded 
until  he  feels  faint  or  sees  blood  flowing.  We  may  say, 
putting  the  phenomena  of  the  spinal  cord  into  terms  of  con- 
sciousness, that  its  ganglion  cells  are  so  much  occupied  with 


608  PHYSIOLOGY 

the   stronger   stimulus   that   they   do   not    notice   a   weaker 
stimuhis  applied  to  some  other  part. 

Spinal  Beflexes  iji  Higher  Animals 

When  we  come  to  the  highest  animals— monkey  and  man 
— there  seems  to  be  a  striking  difference  between  their  spinal 
cord  and  that  of  the  frog,  in  that  the  reflex  actions,  which 
can  be  carried  out  by  the  cord  severed  from  the  medulla  and 
brain,  are  limited  to  those  of  the  simplest  nature.  If  a  man 
has  had  his  cord  crushed  in  the  dorsal  region,  tickling  the 
soles  of  his  feet  will  cause  him  to  draw  up  his  legs,  although 
he  is  perfectly  unconscious  that  his  feet  are  being  touched. 
But  beyond  one  or  two  simple  reflexes  of  this  description, 
the  spinal  cord  seems  to  have  no  power  of  carrymg  out 
co-ordinated  acts.  It  is  however  difficult  in  these  cases,  and 
in  experiments  on  the  spinal  cord  in  mammals,  to  eliminate 
the  effects  of  shock.  After  total  transverse  section  of  the 
spinal  cord  high  up,  the  animal  is  in  a  condition  of  shock, 
which  lasts  a  considerable  time ;  his  vital  activities  are  pro- 
foundly depressed,  and  it  may  be  impossible  to  evoke  even 
the  simplest  reflex  action  by  stimulation  of  any  sensory  sur- 
face below  the  lesion.  But  if  the  experiment  be  carefully 
conducted,  and  the  animal  be  kept  alive  for  a  considerable 
time,  the  cord  little  by  little  recovers  its  powers,  and  we  then 
find  that  the  spinal  cord  of  the  dog  can  carry  out  the  most 
complicated  reflex  movements  without  any  connection  with 
the  higher  centres.  In  a  dog  whose  cord  has  been  divided 
in  the  dorsal  region,  the  reflex  movements  required  for 
micturition,  defsecation,  impregnation,  and  parturition  may 
be  normally  carried  out.  If  the  dog,  which  usually  squats 
on  the  ground  owing  to  the  paralysis  of  its  hinder  extremities, 
be  raised  on  its  hind  legs  by  the  hands  being  placed  under 
the  fore-legs,  and  given  a  little  push  forwards,  the  animal 
may  run  along  for  a  few  steps  before  it  collapses  again  into 
a  sitting  posture.  In  this  case  the  reflex  runnmg  movements 
of  the  hind  legs,  carried  out  by  the  separated  spinal  cord, 
are  started  by  the  sudden  stretching  of  the  anterior  thigh 
muscles. 

The  vascular  tone  in  the  lower  part  of  the  body,  which  is 
ost  for  some  time  after  the  operation,  is  also  regamed. 

Muscular  tone. — Every  muscle  in  the  body  is  normally  in 


THE   SPINAL   COED  609 

a  condition  of  slight  continued  contraction,  which  is  known 
as  muscular  tone.  If  a  frog  with  intact  spinal  cord  l)e  sus- 
pended by  the  jaw,  and  the  nerves  going  to  the  lower  limb 
be  cut,  this  limb  will  hang  down  straighter  than  the  other  in 
consequence  of  the  abolition  of  its  muscular  tone.  The  same 
result  may  be  produced  if,  instead  of  dividing  all  the  nerves 
of  the  limb,  only  the  sensory  or  only  the  anterior  roots  of 
those  nerves  l)e  divided.  This  shows  that  muscular  tone 
is  reflex,  and  depends  for  its  maintenance  on  the  intact  con- 
dition of  afferent  paths,  centre,  and  efferent  paths.  On  the 
presence  of  this  tone  depends  the  phenomenon  known  as 
'  tendon-reflex '  or  knee-jerk.  If  the  leg  be  allowed  to  hang 
loosely  and  the  patellar  tendon  be  struck,  the  extensor 
muscles  of  the  thigh  contract  and  raise  the  leg.  This  con- 
traction is  probably  due  to  the  direct  stimulation  of  the 
muscle  by  the  sudden  stretching  produced  on  striking  its 
tendon.  If  the  muscle  has  lost  its  tone  by  disease  of  the 
spinal  cord,  or  through  section  of  the  afferent  or  efferent  fibres, 
striking  the  tendon  will  no  longer  stretch  the  flabby  muscle, 
and  the  tendon-reflex  will  be  abolished.  The  mere  stretching 
however  is  not  the  only  factor,  since  under  these  conditions 
no  knee-jerk  is  produced,  however  much  the  muscle  may  be 
passively  stretched.  The  impulses  descending  the  nerves 
to  the  muscles  seem  to  keep  them  in  a  state  of  wakefulness, 
ready  to  respond  to  the  slightest  local  stimulus.  The  cha- 
racters of  these  reflex  tonic  impulses  are  much  affected  by 
the  influence  of  other  afferent  stimuli.  Especially  interestmg 
is  the  relation  shown  by  Sherrington  to  exist  between  the 
tonic  condition  of  antagonistic  muscles,  e.g.  between  the 
hamstrings  and  the  vastus  internus  of  the  quadriceps  extensor 
muscle.  Section  of  the  hamstrmg  muscles  (so  as  to  relax 
them),  or  even  section  of  their  nerve,  causes  at  once  great 
increase  in  the  jerk  elicited  by  tapping  the  patellar  tendon. 
On  the  other  hand  the  knee-jerk  is  at  once  abolished  by 
stretching  the  hamstring  muscles,  or  by  weak  stimulation  of 
the  central  end  of  the  cut  nerve  to  the  hamstrings  (Fig.  278). 
Every  sensory  stimulus  which  evokes  contraction  of  one  set 
of  muscles  will  therefore  produce  inhibition  of  the  antago- 
nistic set. 

Very  great  exaggeration  of  the  tendon  phenomenon  is  ob- 
served in  cases  where  the  pyramidal  tracts  are  degenerated, 

39 


610  PHYSIOLOGY 

and  indicates  a  heightened  reflex  excitabihty  of  the  lower 
spinal  centres,  perhaps  reinforced  hy  impulses  from  the  cere- 
bellum. 

The  value  of  the  tendon  phenomena  as  a  means  of  dia- 
gnosis has  tended  to  obscure  their  great  importance  in  the 
normal  individual.  Every  joint  is  protected  by  inextensible 
ligaments  and  by  muscles.  A  sudden  strain  on  a  ligament 
either  will  have  no  effect,  or  will  rupture  some  of  its  fibres 
and  perhaps  injure  the  adjacent  joint  surfaces.     An  ordinary 


Fig.  278. 


L.3 


Ant?  crur  .n. 


Diagram  to  show  muscles  and  nervei3  concerned  in  Sherrington's 
experiment  on  tlie  reciprocal  innervation  of  antagonistic 
muscles.  L.3,  L.4,  L.5,  third,  fourth,  and  fifth  lumbar  roots; 
S.l,  S.'2,  first  and  second  sacral  roots. 

reflex  contraction  would  be  powerless  to  prevent  this,  since 
the  mischief  would  be  done  before  the  reaction  could  take 
place.  But  the  central  nervous  system  confines  itself  to 
keeping  the  muscles  awake,  so  that  they  themselves  may 
react  to  any  sudden  increase  in  their  tension  by  an  equally 
sudden  contraction,  which  saves  the  joint  before  the  central 
nervous  system  has  even  become  aware  of  the  strain. 

Co-ordmation  of  Movements  hy  the  Cord. — Since  the  act 
of  stretching  a  muscle  in  a  normal  state  of  tone  acts  as  an 
excitant,  and  causes  increased  tone  or  even  contraction,  every 


THE   SPINAL   COED  611 

movement  of  a  limb,  by  putting  the  muscles,  antagonistic  to 
the  movement,  on  the  stretch,  would  tend  to  prevent  the 
carrying  out  of  the  movement.  Thus  flexion  of  the  knee 
would  stretch  the  extensor  muscles  and  thus  increase  their 
tone  and  resistance  to  the  movement  of  flexion.  Sherrington 
has  shown  however  that  every  movement  of  a  limb,  worked 
reflexly,  involves  not  only  the  contraction  of  the  muscles 
eftecting  the  movement,  l)ut  also  a  simultaneous  inhibition  of 
tone  and  relaxation  of  the  antagonistic  muscles,  and  this 
occurs  in  the  '  spinal  animal,'  i.e.  in  one  whose  spinal  centres 
are  entirely  cut  oft'  from  the  brain.  Thus  a  '  painful ' 
stimulus,  such  as  the  prick  of  a  needle  applied  to  the  pad  of 
the  foot,  causes  retraction  of  the  foot,  and  this  movement 
involves  contraction  of  the  flexors  and  relaxation  of  the 
extensors  of  the  limb.  On  the  other  hand  gentle  pressure 
applied  to  the  pad  causes  inhibition  of  the  flexors  and  con- 
traction of  the  extensors,  so  that  the  limb  is  extended. 

The  reflex  extends  also  to  the  other  side  of  the  cord,  but 
here  the  eftbrts  are  reversed :  extension  of  the  stimulated 
limb  being  associated  with  flexion  of  the  opposite  limb,  i.e.  the 
ordinary  association  of  movements  involved  in  the  diagonal 
action  of  the  limbs  in  progression. 

The  spinal  cord  is  thus  able  to  carry  out  co-ordinated  acts 
of  considerable  complexity,  these  acts  being  adapted  to  the 
character  and  situation  of  the  stimulus,  and  also  to  the 
previous  state  of  activity  of  the  cord  and  of  the  muscles 
involved. 

Other  reflex  functions  of  the  cord  are  concerned  with  the 
carrying  out  of  the  following  actions  :  — 

Micturition. 

Defaecation. 

Impregnation  and  parturition. 

Vascular  tone. 


612  PHYSIOLOaY 


CHAPTER    XV 

THE   BRAIN 

Section  1 
THE    RTRUCTUEE    OF    THE    BRAIN 

The  physiology  of  the  brain  falls  naturally  into  two  main 
divisions,  viz. — the  cerebral  hemispheres,  and  the  rest  of  the 
brain,  including  medulla,  pons,  iter,  and  corpora  quadrige- 
mina,  and  third  ventricle. 

This  second  part  may  be  considered  as  a  prolongation  of 
the  spinal  cord  forwards,  consisting  like  this  of  a  central 
tube  of  grey  matter,  surrounded  by  a  tube  of  white  matter. 
Owing  to  the  importance  and  complex  connections  of  the 
nerve-roots  which  arise  from  this  part  of  the  neural  axis 
(cranial  nerves),  the  typical  division  of  grey  matter  into 
cornua  becomes  lost ;  and  we  find  that,  while  some  nerves 
take  their  origin  from  the  central  tube  of  grey  matter,  in 
other  cases  the  collection  of  cells  forming  the  nucleus  has 
become  more  or  less  separated  from  the  central  axis. 

Moreover  masses  of  grey  matter,  which  have  no  repre- 
sentative in  the  cord,  make  their  appearance  in  the  white 
matter. 

The  roof  of  the  neural  canal,  which  over  the  third  ven- 
tricle and  the  posterior  part  of  the  fourth  is  greatly  thinned, 
consisting  merely  of  a  layer  of  epithelial  cells,  is  thickened 
over  the  iter  (second  cerebral  vesicle)  to  form  the  corpora 
quadrigemina,  and  over  the  pons  and  anterior  part  of  the 
fourth  ventricle  it  grows  out  into  a  large  excrescence  with 
complicated  structure— the  cerebellum  fcf.  Fig.  285).  Cover- 
ing the  corpora  quadrigemina  and  cerebellum  is  a  layer  of 
grey  matter  outside  a  central  mass  of  white  fibres.  The 
lateral  walls  of  the  third  ventricle  (first  cerebral  vesicle)  are 
thickened  to  form  the  optic  thalami,  which  contain  masses 


THE   i3RAIN 


613 


of  grey  matter.  The  cerebral  hemispheres  are  formed  by 
hollow  outgrowths  from  the  first  cerebral  vesicle.  These  in 
course  of  development  become  as  large  as  the  whole  of  the 
rest  of  the  brain  put  together,  and  grow  backwards  over  the 
rest  of  the  brain  as  far  as  the  middle  of  the  cerebellum 
(Figs.  9, 10,  pp.  16, 17).  Their  upper  walls  become  very  much 
thickened,  and  consist  of  white  matter  internally  and  grey 
matter  externally.  Their  lower  walls  remain  as  a  thin  layer 
of  undifferentiated  epithelial  cells ;  this  becomes  closely 
applied  to  the  epithelial  layer  forming  the  roof  of  the  third 


Human  brain  viewed  from  the  bide,  to  display  the  various  divisions 
(Quain).  A,  cerebral  hemisphere ;  B,  cerebellum ;  C,  pons 
Varolii ;  D,  medulla  oblongata ;  a,  crura  cerebri ;  b,  superior, 
c,  middle,  and  </,  inferior  peduncles  of  cerebellum. 

ventricle,  from  which  it  is  separated  only  by  a  process  of  the 
pia  mater  carrying  numerous  blood-vessels  (the  velum  inter- 
positum).  The  lower  and  outer  wall  of  the  cerebral  hemi- 
spheres becomes  very  much  thickened,  and  forms  the  corpus 
striatum,  which  is  closely  applied  to  the  front  and  outer  part 
of  the  optic  thalamus.  In  it  two  masses  of  grey  matter  are 
developed,  the  nucleus  caudatus  and  nucleus  lenticularis, 
separated  from  one  another  by  a  layer  of  white  fibres,  which 
is  continuous  with  a  similar  layer  separating  the  corpus 
striatum  from  the  optic  thalamus,  and  is  called  the  internal 
capsule. 


614 


PHYSIOLOGY 


It  will  be  convenient  to  trace  first  the  modifications  under- 
gone by  the  axial  part  of  the  nervous  system  in  the  brain, 
and  then  to  deal  with  the  new  masses  of  grey  matter  which 
have  no  homologies  in  the  spinal  cord,  as  well  as  the  long 
tracts  of  white  matter  serving  to  connect  different  levels  or 
different  sides  of  the  brain. 

In  examining  successive  sections  from  the  spinal  cord 
up  through  the  medulla,  the  first  change  which  makes 
its  appearance   is  due  to  the   decussation   of   the  pyramids 


Bection  IIuoukIi  the  lower  bolder  of  the  nieduUii  oblongata,  at 
the  pyramidal  decussation  (Bechterew).  f.l.a.  anterior  fissure  ; 
cl,  decussation  of  the  pyramids ;  V,  anterior  columns  ;  C.a, 
anterior  cornu  ;  cc,  central  canal ;  S,  lateral  columns ;  f.r, 
formatio  reticularis ;  cc,  neck,  and  g,  head  of  the  posterior 
cornu;  r.p.C.I,  posterior  root  of  first  cervical  nerve;  n.c, 
beginning  of  nucleus  cuneatus ;  n.cj,  nucleus  gracilis;  H\ 
funiculus  gracilis;  H'-,  funiculus  cuneatus;  s.l.p,  posterior 
fissure. 


(Fig.  280).  Throughout  the  spinal  cord,  fibres  have  been 
crossing  from  one  side  to  the  other  through  the  anterior 
white  commissure,  many  of  them  belonging  to  the  pyramidal 
system.  But  at  the  lower  border  of  the  medulla  we  see 
a  large  mass  of  fibres  crossing  between  the  anterior  columns 
and  the  postero-lateral  columns,  at  first  cutting  off  the  head 
of  the  anterior  horn  and  later  on  breaking  this  up  altogether, 
so  that  the  only  definite  collection  of  grey  matter  left  in  this 
situation  is  a  small  part  of  the  lateral  column  of  grey  matter 


THE   BRAIN  615 

known  as  the  lateral  nucleus.  In  this  way  are  formed  the 
big  anterior  columns  of  the  medulla,  which  are  known  as  the 
pyramids,  and  which  contain  all  the  fibres  that  in  the  cord 
are  represented  by  the  direct  and  crossed  pyramidal  tracts. 

The  next  change  is  due  to  the  ending  of  the  posterior 
columns.  It  will  be  remembered  that  these  are  the  central 
ascending  branches  of  dorsal  nerve-roots,  having  therefore 
an  origin  outside  the  cord.  All  the  way  up  the  cord  they 
send  in  collaterals  to  end  in  the  grey  matter  of  the  posterior 
horn.  The  main  mass  however  terminates  in  the  medulla 
just  above  the  pyramidal  decussation  in  two  collections  of 
grey  matter —the  nucleus  gracilis  and  the  nucleus  cuneatus, 
which  seem  to  be  formed  by  a  great  hypertrophy  of  the  grey 
matter  at  the  root  of  the  posterior  horn.  The  effect  of  this 
development  in  the  dorsal  region  of  the  medulla  is  to  push 
the  head  of  the  posterior  horn  outwards.  At  the  same  time 
this  mass  of  gelatinous  substance  becomes  enlarged,  so  that 
in  section  we  have  three  grey  masses  from  within  outwards, 
the  nucleus  gracilis,  the  nucleus  cuneatus,  and  the  nucleus  of 
Kolando  (Fig.  281). 

With  the  disappearance  of  the  posterior  columns  and  the 
development  of  the  pyramids  we  get  a  practical  obliteration 
of  the  anterior  fissure  and  a  displacement  of  the  central 
canal  towards  the  dorsal  surface.  A  little  higher  up  the 
canal  opens  out  altogether,  forming  the  fourth  ventricle, 
covered  on  its  dorsal  surface  only  by  a  thin  layer  of  epen- 
dyma,  a  simple  epithelium  representing  all  that  is  left  of 
the  dorsal  wall  of  the  primitive  hind  brain.  The  appearance 
of  the  section  is  now  modified  by  two  structures.  In  the 
first  place  a  new  mass  of  grey  matter  consistmg  of  a  thin 
layer  shaped  like  a  flask  with  its  orifice  directed  inwards 
(Fig.  282,  o)  is  developed  in  the  lateral  part  of  the  medulla, 
between  the  pyramids  in  front  and  the  tubercle  of  Rolando 
behind.  This  is  the  olivary  body,  and  has  on  its  inner  and 
dorsal  sides  two  other  little  grey  masses  which  are  the  acces- 
sory olivary  bodies.  The  other  feature  is  the  new  relay  of 
sensory  fibres  which  start  from  the  dorsal  nuclei,  the  nuclei 
gracilis  and  cuneatus.  These  fibres  run  outwards  and  for- 
wards from  the  nuclei  right  round  the  medulla,  some  passing 
superficially  to  the  olivary  body  to  join  the  restiform  body  of 
the  opposite  side,  while  others  pass  deeply  to  the  olives,  and 


616 


PHYSIOLOGY 


crossing  in  the  median  raphe  turn  upwards  in  the  broken 
mass  of  grey  and  white  matter  which  Hes  between  the  olives 
and  the  superficial  grey  matter  of  the  fourth  ventricle. 
Some  fibres  also  pass  into  the  restiform  body  of  the  same 
side. 

It  will  be  remembered  that  the  anterior  half  of  the  fourth 
ventricle  is  covered  in  by  the  cerebellum,  which  is  attached 


nM. 


Section  of  the  medulla  oblongata  in  the  region  of  the  superior 
pyramidal  decussation  (Schwalbe).  *.  a.«t./.,  anterior  median 
fissure ;  f.a.,  superficial  arciform  fibres  emerging  from  the 
fissure ;  j^y.,  pyramid  ;  n.ar.,  nucleus  of  the  arciform  fibres ; 
f.a.',  deep  arciform  fibres  becoming  superficial ;  o,  lower  end 
of  olivary  nucleus  ;  o',  accessory  olivary  nucleus  ;  n.l.,  nucleus 
lateralis ;  f.r.,  formatio  reticularis ;  f.a.-,  arciform  fibres  pro- 
ceeding from  formatio  reticularis ;  g,  substantia  gelatinosa 
of  Kolando;  a.  F.,  descending  root  of  fifth  nerve;  ii.c,  nucleus 
cuneatus ;  7i.c.',  external  cuneate  nucleus ;  f.c,  funiculus 
cuneatus  ;  n.g.,  nucleus  gracilis;  f.g.,  funiculus  gracilis;  p.m.f., 
posterior  median  fissure ;  c.c,  central  canal  surrounded  by 
grey  matter,  in  which  are  n.XT.,  nucleus  of  the  spinal  ac- 
cessory, and  ii.XII.,  nucleus  of  the  hypoglossal ;  s.cl.,  superior 
pyramidal  decussation  (decussation  of  fillet). 


to  the  axial  part  of  the  brain  hy  three  peduncles,  the  inferior 
peduncle  or  restiform  body,  the  lateral  peduncles  which  form 
the  great  mass  of  transverse  fibres  known  as  the  pons  Varolii, 
and  the  superior  peduncles  which  run  forward  to  the  pos- 
terior corpora  quadrigemina.      The  restiform  bodies  can  be 


THE    BRAIN 


617 


regarded  as  the  direct  continuation  forwards  of  the  postero- 
lateral columns  of  the  cord,  mi)ius  the  pyramidtil  tracts,  the 
chief  remaining  tract  therefore  being  the  posterior  or  direct 
cerebellar  tract.  In  the  region  of  the  dorsal  nuclei,  however, 
it  receives  accession  of  fibres  from  the  gracile  and  cuueate 


|)   n.ar 

bectioii  of  the  inedulla  oblongata  at  about  the  midclle  of  the 
olivary  body  (Schwalbe).  j.  f.l.a.,  anterior  median  fissure ; 
7i.ar.,  nucleus  arciformis ;  i),  pyramid ;  XII..  bundle  of  hypo- 
glossal nerve  emerging  from  the  surface ;  at  b  it  is  seen 
coursing  between  the  pyramid  and  the  olivary  nucleus,  o ; 
/.rt.c,  external  arciform  fibres;  «./.,  nucleus  lateralis;  a,  arci- 
form  fibres  passing  towards  restiform  body,  partly  through  the 
substantia  gelatinosa,  </,  partly  superficial  to  the  descending 
root  of  the  fifth  nerve,  a.V.;  A',  bundle  of  vagus  root,  emerg- 
ing ;  f.r.,  formatio  reticularis ;  c.»".,  corpus  restiforme.  begin- 
ning to  be  formed,  chiefly  by  arciform  fibres,  superficial  and 
deep;  Ji.c,  nucleus  cuneatus ;  7!. r/.,  nucleus  gracilis;  f,  attach- 
ment of  the  taenia ; /.s.,  funicuhis  solitarius;  nX,  nX',  two 
parts  of  the  vagus  nucleus ;  n.XII,  h\-poglossal  nucleus ;  n.t., 
nucleus  of  the  funiculus  teres ;  ii.am.,  nucleus  ambiguus ; 
r,  raphe  ;  A,  formatio  reticularis  alba;  o',  o",  accessory  olivary 
nuclei ;  o,  olivary  nucleus  ;  p.o.L,  pedunculus  olivaD. 


nuclei  of  the  same  side  and,  through  the  superficial  arcuate 
fibres,  from  the  nuclei  of  the  opposite  side,  and  thus  passes 
as  a  thick  white  bundle  into  the  cerebellum.  On  its  way  it 
is  joined  by  a  smaller  bundle,  the  internal  restiform  body, 
which  conveys  fibres  from  the  vestibular  division  of  the 
eighth  nerve  and  from  Deiters'  nucleus  to  the  cerebellum. 


618 


PHYSIOLOGY 


A  section  through  the  pons  shows  the  fourth  ventricle 
widely  dilated,  with  a  floor  formed  of  grey  matter  as  in  the 
medulla.  The  chief  difference  in  the  appearance  of  the  sec- 
tion is  due  to  the  great  masses  of  transverse  fibres  which 
pass  into  the  pons  by  the  lateral  peduncles  of  the  cerebellum, 
cross  by  the  median  raphe,  and  either  turn  upwards  or  down- 
wards on  the  opposite  side  or  end  in  connection  with  the 


Section  across  llie  pons  at  about  the  middle  of  the  fourth  ventricle 
(Schwalbe).  ~.  pij.  ijyramid  bundles  continued  up  from  the  medulla  ;  ^'o, 
transverse  fibres  of  the  pons  passing  from  themiddle  crus  of  the  cerebellum, 
before  (po^)  and  behind  ( j»'-)  the  chief  pyramid  bundles  ;  t,  deeper  fibres  of 
the  same  set,  constituting  the  trapezium ;  the  grey  matter  between  the 
transverse  fibres  is  not  represented ;  r,  raphe ;  o.s.,  superior  olivary 
nucleus  ;  a.V,  bundles  of  the  descending  sensory  root  of  the  fifth  nerve 
enclosed  by  a  prolongation  of  the  grey  substance  of  Rolando  ;  VI,  root 
bundle  of  the  sixth  nerve;  7i. FJ,  its  nucleus;  VII,  root  bundle  of  the 
facial  nerve  ;  Vila,  longitudinal  portion  of  the  same  ;  n.VII,  its  nucleus  ; 
VIII,  (superior)  root  of  the  auditory  nerve;  n.VIII,  its  nucleus;  v, 
section  of  a  vein. 


nerve-cells  which  are  scattered  throughout  the  white  fibres. 
The  pyramids  can  still  be  seen  as  thick  longitudinal  bundles 
on  each  side  in  the  midst  of  the  transverse  fibres.  Dorsall}" 
to  these  transverse  fibres  is  a  special  mass  of  grey  matter 
known    as    the    superior    olive.     Even    in    the  pons  we  can 


THE   BRAIN  619 

divide  the  nervous  mass,  lying  below  or  anteriorly  to  the 
central  grey  matter,  into  two  parts — the  formatio  reticularis 
behind  and  the  transverse  fibres  and  pyramids  in  front.  A 
little  further  forward  a  section  will  escape  the  cerebellum 
altogether,  and  will  cut  through  the  fourth  ventricle,  bounded 
ventrally  by  the  upper  part  of  the  pons  and  dorsally  by  the 
thin  mass  of  grey  matter,  the  valve  of  Vieussens ,  (Fig.  283). 
On  each  side  will  be  seen  the  superior  peduncles  of  the  cere- 
bellum, made  up  of  fibres  runnmg  from  cerebellum  to  posterior 
corpora  quadrigemma,  as  well  as  the  continuation  upwards 
of  the  antero-lateral  ascending  tract,  which  passing  up  in 
this  peduncle  bends  dorsally  round  the   fourth   nerve,  and 


Diagrammatic  transverse  section  tlirough  mid-brain  to  show  position  of 
fillet  and  pyramid.  A.Q.  Anterior  corpus  qnadrigeminum.  d  V.  Descend- 
ing motor  root  of  fifth  nerve.  F.  Fillet  (I,  lateial,  and  m,  mesial  fillet). 
Pyr.  Pyramid.  Fr.  Fibres  from  frontal  lobe  to  pons.  T.O.  Fibres  from 
occipital  lobe  to  pons.  Ne.  Fibres  from  nucleus  caudatus  to  pons.  III. 
Boot  of  third  nerve.     S.  Sylvian  iter.     En.  Red  nucleus. 

running  backwards  ends  in  the  superior  vermis  of  the  cere- 
bellum. 

A  little  further  forwards  the  fourth  ventricle  comes  to  an 
end,  and  the  section  passes  through  the  mid-brain,  the  cavity 
of  the  second  cerebral  vesicle  being  represented  by  the  narrow 
Sylvian  aqueduct,  bounded  dorsally  by  the  corpora  quadri- 
gemina  and  ventrally  by  the  crura,  the  stalks  of  the  brain. 
The  crura  are  divided  by  an  irregular  mass  of  grey  matter, 
the  substantia  nigra,  into  two  parts.  Anteriorly  is  the  pes 
or  crusta,  composed  almost  entirely  of  longitudinal  white 
fibres.  The  dorsal  part,  the  tegmentum,  is  a  direct  prolonga- 
tion forwards  of  the  formatio  reticularis  of  the  medulla  and 


620 


PHYSIOLOGY 


pons,  and  like  this  contains  much  scattered  gi'ey  matter, 
besides  one  large,  more  distinct  mass  known  as  the  red 
nucleus. 

The  cerebral  axis  comes  to  an  end  at  the  third  ventricle, 
which  corresponds  to  the  first  cerebral  vesicle.  It  terminates 
in  the  following  way :  the  roof  ceases  to  exist,  being  reduced 
to  a  mere  layer  of  epithelium.  The  floor  is  also  thinned,  but 
still  contains  nervous  matter,  forming  the  optic  commissure, 


Fig.  285. 


for.M' 


anav.Jisf. 

irincal  stria 


infaiuhh 

pit  hoilij 

Corp  alb 


posf.  cotntn. 
incal  Iiodi^ 


tab.  ualv. 

Portion  of  a  median  section  of  the  brain,  showing  the  corpus 
callosuni,  third  ventricle,  iter  or  aqueduct  of  Sylvius,  fourth 
ventricle,  cerebellum,  pons,  etc.     (G.  D.  Thane.) 


the  infundibulum,  and  the  corpora  albicantia  (Fig.  285).  The 
lateral  wall  however  undergoes  considerable  development 
and  forms  the  great  cerebral  ganglion  known  as  the  optic 
thalamus.  From  the  front  part  of  the  first  cerebral  vesicle 
the  cerebral  hemispheres  have  been  developed,  and  in  the 
anterior  and  basal  portion  of  these  hemispheres  a  complicated 
mass  of  grey  matter,  the  corpus  striatum,  has  been  developed 
in  close  proximity  to  the  optic  thalamus.  As  the  crura  cerebri 
pass  up,  the  tegmentum  becomes  continuous  with  the  optic 


THE   BRAIN  621 

thalamus  and  tissues  of  tlie  subthalamic  region,  ^Yhile  the 
crusta  diverging  somewhat  sweeps  round  the  outer  surface  of 
the  optic  thalamus,  lying  thus  between  the  posterior  part  of 
the  corpus  striatum  (the  lenticular  nucleus)  and  the  optic 
thalamus.  In  front  the  fibres  pass  as  a  distinct  layer  through 
the  substance  of  the  corpus  striatum  between  the  nucleus 
lenticularis  and  nucleus  caudatus  of  this  body.  The  layer  of 
white  matter  is  thus  fan-shaped,  and  expands  still  further 
after  passing  between  the  basal  ganglia  to  be  distributed  to  all 
parts  of  the  cortex  of  the  cerebral  hemispheres,  forming  an 
important  constituent  of  the  white  matter,  the  corona  radiata, 
underlying  the  superficial  grey  matter.  The  part  between  the 
basal  ganglia  is  known  as  the  internal  capsule  {v.  Figs.  286 
and  287).  The  anterior  and  posterior  parts  (in  horizontal 
section)  are  inclined  to  one  another,  so  that  we  can  distin- 
guish an  anterior  and  a  posterior  limb,  the  junction  of  the  two 
being  known  as  the  genu. 

The  Axial  Grey  Matter 

In  the  spinal  cord  we  could  distinguish  between  the 
anterior  grey  matter  giving  origin  to  the  motor  nerves,  the 
posterior  grey  matter  serving  as  an  end-station  for  a  number 
of  the  sensory  posterior  root-fibres,  and  a  lateral  horn,  less 
well  marked,  probably  giving  origin  to  the  visceral  system  of 
nerves. 

As  the  central  canal  widens  out  to  form  the  fourth  ven- 
tricle, the  relative  position  of  these  various  parts  becomes 
altered,  the  anterior  grey  matter  being  now  nearest  the 
median  line,  while  the  posterior  grey  matter  lies  more  late- 
rally. The  lateral  grey  matter  seems  to  lie  deeper  than  the 
rest,  from  which  it  is  separated  by  a  part  of  the  tangle  of 
fibres  and  cells  known  as  the  form  a  tin  reticularis. 

AU  the  cranial  nerves  from  the  third  to  the  twelfth  arise 
or  end  in  the  axial  grey  matter,  or  in  close  proximity  to  it. 
So  great  however  is  the  complexity  of  this  part  of  the  ner- 
vous system,  and  so  involved  are  the  genetic  relations  of  the 
various  nerves,  that  it  is  difficult  or  impossible  in  many  cases 
to  state  definitely  the  spinal  analogies  of  the  various  nerves. 

The  cranial  nuclei  (of  origin  or  termination)  may  be 
roughly  classed  as  follows  : 


622  PHYSIOLOGY 

1.  Motor  nuclei,  close  to  the  middle  line  in  floor  of  fourth 
ventricle  and  iter.  From  below  upwards  we  get  definite 
groups  of  large  cells  giving  origin  to  the  fibres  of  the  hypo- 
glossal, sixth  nerve,  fourth  nerve,  third  nerve. 

2.  Sensory  nuclei,  i.e.  first  cell-stations  of  afferent  fibres 
having  their  trophic  centres  outside  the  cord.  These  lie 
externally  to  the  motor  nuclei,  and  are  represented  ):)y  the 
combined  nucleus  of  the  glossopharyngeal,  vagus,  and  acces- 
sory, and  also  by  the  nuclei  of  the  eighth  nerve  and  the 
sensory  nucleus  of  the  fifth,  with  its  descending  prolongation 
in  the  suhstantia  gelatinosa. 

3.  Nuclei  of  efferent  neriies  hclonging  to  tlie  splanchnic 
system.  These  lie  more  deeply  at  some  distance  from  the 
middle  line,  and  include  tlie  nucleus  ambiguus  for  the  efferent 
fibres  of  the  vago-giossopharyngeal,  the  nucleus  of  the  seventh 
or  facial,  and  probably  the  motor  nucleus  of  the  fifth  nerve. 

In  a  cross -section  of  the  medulla  at  the  pyramidal  decus- 
sation, before  the  central  canal  has  opened  out  (Fig.  280), 
two  groups  of  cells  are  seen  in  the  central  grey  matter. 
The  anterior  of  these  consists  of  large  multipolar  cells,  and 
gives  origin  to  the  motor  fibres  of  the  hypoglossal  nerve. 
The  posterior  group  is  the  lower  end  of  a  long  column  which 
receives  the  end-arborisations  of  the  afferent  fibres  of  the 
ninth,  tenth,  and  eleventh  nerves  {glossopharyngeal,  vagus, 
and  bulbar  portion  of  sjnnal  accessory). 

In  a  section  a  little  higher  up,  taken  through  the  middle 
of  the  olivary  body  (Fig.  282),  the  central  canal  has  opened 
out  into  the  fourth  ventricle,  and  the  vago-glossopharyngeal 
is  now  seen  lying  laterally  to  the  hypoglossal  nucleus.  The 
nerve-fibres  connected  with  these  nuclei  pass  out  on  either 
side  of  the  olivary  body,  the  hypoglossal  taldng  the  median 
and  the  vago-glossopharyngeal  the  lateral  position.  The 
fibres  of  these  afferent  nerves,  like  all  others  in  the  bulb  or 
cord,  divide  into  ascending  and  descending  branches  on  arriving 
at  their  grey  matter.  The  ascending  branches  are  short,  but 
the  descending  extend  some  distance  down  the  medulla,  as  a 
special  bundle,  the  funiculus  solitarius,  lying  immediately 
under  the  vago-glossopharyngeal  nucleus.  This  bundle  is 
sometimes  called  the  descending  sensory  root  of  the  glosso- 
pharyngeal, or,  from  its  supposed  importance  for  the  respira- 
tory centre,  the  respiratory  bundle  of  Gierke. 


THE   EEAIN  623 

The  motor  fibres  of  the  vago-glossopharyngeal,  \Yhich  are 
splanchnic  in  function,  arise  from  a  mass  of  grey  matter  lying 
deeply  in  the  medulla,  and  corresponding  apparently  to  the 
lateral  horn.  This  nucleus  is  known  as  the  nucleus  amhiguus 
{n.am.,  Fig.  282). 

At  the  upper  part  of  the  medulla,  where  the  fourth  ven- 
tricle attains  its  greatest  diameter,  the  important  eighth 
nerve  enters  (Fig.  295,  p.  639).  This  really  consists  of  two 
nerves,  an  outer  division  derived  from  the  cochlea  and 
carrying  auditory  sensations,  and  an  inner  division  derived 
from  the  vestibule  and  semicircular  canals  and  convej'ing 
afferent  impressions  wdiich  determine  the  maintenance  of 
equilibrium.  These  nerves  are  purely  sensory,  and  are 
•derived  from  Inpolar  cells  situated  in  the  internal  ear.  As 
they  enter  the  medulla  they  are  separated  by  the  mass  of 
white  fibres  forming  the  restiform  body.  Both  sets  of  fibres 
bifurcate  on  entering  the  medulla.  The  branches  of  the 
cochlear  nerve  make  connection  with  two  collections  of  cells, 
the  dorsal  nucleus,  apparently  embedded  in  the  fibres  of  the 
root  itself,  and  the  accessory  nucleus,  a  little  triangular 
mass  of  grey  matter  situated  in  the  angle  between  the 
cochlear  and  vestibular  nerves.  From  these  nuclei  fibres 
are  given  off  which  take  two  courses.  Some,  following  the 
previous  course  of  the  cochlear  nerve,  pass  across  the  sur- 
face of  the  fourth  ventricle  as  the  stricB  meduUares  or  stricB 
acousticce,  and  then  bending  inwards  pass  into  the  tegmentum 
of  the  opposite  side.  Others  pass  deeply  and  form  a  mass  of 
transverse  fibres  in  the  ventral  part  of  the  tegmentum,  known 
as  the  corpus  trapezoides.  After  making  connections  with 
the  superior  olivary  body  and  a  special  nucleus,  they  join  the 
superficial  set  of  fibres,  and  pass  up  in  the  tegmentum  to  the 
inferior  corpora  quadrigemina,  forming  the  lateral  fillet. 

The  vestihular  nerve  also  has  two  nuclei  of  termination, 
the  median  nucleus  with  small  cells,  and  the  lateral  or 
Deiters'  nucleus  with  large  cells.  The  descending  fibres  end 
chiefl}'  in  the  median  nucleus,  wdiile  the  ascending  fibres  end 
in  Deiters'  nucleus.  From  the  latter  a  distinct  band  of  fibres 
passes  up  to  the  cerebellum,  forming  the  median  division  of 
the  restiform  body,  while  other  fibres  run  across  to  the  teg- 
mentum of  the  opposite  side,  wdiere  they  take  part  in  the 
formation  of  ihe posterior  longitudinal  bundle. 


624  PHYSiOLoaY 

In  a  section  tlirough  tlie  fourih  ventricle  through  the 
middle  of  the  pons,  a  group  of  large  cells  is  seen  in  the 
position  occupied  by  the  nucleus  of  the  hypoglossal  below. 
In  this  situation  however  these  cells  give  rise  to  the  fibres 
of  the  sixth  nerve.  Another  group  is  seen  lying  laterally  and 
more  deeply,  evidently  belonging  to  the  lateral  horn  system. 
This  is  the  nucleus  of  the  seventh  or  facial  nerve,  the  fibres 
of  which  pass  dorsally  and  anteriorly,  looping  round  the  sixth 
nerve-nucleus,  before  issuing  as  the  root  of  the  seventh  nerve. 

In  the  upper  part  of  the  pons  we  find  the  big  fifth  nerve 
with  its  two  roots.  The  fibres  of  the  sensory  root  derived 
from  the  cells  of  the  Gasserian  ganglion  bifurcate.  The 
upper  divisions,  which  are  short,  end  in  a  mass  of  grey 
matter  at  the  lateral  part  of  the  formatio  reticularis,  the 
so-called  sensory  root,  while  the  descending  divisions  form  a 
long  strand  of  white  fibres  passing  down  as  far  as  the  second 
cervical  nerve.  They  form  a  sort  of  cap  to  the  siihstantia 
gelatinosa  of  Rolando,  around  the  small  cells  of  which  the 
fibres  finally  terminate.  The  motor  fibres  arise  partly  from 
the  motor  nucleus,  a  mass  of  cells  lying  internally  to  the 
sensory  nucleus  and  belonging  probably  to  the  lateral  horn 
system.  A  large  number  however  are  derived  from  a  long 
column  of  cells,  which  stretches  forward  from  the  nucleus 
as  far  as  the  level  of  the  anterior  corpora  quadrigemina. 
These  fibres  are  known  as  the  descending  motor  root  of  the 
fifth  nerve. 

In  the  regit)n  of  the  mid-brain,  besides  the  root  of  the 
fifth  nerve  just  mentioned,  we  find  only  the  motor  nuclei  of 
the  fourth  and  third  nerves,  which  are  situated  near  the 
median  line  in  the  ventral  part  of  the  central  grey  matter, 
corresponding  in  situation  to  the  sixth  and  twelfth  nerves 
lower  down. 

Intermediate  Grey  Matter  of  the  Cerebral.  Axis 

The  masses  of  grey  matter  which  are  found  throughout 
this  region  may  be  regarded  as  extra  shunting  stations  (or 
association  centres  for  various  systems  of  nuclei  and  conduct- 
ing paths),  which  have  arisen  in  consequence  of  the  great 
complexity  of  reaction  required  of  the  nerve  mechanisms  in 
connection   with    the   organs    of    special   sense.      We   must 


THE   BRAIN  625 

confine  ourselves  here  to  little  more  than  the  enumeration 
of  some  of  the  chief  masses,  though  we  shall  have  occasion 
to  refer  to  some  in  more  detail  when  dealing  with  the  co- 
ordinating mechanisms  of  the  cerebral  axis.  From  below 
upwards  we  may  enumerate  the  following  grey  masses  : 

In  the  medulla  is  the  large  olivary  body,  with  the  accessory 
olive  lying  on  its  inner  side.  Each  olive  sends  fibres  across 
the  middle  line  to  the  opposite  cerebellar  hemisphere,  and 
must  be  regarded  as  connected  with  this  body  in  its  functions, 
since  atrophy  or  removal  of  one  side  of  the  cerebellum  is 
followed  by  atrophy  of  the  opposite  olive. 

In  the  pons  we  find  a  similar  but  smaller  body,  the 
superior  olive,  in  the  neighbourhood  of  the  nucleus  of  the 
seventh  nerve.  The  superior  olive  is  closely  connected  with 
the  co-ordination  of  visual  and  vestibular  impressions  with 
the  eye  movements  (Fig.  290). 

Deiters'  nucleus,  which  occurs  in  the  same  region,  although 
described  as  one  of  the  nuclei  of  the  eighth  nerve,  might 
equally  well  be  included  in  this  class  owing  to  its  manifold 
connections  with  both  afferent  and  efferent  mechanisms. 

In  close  connection  with  Deiters'  nucleus  are  a  number  of 
grey  masses  in  the  cerebellum,  the  so-called  roof-nuclei  in 
the  roof  of  the  fourth  ventricle,  and  the  dentate  nucleus  in 
the  middle  of  each  cerebellar  hemisphere. 

In  the  mid -brain  we  must  mention  the  superficial  grey 
matter  covering  the  corpora  quadrigemina. 

On  the  ventral  side  of  the  Sylvian  iter  are  various  masses 
of  grey  matter  in  the  crura,  the  red  nucleus,  a  large  mass  in 
the  tegmentum  just  below  the  oculo-motor  nucleus,  and  the 
substantia  nigra,  which  divides  each  crus  into  two  parts,  the 
dorsal  tegmentum  and  the  ventral  pes  or  crusta. 

Finally  at  the  fore  part  of  the  cerebral  axis  we  come  to 
the  two  great  ganglionic  masses  already  described,  the  optic 
thalamus  and  the  corpus  striatum.  The  optic  thalamus  is 
connected  by  fibres  with  all  parts  of  the  cortex  and  represents 
the  termination  of  the  whole  tegmental  system,  so  that  in 
many  ways  it  may  be  regarded  as  a  sort  of  foreman  of  the 
central  nervous  system,  bringing  all  parts  of  this  system  in 
relation  with  the  supreme  cerebral  cortex. 

The  corpus  striatum  on  the  other   hand  represents  the 

40 


626 


PHYSIOLOGY 


most  primitive  part  of  the  cerebral  hemispheres.  It  does 
not  act  as  the  intermediary  between  the  cortex  and  the  lower 
parts  of  the  brain,  but  its  fibres  run  to  lower  levels  in  company 
with  those  from  the  cortex.  It  seems  possible  that  it  may 
under  conditions  carry  out  in  a  simple  way  the  functions  which, 
with  more  elaboration,  need  the  co-operation  of  the  cortex. 


Face 


Diagrammatic  vertical  section  through  brain  showing  course  of 
pyramidal  fibres.  M.C.  Cortex  on  which  are  situated  the  motor 
centres.  I.C.  Internal  capsule.  C.N.  Caudate  nucleus.  O.T. 
Optic  thalamus.  L.N.  Lenticular  nucleus.  D.  Point  of  decus- 
sation of  fibres  to  F.N.,  facial  nucleus.  VII.  Seventh  nerve. 
P.  Pyramids  of  medulla.  A.P.  Anterior  pyramidal  tract. 
Lp.  Lateral  pyramidal  tract.  M.O.  Medulla  oblongata.  S.C. 
Spinal  cord.     Cbl.  Cerebellum. 


The  Comiecting  Tracts  of  the  Brain 

The  tracts  of  the  white  matter  of  the  brain  may  be  divided 
into  the  longitudinal  tracts  which  connect  different  levels  of 
the  brain  with  one  another,  and  the  commissural  or  transverse 


THE   BRAIN 


627 


tracts  which  run  across  from  one  side  of  the  brain  to  the 
other  and  serve  to  connect  the  two  sides  of  the  brain  at  the 
same  level.  The  longitudinal  tracts  may  be  divided  into 
those  contained  in  the  crusta  or  pes  and  its  continuations, 
and  those  which  run  in  the  tegmentum. 

The  Pedal  System. — The  most  important  of  these  tracts  is 
the  ijyramidal  tract  of  fibres  which,  arising  from  cells  in  the 
central  portion  of  the  cerebral  cortex  (the  motor  area),  pass 
along  the  corona  radiata  and  converge  to  form  the  genu  and 
anterior   two-thirds   of   the   posterior   limb   of    the   internal 

Fig.  287. 


Horizontal  section  through  basal  ganglion  to  show  arrangement 
of  fibres  in  internal  capsule.  From  before  backwards  the  fibres 
go  to  the  motor  centres  for  the  eye,  head,  tongue,  mouth, 
shoulder,  elbow,  fingers,  abdomen,  hip,  knee,  and  foot.  S.  Fibres 
from  temporo-occipital  region,  oc.  Fibres  to  occipital  lobe. 
op.  Optic  radiations.  Nc.  Nucleus  caudatus.  O.T.  Optic  thala- 
mus.    N.L.  Nucleus  lenticularis.     (After  Sherrington.) 

capsule.  The  arrangement  of  fibres  in  the  mternal  capsule 
according  to  their  function  is  indicated  in  Fig.  287.  Thence 
they  pass  through  the  crura  cerebri,  takmg  up  the  middle 
two-thirds  of  the  crusta,  and  then  through  the  pons. 
Emerging  from  the  lower  border  of  the  pons,  they  form  two 
thick  masses,  the  anterior  pj^amids  of  the  medulla.  At  the 
lower  part  of  the  medulla,  the  major  portion  of  the  fibres 
passes  across  to  the  posterior  part  of  the  lateral  column  of  the 
other  side,  and  is  continued  down  this  side  as  the  crossed 
pyramidal  tract.  A  small  number  of  fibres  of  each  tract  do 
not  cross  at  once,  but  pass  down  in  the  cord  in  the  same 
situation  as  they  occupied  in  the  medulla,  forming  the  direct 


628  PHYSIOLOGY 

or  anterior  pyramidal  tracts.  These  however  also  cross 
gradually  in  the  cord  before  reaching  their  final  destination — 
the  anterior  cornu  of  the  opposite  side. 

The  anterior  limb  of  the  internal  capsule  in  front  of  the 
genu  is  occupied  by  the  frontal  cortical  fibres,  which  are 
derived  from  the  grey  matter  of  the  frontal  lobes  and  which 
proceed  along  the  innermost  portion  of  the  crusta  to  the 
pons,  where  they  end,  probably  in  indirect  connection  with  the 
transverse  fibres  of  the  middle  peduncle  of  the  cerebellum. 

Immediately  behind  the  pyramidal  tracts  in  the  posterior 
limb  of  the  internal  capsule  is  a  collection   of   fibres   (the 

Fio.  288. 


Transverse  section  through  mid-brain  to  show  position  of 
fillet  and  pyramid.  A.Q.  Anterior  corpus  quadrigeminum. 
dV.  Descending  root  of  fifth  nerve.  F.  Fillet  (I,  lateral, 
and  m,  mesial  fillet).  Pyr.  Pyramid.  Fr.  Fibres  from 
frontal  lobe  to  pons.  T.O.  Fibres  from  occipital  lobe 
to  pons.  Ne.  P'ibres  from  nucleus  caudatus  to  pons. 
III.  Pioot  of  third  nerve.     S.  Sylvian  iter.     Kn.  Red  nucleus. 

te7nporo-occi2yital  cortical)  which  run  from  the  temporo- 
occipital  convolutions  of  the  cortex  through  the  internal 
capsule  and  along  the  outer  lateral  border  of  the  crusta  to 
end  in  the  pons  in  the  same  manner  as  the  frontal  fibres  just 
described. 

A  fourth  constituent  of  the  crusta,  which  does  not  appear 
in  the  internal  capsule,  is  the  small  band  of  fibres  derived 
from  the  caudate  nucleus  of  the  corpus  striatum.  These 
firbes  seem  to  end  partly  in  the  substantia  nigra  itself  and 
partly,  along  with  the  others  of  this  system,  in  the  pons 
Varolii. 

The  Tegmental  System. — Whereas  the  pedal  system  con- 
sists of  fibres  which  for  the  most  part  degenerate  downwards 


Fig.  289. 


Cortex  cerebri 


Lateral  fillet - 


Post'  columns'^--,  _ 


Corpora  quadrigemma 
Sup-  cerebellar  peduncle 
Sup-  vermis 

Restiform  body 
Cracile  &  cuneate  nuclei 


--Ant  •  cerebellar  tract 


--Post'  cerebellar  tract 


Diagram  to  show  possible  paths  of  afferent  impulses  from  posterior 
roots  to  cerebral  cortex.  The  fillet  path  is  represented  in  black, 
the  cerebellar  path  in  red. 


THE   BRAIN  631 

and,  in  the  case  of  the  pyramidal  tracts  at  any  rate,  carry 
efferent  impulses,  the  tegmentum  furnishes  the  various 
alternative  paths  for  afferent  impulses  on  their  way  to  the 
cerebral  hemispheres. 

Two  main  courses  are  open  for  these  impulses  (Fig.  289). 
They  may  travel  (1)  by  the  dorsal  column  nuclei  and  tract 
of  the  fillet  or  (2)  indirectly  through  the  intermediation  of 
the  cerebellum. 

1.  The  fillet  path. — The  posterior  nerve-roots  send  direct 
contmuations  into  the  posterior  columns  of  the  cord.  The 
fibres  m  these  columns  end  by  arborisations  round  the 
cells  of  the  gracile  and  cuneate  nuclei.  From  these  cells 
fresh  relays  arise  which  cross  as  the  internal  arcuate  fibres 
•to  the  inter-olivary  layer  on  the  other  side  of  the  medulla  in 
the  supra-pyramidal  or  sensory  decussation.  Hence  they  pass 
up  in  the  tegmental  region  of  the  pons  and  in  the  tegmentum 
of  the  crus  as  a  flat  band  of  fibres — the  fillet,  which  divides 
into  two  portions  known  as  the  lateral  and  median  divisionSc 
The  lateral  fillet  is  formed  almost  entirely  by  fibres  from  the 
cochlear  nucleus  and  ends  in  the  corpora  quadrigemina.  The 
median  division,  which  receives  also  accessions  from  the  sen- 
sory nuclei  of  the  bulb,  ends  partly  in  the  grey  matter  of  the 
anterior  corpus  quadrigeminum,  the  great  majority  of  the 
fibres  however  passing  to  the  thalamus  itself.  From  the 
thalamus  fibres  proceed  to  the  frontal  and  parietal  cortex 
through  the  extreme  anterior  end  of  the  internal  capsule, 
while  others  pass  through  the  posterior  third  of  the  hind  limb 
to  be  distributed  to  all  parts  of  the  temporal  and  occipital 
lobes.  The  latter  fibres,  which  are  associated  with  others 
derived  from  the  anterior  corpus  quadrigeminum  and  concerned 
largely  with  the  passage  of  visual  impulses,  are  often  spoken 
of  as  the  optic  radiations. 

2.  The  cerebellar  path. — The  posterior  roots,  as  they 
enter  the  cord,  send  collaterals  to  arborise  round  the  cells  of 
Clarke's  column  and  other  cells  of  the  grey  matter.  From 
these  cells  fibres  arise  which  form  the  anterior  and  posterior 
cerebellar  tracts.  The  anterior  (antero-lateral  ascending) 
tract  passes  up  in  the  lateral  columns  through  the  pons  into 
the  superior  peduncle  of  the  cerebellum,  where  it  loops  round 
to  end  in  the  superior  vermis.  The  posterior  (direct)  cere- 
bellar tract  also  passes  up  into  the  superior  vermis  of   the 


632 


PHYSIOLOGY 


cerebellum  by  the  restiform  body.  By  the  same  path  the 
cerebellum  also  receives  fibres  from  the  gracile  and  cuneate 
nuclei  of  both  sides. 

The  continuation  forwards  of  these  impulses  is  by  means 
of  the  superior  cerebellar  peduncles,  which,  arising  chiefly  in 
the  dentate  and  roof  nuclei  but  also  to  a  certain  extent  in 
the  superficial  grey  matter  of  the  cerebellum,  pass  forwards 

Fig.  290. 

Ant.  Corp.  quad. 


Optic  tract. 


Sup.oliv 


Sup.  olive. 
Post.  longitudinal  bundle. 


Diagram  to  illustrate  some  of  the  connections  of  the  nuclei  of 
the  nerves  to  the  ocular  muscles  (after  Held). 


to  the  corpora  quadrigemina.  Converging  below  these  bodies 
they  decussate,  and  finally  end  in  the  tegmentum,  chiefly  in 
connection  with  the  red  nucleus.  The  further  continuation 
to  the  cerebral  hemispheres  must  lie  in  the  tracts  already 
described  connecting  them  with  the  tegmentum. 

The  posterior  lo7igitiidinal  bundle  (Fig.  290)  is  a  distinct 
tract  of  white  fibres  lying  dorsally  to  the  reticular  formation. 
In  front  it  runs  just  below  the  oculo-motor  nucleus,  with  which 


THE   BRAIN  633 

it  is  connected,  and  can  be  traced  backwards  (or  downwards)  as 
far  as  the  first  cervical  nerve,  where  it  merges  in  the  ground 
fibres  (anterior  basis  bundle)  of  the  anterior  columns.  It  is 
made  up  partly  of  descending  fibres  from  the  anterior  corpus 
quadrigeminum  and  nuclei  of  the  floor  of  the  third  ventricle, 
partly  of  ascending  fibres  from  the  various  sensory  nuclei  of  the 
medulla.  One  of  its  chief  functions  is  to  serve  as  a  means  of 
communication  between  the  various  oculo-motor  nuclei  (third, 
fourth,  and  sixth  nerves)  around  the  cells  of  which  many  of 
its  fibres  end. 

The  Commissural  Fibres  of  the  Central  Nervous  System 

The  chief  mass  of  these  is  contained  in  the  corpus  callo- 
sum  which  connects  the  two  opposite  cerebral  hemispheres. 

Other  smaller  commissures  are  the  anterior  white  com- 
missure connecting  chiefly  the  olfactory  lobes  and  temporo- 
sphenoidal  convolutions,  and  the  posterior  commissure  between 
the  two  thalami. 

The  great  mass  of  transverse  fibres  forming  the  pons, 
which  serve  to  connect  the  cerebellar  hemispheres,  are  not 
directly  continuous  across  the  middle  line  but,  starting  from 
the  superficial  grey  matter  of  one  hemisphere,  end  in  the  grey 
matter  of  the  opposite  side  of  the  pons. 


634  PHYSIOLOGY 


Section  2 

SUMMAEY   OF  THE   CONNECTIONS  AND    FUNCTIONS 
OF   THE    CEANIAL   NEEVES 

Cranial  nerves. — The  cranial  nerves  are  generally  reckoned 
as  twelve  in  number :  1st,  olfactory  ;  2nd,  optic  ;  3rd,  oculo- 
motor; 4th,  or  trochlear;  5th,  or  trigeminus;  6th;  7th,  or 
facial ;  8th,  auditory  ;  9th,  glossopharyngeal ;  10th,  vagus  or 
pneumogastric  ;  11th,  spinal  accessory  ;  12th,  hypoglossal. 

Of  these  the  first  two  stand  on  a  different  footing  from 
the  rest  which,  like  the  spinal  nerves,  are  outgrowths  of 
nerve-fibres  from  the  central  tube  of  grey  matter  surrounding 
the  neural  canal  or  from  ganglia  corresponding  to  the  spinal 
posterior  root-ganglion.  The  olfactory  and  optic  nerves  are 
not  peripheral  nerves  at  all,  but  are  actual  outgrowths  from 
the  brain.  The  olfactory  bulb  and  the  retina  are  morpho- 
logically distinct  lobes  of  the  brain. 

The  first  or  olfactory  nerve  (or,  more  properly,  olfactory 
lobe)  has  ten  or  twelve  filaments,  which  pierce  the  cribriform 
plate,  and  are  distributed  to  the  olfactory  mucous  membrane. 
It  is  the  central  organ  of  smell.  It  is  supposed  that  impulses 
do  not  cross  from  one  side  of  the  body  to  the  cerebral  hemi- 
sphere of  the  opposite  side,  as  is  the  case  with  all  the  other 
nerves  of  the  body. 

The  second  or  optic  nerve  subserves  the  function  of  vision, 
and  that  alone.  The  two  nerves  join  at  the  optic  cliiasma. 
Here  there  is  a  partial  exchange  of  fibres,  and  each  of  the 
optic  tracts,  which  are  the  continuations  behind  the  chiasma 
of  the  optic  nerves,  contains  fibres  from  both  optic  nerves. 

Thus  the  right  optic  tract  contains  the  fibres  from  the 
external  half  of  the  right  retma  and  the  internal  half  of  the 
left  retina.  Since  these  are  corresponding  parts  of  the  two 
retinae,  and  are  excited  by  light  commg  from  the  left  of  the 
person,  section  of  one  optic  tract  causes,  blindness  on  the 
opposite  side  (hemianopia) .  Each  optic  tract  is  connected 
behind  with  the  back  part  of  the  optic  thalamus  (pulvmar), 
the  external  corpus  geniculatum,  and  the  anterior  corpus 
quadrigeminum  of   the  same  side    (Fig.   291).     From   these 


THE   BRAIN 


635 


points  fibres  pass  through  the  posterior  part  of  the  internal 
capsule  and  corona  radiata  {'optic  radiations  ')  to  the  cortex 
of  the  occipital  lobe. 

The  fibres  forming  the  optic  tracts  are  the  axons  of  the 
nerve-cells  in  the  ganglion-cell  layer  of  the  retina.  Section 
of  the  optic  nerve  therefore  is  followed  by  degeneration  of 
these  fibres  in  a  central  direction.     Mixed  up  with  these  fibres 

Fig.  291. 


Diagram  to  show  connections  of  optic  tracts  (after  Sherrington). 
L,  left,  and  K,  right  retina.  OD.  Optic  decussation  (chiasma). 
OpT.  Optic  tract.  NO.  Nucleus  caudatus.  LN.  Lenticular 
nucleus.  Th.  Optic  thalamus.  G.  External  geniculate  body. 
AQ.  Anterior  corpus  quadrigeminum.  P.  Pulvinar.  OpE. 
Optic  radiations  running  to  0  C,  the  occipital  cortex.  Ill.n. 
Nucleus  of  3rd  nerve  in  tloor  of  Sylvian  aqueduct.  IV.  Fourth 
ventricle. 


however  are  others  which  run  in  a  centrifugal  direction,  start- 
ing from  cells  in  the  optic  thalamus  or  adjacent  masses  of 
grey  matter  and  terminating  in  the  inner  nuclear  layer  of  the 
retina  (Fig.  292). 

The  third  or  oculo-motor  nerve  arises  from  a  column  of 
nerve-cells  situated  at  the  extreme  hind  part  of  the  floor  of  the 
third  ventricle,  and  from  the  front  part  of  the  floor  of  the 


636 


PHYSIOLOGY 


aqueduct  of  Sylvius,  below  the  anterior  corpus  quadrigeminum. 
It  emerges  from  the  inner  side  of  the  crus  cerebri.  It  is  the 
motor  nerve  for  the  levator  palpebrarum,  superior,  inferior, 
and  internal  recti,  and  inferior  oblique  muscles.  It  also 
supplies  the  constrictor  iridis  and  the  ciliary  muscle.  Stimu- 
lation of  it  therefore  causes  the  eye  to  look  upwards  and 
inwards,  with  contraction  of  the  pupil  and  spasm  of  accommo- 
dation. By  careful  stimulation  of  various  parts  of  its  nucleus 
the  different  movements  of  these  muscles  may  be  produced 
separately. 

The  nucleus  of  the  fourth  nerve  is  situated  just  behind 
that  for  the  third  in  the  floor  of  the  Sylvian  aqueduct.     The 

Fi(i.  292. 


Schema  of  the  course  of  the  visual  impulse  from  rods  and  cones  to 
brain  (Bechterew  after  Ramon  y  Cajal).  A,  retina  ;  B,  optic  nerve  ; 
C,  external  geniculate  body,  a,  cone  ;  b,  rods  ;  c,  d,  bi-polar  cells  ; 
e,  ganglion  cells ;  /,  centrifugal  fibre ;  g,  spongioblast ;  h,  terminal 
ramifications  of  retinal  nerve-fibres ;  j,  cells  transmitting  impulse 
to  cortex ;  T,  cell  giving  off  centrifugal  fibres. 


fibres  run  from  here  round  the  aqueduct,  and  take  their  super- 
ficial origin  from  the  valve  of  Vieussens,  a  thin  plate  of  grey 
matter  forming  the  roof  of  the  fourth  ventricle  just  in  front 
of  the  cerebellum. 

This  nerve  supplies  only  the  superior  oblique  muscle  of 
the  eyeball. 

The  fiftli  or  trigeminus  resembles  a  spinal  nerve  in  that  it 
has  a  motor  as  well  as  a  sensory  root,  the  latter  being  provided 
with  a  ganglion  (Gasserian  ganglion).  It  has  a  very  extensive 
origin,  owing  to  the  fact  that  its  sensory  part  represents  the 
sensory  roots  of  all  the  motor  cranial  nerves  from  the  third 


THE    BRAIN 


637 


to  the  hypoglossal.  The  middle  or  motor  root  arises  from  a 
collection  of  nerve -cells  in  the  floor  of  the  fourth  ventricle. 
It  also  receives  fibres  from  the  descending  motor  root,  which 
can  be  traced  forwards  from  the  nucleus  as  far  as  the  level  of 
the  anterior  corpora  quadrigemina. 

The  sensory  root  may  also  be  traced  mto  a  nucleus  lying 
lateral  to  the  motor  nucleus,  though  in  this  case  the  fibres  of 
the  nerve  end  in  the  nucleus,  having  their  trophic  centre  in 
the  cells  of  the  Gasserian  ganglion.  Connected  with  the 
sensory  nucleus  is  the  descending  root  of  the  fifth  nerve,  which 
can  be  traced  downwards  through  the  medulla  as  far  as  the 


Fm.  293. 


Corpora  qn.-idri- 
getnlna 


Eminentia  teres 

Strife  acousticaj 
Ala  cinerea 


Calamus  scriptoriiis 


Xucleiis  of  iii 
Xncleus  of  iv 

Motor  nncleiis  of  v 
.sensory  niicleu?;  of  v 
Nucleus  of  vi 
Xucleus  of  facial 
Principal  nucleus  of 

auditory 
Nucleus  of  glosso- 

pharyiigeal 
Nucleus  of  vagus 

Nucleus  of  spinal 
accessory 

Nucleus  of  hypo- 
glossal 


Diagram  of  fourth  ventricle  and  adjacent  parts,  as  seen  from  dorsal 
aspect,  to  show  positions  of  nerve  nuclei.  These  are  marked  on  the 
right-hand  half.     (After  Erb.) 


level  of  the  second  cervical  nerve,  where  it  appears  as  a  little 
cap  of  white  fibres  on  the  mass  of  gelatinous  substance  forming 
the  head  of  the  posterior  horn,  and  known  in  the  medulla  as 
the  tubercle  of  Eolando. 

This  nerve  is  the  motor  nerve  for  the  muscles  of  mastica- 
tion, and  for  the  tensor  tympani  and  tensor  palati.  It  is  the 
sensory  nerve  for  the  whole  of  the  face  (including  eyeball, 
mouth,  and  nose).  It  also  contains  dilator  fibres  to  blood- 
vessels derived  from  the  chorda  tympani,  and  is  said  to  have 
trophic  functions.  The  latter  conclusion  is  from  the  fact  that 
section  of  the  fifth  nerve  in  the  skull  is  followed  by  ulceration 
and  sloughing  of  the  cornea,  and  finally  destructive  changes 


638 


PHYSIOLOGY 


involving  the  whole  eyeball.  Since  however  these  results  may 
be  prevented  by  carefully  shielding  the  eye  from  all  dust  and 
deleterious  influences,  it  is  probable  that  the  ulceration  is 
merely  a  secondary  consequence  of  the  anaesthesia.  The 
cornea  being  anaesthetic,  foreign  objects  that  fall  on  its  surface 
are  allowed  to  remain  there,  and  so  give  rise  to  injurious 
changes  and  ulceration. 

The  fifth  is  also  said  to  be  the  nerve  of  taste  for  the 
anterior  third  of  the  tongue,  but  it  is  possible  that  the  taste - 
fibres  which  run  in  the  fifth  are  derived  from  the  glosso- 
pharyngeal. 


Fig.  294. 


Uoot-flbres  of  v 


Superior  olive 


Root-fibres  of  facial 

Root-fibres  of  vi 
Olivary  body 

Pyramidal  tract 


Motor  nucleus  of  V 


Sensory  nucleus  of  v 
Nucleus  of  vi 
Genu  of  facial 
Nucleus  of  facial 
Strife  acousticiB 
Nucleus  of  auditory 
Nucleus  of  glosso- 
pharyngeal 
Nucleus  of  vagus 
Nucleus  of  spinal 

accessory 
Nucleus  of  hypo- 
glossal 


Diagram  to  show  positions  of  principal  nerve  nuclei  in  pons  and 
medulla,  side  view.  The  organ  is  supposed  to  be  split  down  the 
middle  line  and  the  right  half  viewed  from  the  mesial  side. 
The  most  mesially  situated  nuclei  are  shaded,  the  others  stippled. 
(After  Erb.) 


The  sixth  nerve,  the  motor  nerve  for  the  external  rectus, 
rises  from  a  small  group  of  cells  in  the  floor  of  the  fourth 
ventricle  near  the  middle  line. 

The  nucleus  for  the  facial  or  seventh  nerve  is  situated 
more  deeply  and  laterally  than  the  preceding.  It  supplies 
all  the  muscles  of  the  face,  and  is  therefore  the  nerve  of 
expression. 

The  chorda  tympani,  which  is  a  branch  of  this  nerve, 
contains  secretory  and  vaso-dilator  fibres  for  the  submaxillary 
gland.  The  fibres  of  taste  which  are  said  to  run  in  this  nerve 
are  probably  derived  from  the  glosso-pharyngeal. 

The  eighth  or  auditory  rises  on  a  level  with  the   hind 


THE   BKAIN  639 

margin  of  the  pons  by  two  roots,  one  of  which,  the  dorsal 
root,  winds  round  the  restiform  body,  while  the  other,  the 
ventral  or  median  root,  sinks  into  the  substance  of  the  bulb 
to  the  inner  side  of  the  restiform  body.  The  fibres  composing 
the  dorsal  roots  come  from  the  cochlea  and  convey  sensations 
of  sound.  The  fibres  bifurcate,  the  ascending  branches  ending 
in  the  dorsal  nucleus  (acoustic  tubercle),  while  the  descending 
make  connection  vvith  a  mass  of  cells  in  the  angle  between  the 
two  roots,  known  as  the  accessory  nucleus.  The  ventral  root 
consists  of  fibres  derived  from  the  vestibule  and  semicircular 
canals,  and  carries  impulses  connected  with  the  reflex  main- 
tenance of  equilibrium.     It  ends  partly  in  the  lateral  auditory 

Fig.  295. 
Cbni 


Section  through  medulla  just  behind  the  pons?,  to  show  origin  of 
anditoiy  neive.  Cbm.  Cerebellum.  V.  Fourth  ventricle.  E.  Resti- 
form body.  Py.  Pyramid.  Va.  Ascending  root  of  fifth  nerve. 
VII.  Nucleus  of  seventh  nerve.     VIII.  Auditory  nerve. 

nucleus  (of  Deiters),  whence  a  number  of  fibres  pass  up  with 
the  restiform  body  into  the  cerebellum,  and  partly  in  the 
median  nucleus. 

The  ninth,  tenth,  and  eleventh  nerves  are  connected  with 
an  elongated  nucleus  in  the  lateral  part  of  the  lower  half  of 
the  fourth  ventricle,  the  motor  fibres  of  the  first  two  nerves 
arising  from  the  nucleus  ambiguus. 

The  ninth  is  probably  a  pure  sensory  nerve,  and  conveys 
sensations  from  tongue  (taste?)  and  pharynx.  Eunning  in 
it  are  also  motor  fibres  to  the  stylo-pharyngeus  and  middle 
constrictor  of  the  pharynx. 

The  tenth  or  vagus  is  joined  by  the  accessory  part  of  the 
spinal  accessory,  so  that  the  two  nerves  may  be  considered 


640  PHYSIOLOGY 

together.     It  has  both  afferent  and  efferent  functions,  most  of 
which  we  have  already  considered. 

Efferent  functions  : 

Motor    to    levator    palati    and    three    constrictors    of 

pharynx. 
Motor  to  muscles  of  larynx. 
Inhibitory  to  heart. 
Motor  to  muscular  walls  of   oesophagus,  stomach,  and 

small  intestine. 
Motor  to  unstriated  muscle  in   walls   of    bronchi   and 

bronchioles. 
Secretory  to  glands  of  stomach  and  possibly  to  pancreas. 

Afferent  functions  : 

Eegulate  respiration.  Stimulation  of  central  end  may 
quicken  respiration  and  promote  inspiration,  or  may 
inhibit  inspiration.  Stimulation  of  central  end  of 
superior  laryngeal  causes  stoppage  of  inspiration, 
expiration,  cough. 

Depressor  (from  heart  to  vaso-motor  centre). 

Reflex  inhibition  of  heart. 

The  external  branch  of  the  spinal  accessory  is  the  motor 
nerve  of  the  sterno-mastoid  and  trapezius  muscles. 

The  hyjjoglossal  {twelfth,  nerve)  arises  from  a  nucleus  in 
the  floor  of  the  fourth  ventricle  at  its  lower  end  close  to  the 
middle  line. 

It  is  a  pure  motor  nerve,  and  supplies  the  intrinsic  and 
extrinsic  muscles  of  the  tongue. 

Since  the  integrity  of  the  nuclei  of  the  cranial  nerves  is  a 
necessary  condition  for  the  carrying  out  of  various  reflex  acts 
in  which  these  nerves  are  involved,  the  grey  matter  of  the 
fourth  ventricle  and  aqueduct  is  often  spoken  of  as  if  it  were 
cut  up  into  a  series  of  centres  distinct  for  every  act.  It  must 
be  remembered  however  that,  when  a  dozen  or  more  centres 
are  enumerated  as  being  situated  in  the  fourth  ventricle,  it  is 
not  meant  that  we  can  anatomically  distinguish  a  group  of 
cells  for  each  act  or  group  of  actions  named.  When  we  say 
that  a  part  of  the  nervous  system  is  a  centre  for  any  action, 
we  merely  mean   that   this  part  forms  a  necessary  link,  or 


THE   BRAIN  641 

meeting  of  the  ways,  in  the  complicated  directing  of  nerve 
impulses  that  takes  place  in  every  co-ordinated  act. 

The  chief  centres  are  the  respiratory  and  the  vaso-motor. 
These  we  have  already  considered.  Other  centres  that  may 
be  enumerated  are — 

Centres  for  movements  of  intrinsic  and  extrinsic  ocular 
muscles. 

Cardiac  inhibition. 

Mastication,  deglutition. 

Sucking. 

Convulsive  (connected  with  respiratory). 

Vomiting. 

Diabetic  (connected  with  vaso-motor,  v.  p.  492). 

Salivary. 

Centres  of  phonation  and  articulation. 


41 


642  PHYSIOLOGY 


Section  3 
FUNCTIONS   OF    THE   CEREBRAL   AXIS 

We  have  already  studied  the  phenomena  exhihited  by  an 
animal  (frog)  possessing  spinal  cord  alone.  We  can  now  by 
a  study  of  the  same  or  a  higher  animal,  deprived  only  of  its 
cerebral  hemispheres,  come  to  some  conclusion  concerning 
the  functions  of  the  lower  parts  of  the  brain  and,  by  com- 
parison of  these  phenomena  with  those  exhibited  by  an  intact 
animal,  of  the  cerebral  hemispheres  themselves. 

When  a  frog's  cerebral  hemispheres  have  been  excised,  a 
casual  observer  would  not  at  first  notice  anything  abnormal 
about  the  animal.  He  sits  up  in  his  usual  position,  and  on 
stimulation  may  be  made  to  jump  away,  guiding  himself  by 
sight,  so  that  he  avoids  any  obstacles  in  his  path.  Move- 
ments of  swallowing  and  breathing  are  normally  carried  out. 
The  animal,  thrown  on  to  his  back,  immediately  turns  over 
again.  If  put  into  water,  he  swims  about  until  he  comes  to 
a  floating  piece  of  wood  or  any  support,  when  he  crawls  out 
of  the  water  and  sits  still.  If  he  be  placed  on  a  board  and 
the  board  be  inclined,  he  begins  to  crawl  slowly  up  it,  and  by 
gradually  increasing  its  inclination  he  may  be  made  to  crawl 
up  one  side  and  down  the  other.  But  a  striking  difference 
between  him  and  a  normal  frog  is  the  almost  entire  absence 
of  spontaneous  motion — that  is  to  say,  motion  not  reflexly 
provoked  by  changes  immediately  taking  place  in  his  environ- 
ment. All  psychical  phenomena  seem  to  be  absent.  He  feels 
no  hunger  and  shows  no  fear,  and  will  suffer  a  fly  to  crawl 
over  his  nose  without  snapping  at  it.  '  In  a  word,  he  is  an 
extremely  complex  machine,  whose  actions,  so  far  as  they  go, 
tend  to  self-preservation  ;  but  still  a  machine  in  this  sense, 
that  it  seems  to  contain  no  incalculable  element.  By  applying 
the  right  sensory  stimulus  to  him,  we  are  almost  as  certain  of 
getting  a  fixed  response  as  an  organist  is  when  he  pulls  out  a 
certain  stoj).'  ^ 

The  effects  of  ablation  of  the  cerebral  hemispheres  of  the 
pigeon  are  very  similar.     If  left  to  itself,  the  bird  remains 

'  James,  '  PsycboloKy.' 


THE   BRAIN  643 

perfectly  still  and  seems  fast  asleep  ;  if  stimulated,  it  may  be 
made  to  fly  normally,  and  is  then  observed  to  avoid  obstacles, 
guided  by  the  sense  of  sight.  Like  the  frog,  it  shows  no 
signs  of  fear  or  hunger,  and  will  starve  to  death  on  a  heap 
of  corn,  although  it  will  begin  to  eat  the  corn  if  its  beak 
be  plunged  into  it.  In  the  higher  mammals  ablation  of  the 
cerebral  hemispheres  produces  such  severe  shock  that  nothing 
can  be  said  with  regard  to  the  working  of  the  lower  part  of 
the  brain.  The  rabbit  however  will  survive  the  operation  for 
a  few  hours,  and  during  this  time  it  can  be  made  to  run,  and 
no  symptoms  of  paralysis  are  observed.  When  a  sensory 
surface  or  nerve  is  stimulated,  the  animal  gives  forth  a 
prolonged  and  plaintive  cry.  We  thus  see  that  the  axial 
■parts  of  the  brain,  together  with  the  corpora  quadrigemina 
and  cerebellum,  contain  all  the  necessary  nervous  mechanisms 
for  the  carrying  out  of  the  co-ordinated  muscular  actions 
involved  in  standing,  running,  flying,  mastication,  deglutition, 
and  expression  of  the  emotions. 

Co-ordination  of  Muscular  Actions 

The  first  two  of  these  functions  must  be  considered  a  little 
more  fully.  The  maintenance  of  equilibrium  depends  on  a 
series  of  complex  reflex  acts,  which  are  dependent  on  incoming 
or  afferent  impulses,  on  various  centres  or  interlacements  of 
nerve-paths  in  the  grey  matter,  and  on  efferent  impulses  to 
the  muscles. 

The  chief  afferent  impulses  which  guide  the  maintenance 
of  equilibrium  are  those  from  the  skin,  eyes,  semicircular 
canals,  and  muscles  themselves.  The  importance  of  impulses 
from  the  skin  is  shown  by  those  cases  in  which,  from  disease 
of  the  sensory  tracts,  there  is  anaesthesia  of  the  soles  of  the 
feet.  In  these  cases  the  patient  is  unable  to  stand  with 
his  eyes  shut,  and  indeed  may  first  discover  that  anything 
is  wrong  with  him  from  the  fact  that  he  is  apt  to  fall  down 
whenever  he  is  washing  his  face.  The  same  effect  may 
be  experimentally  produced  by  freezing  the  soles  of  the  feet, 
so  as  to  make  them  ans'sthetic.  If  in  a  brainless  frog  the 
skin  be  stripped  from  the  hind  limbs,  it  no  longer  sits  up 
in  a  normal  posture,  and  is  unable  to  climb  up  an  inclined 
board. 


644  PHYSIOLOGY 

The  use  of  visual  impulses  in  guiding  the  nervous  centres 
in  the  maintenance  of  equilibrium  is  shown  by  the  preceding 
experiment,  in  which  no  loss  of  equilibrium  occurred  till  the 
patient  closed  his  eyes.  Sudden  destruction  of  the  eye  in 
pigeons  or  rabbits  causes  these  animals  to  spin  round  and 
round  in  a  circle,  or  may  cause  them  to  fall  over.  The 
giddiness  too,  that  is  often  produced  by  looking  at  rapidly 
moving  objects,  such  as  a  train  or  waterfall,  shows  the  con- 
nection of  this  sense  with  equilibration. 

By  far  the  most  important  afferent  impulses  are  those 
coming  from  the  semicircular  canals.  A  full  account  of  the 
effects  of  interference  with  these  structures  has  been  already 
given  in  the  chapter  on  the  organs  of  special  sense,  where 
too  the  intimate  connections  of  the  semicircular  canals  with 
the  movements  of  the  eyes  and  head  have  been  described. 

Finally,  all  the  muscular  actions  required  for  the  main- 
tenance of  equilibrium  are  guided  and  regulated  by  afferent 
impulses  from  the  muscles  themselves.  The  muscular  sense 
however  is  still  more  important  in  the  co-ordination  of  the 
muscular  contractions  by  which  locomotion  is  carried  out. 
This  is  well  exemplified  in  certain  cases  where,  owing  to 
disease  of  the  intra-muscular  afferent  nerves,  or  of  the  sensory 
channels  of  the  cord,  there  is  a  loss  of  muscular  sense  accom- 
panied by  loss  of  muscular  tone  and  tendon  reflexes.  Such 
cases  are  said  to  suffer  from  ataxy.  There  is  no  propor- 
tionality between  the  contractions  of  the  various  muscles 
used,  so  that  some  muscles  act  too  strongly  and  others  too 
feebly.  In  this  way  a  vast  amount  of  energy  is  expended 
with  very  little  practical  effect  in  moving  the  patient  along. 

It  is  difficult  to  say  that  the  function  of  co-ordinating 
movements  or  maintaining  equilibrium  is  limited  to  any  dis- 
tinct part  of  the  cerebro-spinal  axis.  We  have  already  seen 
that  all  the  machinery  necessary  for  carrying  out  some  of  the 
most  complicated  movements  of  locomotion  is  present  in  the 
spinal  cord  ;  and  it  is  probable  that  under  normal  circum- 
stances all  that  the  higher  centres  do  is  to  set  this  machinery 
going.  We  may  say  that  these  functions  are  served  by  the 
whole  cerebro-spinal  axis  from  the  tliird  ventricle  to  the 
lower  end  of  the  spinal  cord,  together  with  the  outgrowths 
forming  the  corpora  quadrigemina  and  the  cerebellum. 
There  is  a  good  deal  of  evidence  connecting  the  latter  organ 


THE   BRAIN  645 

more  closely  with  the  co-ordination  of  movements  than  with 
any  other  function.  We  may  therefore  take  this  opportunity 
of  considering  the  chief  points  in  the  structure  and  connec- 
tions of  this  organ,  as  well  as  some  suhsidiary  co-ordinating 
mechanisms  in  close  connection  with  the  cerehellum. 

Cerebellum 

Structure. — As  we  have  already  seen,  the  cerebellum  is 
formed  by  an  outgrowth  from  tlie  dorsal  wall  of  the  anterior 
part  of  the  fourth  ventricle,  and  consists  of  a  superficial 
layer  of  grey  matter  enclosmg  white  matter.  It  is  divided 
into  three  lobes,  two  lateral  lobes,  and  one  middle  lobe 
forming  the  superior  and  inferior  vermis.  The  surface  of 
the'  cerebellum  is  increased  by  being  thrown  into  leaf-like 
folds,  so  that  a  section  of  this  organ  has  a  tree-like  appear- 
ance. A  section  through  a  lamina  shows  three  distinct  zones 
— an  outer  molecular  presenting  a  granular  appearance  with 
a  few  nuclei ;  internal  to  this,  a  granular  layer  composed  of 
many  nuclei  of  nerve-cells  ;  and  most  deeply  a  central  core  of 
white  matter.  Between  the  molecular  and  granular  layers 
are  situated  the  cells  of  Purkinje,  large  flask-shaped  cells 
each  with  one  apical  dendrite,  distinguished  above  all  other 
dendrites  of  the  central  nervous  system  by  the  richness  of 
its  l)ranching,  and  with  one  axon,  which  leaves  the  base  of 
the  cell  and  passes  down  into  the  central  white  matter,  giving 
off  collaterals  in  its  course. 

In  preparations  made  l)y  Golgi's  method  we  are  able  to 
distinguish  the  various  elements  composing  these  layers 
and  their  relations.  The  molecular  layer,  besides  neuroglia 
cells  and  the  branching  dendrites  of  the  cells  of  Purkmje, 
cojitains  certain  star-shaped  cells  (a.  Fig.  296),  which  give  off 
an  axon  running  parallel  with  the  surface  in  the  molecular 
layer.  From  this  axon  branches  dip  down  towards  the  cells 
of  Purkinje,  where  they  end  in  a  rich  basket-work  of  fibres 
around  the  body  and  beginning  of  the  axon  of  these  cells. 
The  nuclear  layer  presents  two  kinds  of  cells.  The  most 
numerous  is  a  small  cell  with  a  few  short  dendrites,  each  of 
which  terminates  in  a  claw-shaped  arborisation,  and  a  single 
long  axon,  which  passes  straight  up  into  the  molecular  layer, 
where  it  bifurcates,  forming  two  branches  which  run  parallel 


646 


PHYSIOLOGY 


with  the  surface  m  a  direction  at  right  angles  to  the  plane  of 
expansion  of  the  dendrites  of  Purkinje's  cells,  apparently 
resting  against  the  serrations  on  the  edges  of  these  processes. 
The  second  kind  of  cell  in  the  granular  layer  is  the  so- 
called  Golgi's  cell — a  large  cell  with  many  dendrites  and 
an  axon  which  terminates  by  frequent  branches  in  the  neigh- 
bouring grey  matter. 

Fig.  296. 


Molecular 
layer. 


Cells  of 
Puikinje 


Schema  of  constituent  elements  of  cerebellum  (modified  from  Bolim 
and  Davidoff).  On  the  left  is  a  section  of  the  cortex  as  it  appears 
when  stained  by  ordinary  methods.  The  middle  portion  represents 
diagrammatically  a  section  at  right  angles  to  the  laminse,  while  to 
the  right  of  the  dotted  line  the  section  is  taken  in  the  same  plane  as 
the  laminte.  a,  star-shaped  cells  of  molecular  layer ;  b,  b,  cells  of 
Purkinje  ;  c,  '  Golgi  cell ' ;  d,  small  cells  of  nuclear  layer  ;  e, '  tendril 
fibre  ' ;  f ,  '  moss  fibre  ' ;  g,  axon  of  cell  of  Purkinje. 


The  fibres  making  up  the  white  matter  are  of  three 
kinds — two  afferent  and  one  efferent.  The  moss  fibres,  so 
called  from  the  curious  thickenmgs  they  present  in  the 
nuclear  layer,  pass  up  into  the  grey  matter  and  terminate 
by  frequent  branches  in  this  layer.  The  tendril  fibres, 
also  afferent,  end  in  a  rich  arborisation  which  surrounds  the 
distal  part  of  the  cell  and  the  bases  of  the  dendrites  of  the 


THE   BEAIN  647 

cells  of  Purkinje.  The  efferent  fibres  are  represented  by 
the  axons  of  the  cells  of  Purkinje,  which  acquire  a  medullary 
sheath  and  run  down  into  the  white  matter. 

This  slight  sketcli  of  the  anatomy  gives  us  a  conception 
of  the  extreme  complexity  of  choice  presented  to  nervous 
impulses  traversing  the  cerebellar  cortex.  Thus  a  discharge 
along  an  axon  of  the  cell  of  Purkinje  ma}'^  be  excited  (1)  by 
an  impulse  ascending  the  tendril  fibres,  or  (2)  by  one 
ascending  the  moss  fibres  through  the  granule  cells,  and 
then  passing  by  their  bifurcating  axon  to  the  dendrites  of 
the  cells  of  Purkinje,  or  (3)  by  the  star-shaped  cells  of  the 
molecular  layer  and  their  basket-work  round  the  body  of 
Purkinje's  cells. 

Conuectionfi  of  the  Gerehdlnm. — Although  these  have  been 
already  touched  upon  in  dealing  with  the  brain  as  a  whole, 
they  may  be  summarised  here  in  order  to  guide  us  in  the 
discussion  of  the  cerebellar  functions. 

By  the  Inferior  j^fduncles  afferent  fibres  pass  to  superior 
vermis  — 

1.  From  Clarke's  column  of  the  same  side  by  the  posterior 
cerebellar  tract. 

2.  From  both  sets  of  dorsal  nuclei,  and  therefore  in- 
directly from  the  posterior  columns  and  posterior  spinal 
roots  of  both  sides. 

3.  By  the  internal  restiform  body  (C.V.T.,  Figs.  297  and 
298)  from  the  vestibular  division  of  the  eighth  nerve,  part  of 
the  fibres  passing  through  Deiters'  nucleus. 

By  the  middle  jjediincles  fibres  pass  to  the  other  side  of 
the  pons,  where  they  enter  into  relation  with  the  frontal 
cortical  and  temporo-occipital  fibres,  which  pass  from  the 
cerebral  cortex  of  the  opposite  side  by  the  lateral  portions 
of  the  crusta  to  end  in  this  region. 

By  the  superior  peduncles  the  dentate  nucleus  of  the  cere- 
bellum, and  to  a  certain  extent  the  cortex,  are  connected 
with  the  red  nucleus  and  thalamus  of  the  opposite  side. 
Moreover  the  fibres  of  the  antero-lateral  ascending  tract 
enter  the  cerebellum  by  this  peduncle  and  terminate  in  the 
superior  vermis. 

It  will  be  observed  that,  ji.dging  from  the  direction  of 
degeneration,  most  of  these  tracts,  so  far  as  the  cerebellum 
is   concerned,  are  chiefly  afferent.     No  direct  connection  of 


648 


PHYSIOLOGY 


tlie  cerebellum  with  the  nuclei  of  motor  nerves  has  been 
described,  but  there  is  a  very  important  indirect  connection 
by  means  of  the  so-called  auditory  nucleus  or  nucleus  of 
Deiters.  The  connections  of  this  nucleus  are  shown  in 
Fig.  298.  Receiving  fibres  from  the  roof-nuclei  of  the  cere- 
})ellum,  as  w^ell  as  directly  from  the  semicircular  canals  by 
the  vestibular  nerve,  it  gives  off  a  number  of  efferent  fibres. 
These  may  be  divided  into  two  classes.  One  set,  the  vesti- 
bulo-spinal  fibres,  pass  down  the  cord  as  the  antero-lateral 
descending  tract,  the  fibres  of  which  terminate  in  the  anterior 
cornu   on  the  same  side  of   the   cord.     The  second  set   of 


Fig.  297. 

Sup. Vermis 


r2p. 


Sebeniatic  representation  of  some  of  the  connections  of  the 
cerebellum  (Bruce).  C.E.,  inferior  peduncle  or  restiform  l)ody; 
S.C.P.,  superior  peduncle ;  CD.,  corpus  dentatum ;  E.N.,  roof 
nuclei ;  S.F.,  sagittal  fibres  from  cortex  to  roof  nuclei ;  C.V.T., 
cerebello- vestibular  tract ;  D.N.,  Deiters'  nucleus  ;  VIII,  vesti- 
bular nerve. 


fibres,  passing  inwards,  terminate  partly  in  the  nucleus  of  the 
sixth  nerve  and  partly  turn  upwards  and  downwards  in  the 
posterior  longitudinal  bundle,  making  connections  with  the 
nuclei  of  the  fourth  and  third  nerves,  while  the  descending 
fibres  run  down  in  the  anterior  column  and  apparently  end 
in  the  anterior  cornu. 

Functions  of  the  CereheUum. — We  thus  see  that  the  cere- 
bellum is  connected  directly  with  the  sensory  mechanisms 
chiefly  of  the  same  side  of  the  body,  with  the  cortex  of  the 
opposite  cerebral  hemisphere,  and  indirectly  with  the  motor 
mechanisms  of  the  same  side  of  the  body,  as  well  as  of  the 


TEE  BRAIN 


649 


eye  muscles.  It  is  moreover  specially  connected  with  the 
semicircular  canals.  By  its  complexity  and  connections  it 
is  therefore  eminently  fitted  to  take  its  part  in  controlling 
and  guiding  all  hodily  movements,  especially  those  of  the  same 
side   of   the    body.     It  is  therefore   surprising   to   find  how 


C.R.v 


Fia.  298. 

Sup.Vermls 


Schema  of  connections  of  Deiters'  nucleus  (Bruce).  C.R., 
restiform  body;  E.N.,  roof  nuclei;  S.F.,  sagittal  fibres  from 
cortex  to  roof  nuclei ;  C.V.T.,  eerebello-vestibular  tract ; 
D.N.,  Deiters'  nucleus;  III,  VI,  nuclei  of  third  and  sixth 
nerves ;  P.L.F.,  posterior  longitudinal  bundle ;  VIII,  vesti- 
bular division  of  eighth  nerve ;  S.C.,  semicircular  canals ; 
V.S.T.,  vestihulo-spinal  fibres. 

very  slight  relatively  are  the  disturbances  produced  by 
its  complete  ablation.  In  fact  we  learn  more  from  experi- 
ments in  which  only  one  half  of  the  cerebellum  has  been 
removed,  as  we  are  then  able  to  compare  the  affected  with 
the  relatively  sound  side.  After  such  an  operation  there  is 
at  first  marked  loss  of  co-ordinating  power.  The  animal  tends 
to    fall  towards  the    side    of   the    lesion,  and   we   may   get 


(;50  PHYSIOLOGY 

forced  inovenients  of  the  body  or  of  the  e.yes  to  the  opposite 
8ide.  After  some  time  these  acute  disturljances  pass  away, 
and  the  only  point  to  be  noticed  is  a  loss  of  power  and  of 
tone  in  the  muscles  of  the  side  of  the  body  from  which 
the  cerebellum  has  been  removed.  Complete  or  extensive 
destruction  of  the  cerebellum  in  man  has  a  more  evident 
effect,  owing  to  the  greater  co-ordinating  capacity  necessary 
for  maintaining  the  erect  posture  whilst  standing  and  walk- 
ing. In  such  cases  we  observe  an  inco-ordination  of  move- 
ment exactly  resembling  that  of  a  drunken  man,  and  this 
may  at  times  be  associated  with  nodding  movements  of  the 
head  or  forced  lateral  movements  (nystagmus)  of  the  eye- 
balls. 

It  has  been  suggested  that  the  chief  action  of  the  cere- 
bellum is  to  exert  a  moderating  restrahiing  influence  on  the 
cortex  cerebri.  But  the  facts  mentioned  above  seem  to  show 
that  its  action  is  rather  to  keep  the  motor  mechanisms  both 
of  the  cortex  and  the  spinal  cord  in  a  state  of  wakefulness, 
ready  to  contract  in  co-ordinated  sequence  to  peripheral 
stimuli. 

More  important  is  tlie  connection  of  the  cerebellum  with 
the  organs  of  static  sense  (the  semicircular  canals  and  the 
otolith  organs  in  the  saccule  and  utricle).  We  have  seen 
that  the  spinal  cord  contains  all  the  apparatus  required  for 
the  carrying  out  of  co-ordinated  movements,  with  their  com- 
plicated play  of  afferent  and  efferent  impulses.  Any  such 
movement  requires — 

(1)  The  appropriate  peripheral  stimulus,  usually  applied 
to  the  skin,  and,  according  to  its  character,  exciting  different 
sets  of  nerve  fibres. 

(2)  The  centre  or  series  of  centres  in  the  spinal  cord. 
The  afferent  impulse  on  arrival  at  the  cord  causes  discharge 
of  motor  impulses  to  certain  muscles,  and  a  central  inhibition 
of  the  tone  of  the  antagonistic  muscles. 

(3)  The  adaptation  of  the  extent  of  the  contraction  to  the 
original  stimulus  involves  a  secondary  reflex,  from  muscle 
through  cord  ;  the  afferent  nerves  of  the  muscle  conveying  to 
the  centres  accurate  information  as  to  the  strength  of  con- 
traction of  the  muscle. 

A  movement  therefore  is  started  by  a  skin  impression 
involves    reciprocal   innervation    (motor   and    inhibitory)    of 


THE   BRAIN  651 

antagonistic  muscles,  and  these  responses  are  in  theiv  turn 
guided  and  controlled  by  afferent  impulses  excited  by  the 
movements  and  arising  chiefly  in  the  muscles  themselves.  If 
this  apparatus  is  all  in  working  order,  why  cannot  an  animal 
stand  or  walk  (except  with  great  attention  and  training)  after 
the  destruction  of  the  cerebellum  ?  The  answer  is  to  be 
found  in  the  fact  that  every  animal  (quadruped  or  biped)  is, 
when  erect,  in  a  state  of  unstable  equilibrium,  the  slightest 
movement  tending  to  displace  the  centre  of  gravity,  so  that  the 
animal  would  fall  if  the  displacement  were  not  at  once  reme- 
died by  muscular  contractions.  It  is  the  special  office  of  the 
cerebellum  to  co-ordinate  the  movements  (already  co-ordinated 
with  one  another  by  the  spinal  cord)  with  the  position  of  the 
body  in  relation  to  its  centre  of  gravity.  On  this  account 
it  receives  impulses  from  the  semicircular  canals,  from  the 
eyes,  from  skin,  joints,  and  muscles,  and  sits,  so  to  speak,  on 
the  motor  path  from  cortex  to  spinal  cord,  maintaining 
continually  a  tonic  hold  on  the  muscles,  which  may  be 
strengthened  or  relaxed  according  to  the  position  of  the 
animal. 

The  diagonal  movements  of  the  limbs  in  walking  causes 
an  alternate  shifting  of  the  centre  of  gravity  to  one  side  or 
the  other.  Each  lateral  movement  would  and  does  cause  the 
animal  to  fall,  if  the  position  be  not  immediately  remedied  by 
the  influence  of  the  cerebellum. 

After  extirpation  of  the  cerebellum  in  a  dog,  the  animal 
falls  to  one  side  or  the  other  as  soon  as  it  tries  to  walk. 
Later  it  learns  to  walk  by  an  entirely  new  mode  of  progres- 
sion, viz.  a  series  of  jumps,  in  which  the  hind  limbs  or  fore 
limbs  move  together,  being  sprawled  widely  apart,  so  as  to 
widen  as  much  as  possible  the  basis  of  support  and  avoid  the 
alternate  lateral  displacements  of  the  centre  of  gravity  inci- 
dent on  the  normal  method  of  locomotion. 

E.^roTiONAL  Expression 

By  appropriate  stimuli  it  is  possible  to  elicit  in  animals 
deprived  of  their  cerebral  hemispheres  the  outward  bodily 
manifestations  which  we  usually  regard  as  the  expressions 
of  certain  emotions.  Now  an  emotion  is  a  psychical  state, 
or  state  of  consciousness  ;    but   it  is  dependent  for  its   pro- 


652  PHYSIOLOGY 

duction  on  the  existence  of  certain  bodily  changes— affections 
of  the  heart,  vascular  system,  voluntary  muscles  — which  are 
involuntary  and  reflexly  produced.  The  mechanism  for  this 
reflex  production  is  present  in  the  lower  cerebral  centres,  so 
that  severe  stimulation  of  a  sensory  nerve  in  a  hemisphere- 
less  rabbit  causes  it  to  utter  a  long,  plaintive  scream.  The 
brainless  frog  responds  with  a  croak,  almost  indicative  of 
pleasure,  each  time  its  back  is  gently  stroked.  In  neither 
of  these,  nor  in  any  similar  cases,  are  we  justified  in  speaking 
as  if  an  emotional  state  of  consciousness  were  present,  We 
have  no  reason  to  think  that  the  rabbit  suffers  pain,  or  that 
the  frog  is  pleased,  in  the  two  above-mentioned  experiments, 
but  merely  that  certain  changes  in  the  bodily  condition  are 
reflexly  produced,  which,  if  the  cerebral  hemispheres  were 
present,  would  be  represented  in  consciousness  as  an  emotion 
of  pain  or  pleasure. 

When  a  patient  is  slightly  under  the  influence  of  chloro- 
form, it  frequently  happens  that  all  the  emotional  expressions 
are  preserved,  although  consciousness  is  totally  abolished  ; 
he  may  cry  out  or  struggle  when  cut  with  the  knife,  and 
yet  when  he  recovers  from  the  anaesthetic  he  will  state  that 
he  felt  nothing  whatever  of  the  operation. 


THE   BEAIN  663 

Section  4 
CEREBEAL  HEMISPHEEES 

Structure  of  the  Cerebral  Cortex 

We  have  considered  the  connections  of  the  hemispheres 
so  fully  in  the  previous  sections  that  it  remains  now  only  to 
describe  shortly  the  chief  points  in  their  structure. 

The  cortex,  as  in  the  cerebellum,  consists  of  a  superficial 
layer  of  grey  matter  covering  a  central  mass  of  white  fibres, 
and,  as  in  the  cerebellum,  increased  area  of  surface  with 
growing  intelligence  is  afforded  by  the  folding  of  the  surface 
into,  convolutions  and  fissures.  The  chief  convolutions  are 
indicated  in  Figs.  301,  302. 

On  section  the  grey  matter  is  seen  to  consist  of  many 
layers  of  nerve-cells  embedded  in  a  mass  of  tissue  composed  of 
neuroglia  and  many  fine  nerve-fibres,  both  medullated  and 
non-medullated.  The  nerve-cells  vary  in  size  and  shape,  but 
one  kind  of  cell  is  typical  of  this  part  of  the  nervous  system. 
This  is  the  pyramidal  cell  (Fig.  299,  g),  and  consists  of 
a  cone-shaped  cell-body  with  one  large  apical  dendrite  which 
runs  towards  the  surface  and  breaks  up  in  the  superficial 
molecular  layer  into  a  number  of  branches.  Dendrites  also 
pass  oft"  from  the  sides  and  lower  angles  of  the  cell.  The 
axon  arising  from  the  middle  of  the  base  of  the  cell  passes 
downwards  into  the  white  matter,  giving  oft"  collaterals  in  its 
course.  Some  of  these,  especially  those  from  the  large  cells 
of  the  '  motor  '  area  of  the  cortex,  pass  into  the  pyramidal 
tracts. 

Although  not  so  distinctly  stratified  as  the  cerebellar 
cortex,  the  cerebral  grey  matter  can  be  divided  into  four 
(or  in  some  situations  five)  layers.  Most  superficial  is  the 
molecular  layer,  composed  of  the  dendrites  of  the  pyramidal 
cells,  and  certain  spindle-shaped  '  jjluripolar  '  cells,  i.e.  cells 
with  several  apparently  nervous  processes  running  parallel 
to  the  surface.  These  are  sometimes  spoken  of  as  '  associa- 
tion cells.' 

Next  to  this  comes  a  layer  of  small  pyramidal  cells,  which 
is  succeeded  by  a  layer  of  small  cells  known  as  the  granular 


654 


PHYSIOLOGY 


Fig.  299. 


UL 


Diagrammatic  section  of  cerebral  cortex  (from  Barker,  after  Starr, 
Strong,  and  Learning).  L  molecular  layer  with  o,  bipolar  cell; 
II,  layer  of  small  pyramidal  cells;  III,  layer  of  large  pyramidal 
cells  ;  IV,  polymorphous  layer ;  V,  white  matter. 


THE   BRAIN 


655 


layer.  Below  this,  in  the  motor  cortex,  is  a  layer  of  large 
pyramidal  or  Betz  cells,  while  deepest  of  all  is  the  'poly- 
morphous '  layer,  composed  of  many  types  of  cells,  ordinary 
pyramidal  cells,  pyramidal  cells  with  the  base  and  axon 
tm-ned  towards  the  surface  (cells  of  Martinotti),  and  *  Golgi 
cells '  with  a  freely  branching  axon  which  terminates  in  the 
adjacent  grey  matter.  Among  the  nerve-fibres  passing 
between  grey  and  white  matter,  four  types  (Fig.  300)  can  be 
distinguished : 

a.  Projection  fibres,  passing  from  the  pyramidal  cells  to 
lower  levels  of  the  central  nervous  system.  These  include 
the  various  fibres,  pyramidal  and  otherwise,  that  make  up 
the  internal  capsule  and  the  crusta. 

Fiu.  300. 


Schematic  section  through  cerebral  hemispheres,  to  show  chief 
classes  of  nerve  tracts  (after  Eamon  y  Cajal).  A,  corpus  cal- 
losum ;  B,  anterior  commissure  ;  C,  pyramidal  tract ;  a,  cell 
giving  off  projection  fibre  ;  b,  cell  giving  oil'  commissural  fibre ; 
f ,  cell  with  axon  forming  association  fibres. 


h.  Commissural  fibres  passmg  across  the  corpus  callosum 
and  anterior  commissure  to  the  opposite  hemisphere. 

c.  Association  fibres  passing  from  one  part  of  the  cortex 
to  another  on  the  same  side  of  the  brain. 

d.  Afierent  or  sensory  fibres,  starting  in  most  cases  from 
the  region  of  the  thalamus  and  passing  up  to  termmate  by 
branching  in  the  superficial  layer  of  the  cortex. 

In  a  section  through  the  cortex,  stained  by  some  method  such  as  Weigert's, 
to  display  medullated  nerve-fibres,  in  addition  to  the  bundles  of  radial  fibres 


656  PHYSIOLOGY 

vimning  from  the  white  matter  perpendicularly  to  the  &urface,  bands  of 
tangential  fibres  are  seen  running  parallel  to  the  surface  in  certain  situations, 
viz.: — 

(a)  A  layer  just  under  the  surface  of  the  cortex. 

(b)  A  layer  between  the  molecular  and  small  pyramidal  layer,  the  outer  line 
of  Baillarger. 

(c)  A  layer  betsveen  the  granular  and  large  pyramidal  cell  layers,  the  inner 
line  of  Baillarger. 

(d)  In  addition  to  these  a  special  layer  of  tangential  fibres  is  present  in  the 
occipital  lobe,  running  through  the  middle  of  the  granular  layer,  and  dividing 
it  into  two.     This  is  known  as  the  line  of  Gennari. 

Differences  are  observed  in  the  structure  of  various  parts  of  the  cortex 
corresponding  to  their  functions.  Thus  we  may  say  roughly  that  the  small 
pyramidal  layer  is  chiefly  associational  in  function,  the  large  pyramidal  cells 
are  motor,  the  granular  layer  is  sensory,  while  the  polymorphous  layer 
presides  over  the  lowest  cortical  functions,  such  as  the  getting  of  food,  the 
sexual  instincts,  etc. 

Thus,  in  a  low  animal,  such  as  the  rabbit,  the  polymorphous  layer  has  three 
times  the  thickness  of  the  pyramidal  layer,  whereas  in  man,  with  his  infinitely 
greater  ideational  powers,  it  is  only  one-third  the  thickness  of  this  layer. 

In  man,  in  an  associational  area  such  as  the  prefrontal  convolutions,  the 
pyramidal  layer  is  almost  as  thick  as  all  the  rest  put  together.  In  the  visual 
area  (occipital  lobes)  the  granular  layer  is  the  thickest,  and  is  divided  into  two 
layers  by  the  band  of  tangential  fibres  forming  the  line  of  Gennari.  In  the 
motor  area  (chieMy  the  descending  frontal  convolution),  we  find  below  the 
granular  layer,  the  well-marked  large  pyramidal  cells,  or  Betz  cells,  though  here 
also  the  pyramidal  cell  layer  is  very  thick.  In  this  area  the  actual  average 
thicknesses  of  tlie  different  layers  are  as  follows : — 

Molecular 0-31  mm. 

Small  pyramidal 0-90  mm. 

Granular 0-22  mm. 

Betz  layer 0-22  mm. 

Polymorphous         .....  0'61  mm. 

Functions  of  the  Cortex 
We  may  come  to  some  conclusions  as  to  the  general 
functions  of  the  cerebral  hemispheres  if  we  compare  the 
behaviour  of  a  normal  animal  with  one  that  has  been  deprived 
of  its  cerebral  hemispheres.  In  the  former  case  it  is  quite 
impossible  to  foretell  what  particular  reaction  may  be  evoked 
by  any  stimulus.  It  may  produce  the  same  effect  as  when 
applied  to  a  brainless  animal,  but  in  most  cases  the  reaction 
is  modified  in  various  ways.  The  animal  is  no  longer  a  mere 
machine  that  can  be  played  on  at  will,  but  is  an  individual 
whose  actions  are  ruled  by  a  guiding  intelligence,  who  is 
actuated  by  motives  and  by  feelings  of  fear,  hunger,  pain,  and 
the  like.  In  short,  an  animal,  whose  cerebral  hemispheres 
are  intact,  presents  phenomena  analogous  to  those  which  in 


THE    BEAIN 


657 


ourselves  are  associated  with  changes  in  the  state   of   con- 
sciousness, and  which  we  call  voUtion  or  feehng. 

It  was  formerly  thought  that  in  all  their  functions  the 
cerebral  hemispheres  acted  as  an  entity,  and  that  any 
voluntary  movement  or  any  change  in  the  state  of  con- 
sciousness might  be  considered  as  carried  out  by  all  parts 
of  the  hemispheres  acting  together.  It  has  long  been  known 
however  that  each  cerebral  hemisphere  innervates  the 
opposite  side  of  the  body.  Thus  if  the  cortex  of  one  hemi- 
sphere be  destroyed  or  be  functionally  separated  from  the 
lower  parts  of  the  brain  by  destruction  of  the  internal 
capsule,  paralysis  (hemiplegia)  and  loss  of  sensation  on  the 

.    .  Fig.  301. 


Motor  centres  on  outer  surface  of  monkey's  brain. 
(After  Campbell  and  Sherrington.) 


opposite  side  of  the  body  are  produced.  The  paralysis  is 
limited  to  voluntary,  and  may  not  affect  reflex  or  emotional 
movements.  It  is  interesting  to  note  that  those  movements, 
which  are  usually  carried  out  by  the  muscles  on  both  sides 
of  the  body  at  the  same  time,  are  not  so  affected,  probably 
in  consequence  of  the  close  interdependence  of  the  bulbar  and 
spinal  centres  for  these  movements,  and  partly  because  these 
movements  are  equally  represented  in  both  hemispheres. 

Moreover,  during  the  last  twenty  years,  conclusive  evi- 
dence has  been  brought  forward  of  the  localisation  of  func- 
tion in  the  cerebral  cortex  of  each  side  of  the  brain.  This 
evidence  is  physiological  and  pathological,  but  in  both  cases 
it  falls  under  the  head  of  excitation,  or  of  destruction  and 
consequent   paralysis.     On    exciting    certain    parts    of    the 

42 


658 


PHYSIOLOGY 


cortex,  situated  in  the  neighbourhood  of  the  fissure  of 
Rolando,  definite  co-ordinated  movements  of  certain  muscles 
or  groups  of  muscles  are  produced,  varying  in  their  distri- 
bution according  to  the  exact  spot  stimulated.  Thus  stimula- 
tion with  the  faradic  or  galvanic  current  of  the  convolutions 
at  the  upper  part  of  the  fissure  of  Rolando  causes  co- 
ordinated movements  of  the  lower  limb  ;  of  the  middle  part, 
movements  of  the  upper  limb ;  and  of  the  lower  part 
(including  the  third  frontal  convolution),  movements  of  the 
head  and  face.  In  the  higher  apes  and  in  man  the  motor 
centres  all  lie  in  front  of  the  fissure  of  Rolando. 

These  experiments  are  corroborated  by  others  in  which 
definite  parts  of  the  Rolandic  area  of  the  cortex  are  destroyed. 

Fig.  302. 


Motor  centres  on  mesial  surface  of  monkey's  brain 
(After  Campbell  and  Sherrington.) 

Destruction  of  any  given  zone  of  the  motor  area  produces 
paralysis  of  voluntary  movement  in  the  part  represented  by 
this  portion  of  the  cortex.  Thus  if,  on  stimulation  of  a  spot 
near  the  centre  of  the  fissure  of  Rolando,  we  obtain  a  definite 
movement  of  the  arm  of  the  opposite  side,  and  we  then  excise 
the  cortex  at  this  spot,  we  find  that  after  the  operation  the 
animal  has  lost  voluntary  power  over  this  movement  and  over 
none  other. 

It  was  formerly  thought  that  the  grey  matter  was  not 
directly  excitable,  and  that  the  effects  of  electric  stimuli  were 
due  to  excitation  of  the  underlying  fibres  of  the  corona  radiata. 
The  direct  excitability  of  the  grey  matter  is  however  proved 
by  the  following  considerations. 

1.  There  is  greater  '  lost-time  '  in  the  grey  matter  than  in 


THE   BEAIN  659 

the  underlying  white  matter  ;  that  is,  if  we  first  stimulate  the 
grey  matter,  and  then  shave  this  off  and  stimulate  the  white 
matter  below,  it  is  found  that  the  latent  period,  which  elapses 
between  the  time  when  the  stimulus  is  sent  in  and  the  time  at 
which  the  contraction  takes  place,  is  far  greater  in  the  former 
than  in  the  latter  case. 

2.  It  is  very  common,  as  the  result  of  excessive  stimulation 
of  the  cortex,  to  get,  not  a  single  contraction  of  the  group  of 
muscles  represented  in  the  area  stimulated,  but  an  epileptic 
convulsion,  which  starts  in  this  group  of  muscles  and  spreads 
thence  to  all  the  other  muscles  of  the  body.  The  convulsion 
consists  of  two  stages  : 

.    (a)  The  tonic  stage,  in  which  all  the  muscles  of  the  body 
may  be  in  a  state  of  continued  contraction. 

'{!))  The  clonic  stage,  which  lasts  a  good  deal  longer  than 
the  first  stage,  and  consists  of  rapid  rhythmical  jerking  move- 
ments of  the  muscles.     This  is  followed  by — 

(c)  A  stage  of  exhaustion,  in  which  the  cortex  is  relatively 
inexcitable. 

If  the  grey  matter  oh  both  sides  be  removed,  stimulation 
of  the  white  fibres  of  the  corona  radiata  does  not  produce  a 
typical  epileptic  fit. 

Pathological  evidence  bears  out  the  results  on  localisation 
deduced  from  the  physiological  experiments  just  mentioned. 
Thus  we  have  cases  in  which  a  tumour  pressing  on  a  part  of 
the  motor  area  causes  twitching  movements  of  the  limb,  or 
more  or  less  complete  epileptic  convulsions,  starting  in  the 
particular  limb  represented  in  the  affected  area  of  the  cortex. 
On  the  other  hand,  cases  frequently  occur  in  which  parts  of 
the  cortex  are  destroyed  by  pressure  of  a  growing  tumour, 
or  by  stoppage  of  the  vessels  supplying  that  area,  giving  rise 
to  paralysis  of  definite  muscles  or  groups  of  muscles. 

After  a  movement  has  been  abolished  by  extirpation  of  a 
definite  area  of  the  cortex,  a  considerable  amount  of  recovery 
of  movement  may  take  place.  The  degree  to  which  this  may 
occur  varies  in  different  classes  of  animals,  being  more  com- 
plete in  the  lower  animals,  such  as  the  dog  and  rabbit,  than 
in  the  monkey  and  man.  It  probably  depends  on  the  taking 
up  of  the  functions  of  the  extirpated  area,  partly  by  the  ad- 
joining regions  of  the  cortex,  and  partly  by  the  corresponding 
centre  of  the  opposite  hemisphere,  all  the  centres  of  the  two 


660  PHYSIOLOGY 

sides  of  the  brain  being  functionally  connected  by  means  of 
the  corpus  callosum. 

A  point  of  considerable  importance  is  the  fact  that  move- 
ments rather  than  muscles  are  cortically  represented.  Thus 
stimulation  of  the  right  frontal  lobes  causes  a  movement 
(conjugate  deviation)  of  both  eyes  to  the  left.  In  this  move- 
ment the  external  rectus  of  the  left  side  and  the  internal 
rectus  of  the  right  side  are  both  set  in  action  by  stimulation 
of  the  right  cortex.  Similarly,  we  may  produce  movement 
of  the  head  to  the  left  by  stimulation  of  the  proper  spot  of 
the  right  cortex.  In  this  movement  the  right  sterno-mastoid 
and  the  left  external  oblique  muscles  are  involved. 

When  we  speak  of  motor  centres  it  must  not  be  supposed 
that  motor  impulses  start  de  novo  in  the  pyramidal  cells  of 
these  centres.  Here,  as  in  all  other  parts  of  the  central 
nervous  system,  the  activity  of  the  cells  is  excited  exclusively 
by  impulses  arriving  at  them  from  other  parts,  and  ultimately 
from  the  periphery  of  the  body. 

The  absolute  dependence  of  the  activity  of  the  pyramidal  cells  on  afferent 
stimuli  is  well  shown  by  an  experiment  carried  out  by  Mott  and  Sherrington. 
These  observers  divided  all  the  posterior  roots  of  the  brachial  plexus  in  the 
monkey,  and  found  that  the  effect  was  to  cause  motor  paralysis  of  all  the  finer 
movements  of  the  arm  and  hand.  On  exposing  the  corresponding  area  of  the 
cortex  and  exciting  it  electrically,  all  the  missing  movements  could  be  induced, 
showing  that  the  cortical  cells  needed  only  the  arrival  of  a  stimulus  to  make 
them  discharge.  No  experiment  could  show  better  the  ultimate  reflex  character 
of  those  movements  which  are  generally  distinguished  as  voluntary  or  purposive. 

These  impulses,  since  they  are  afferent  so  far  as  the 
pyramidal  cell  is  concerned,  may  be  spoken  of  as  sensory, 
and  the  collection  of  exalted  reflex  centres  which  make  up 
the  Eolandic  area  may  therefore  be  justly  termed  sensori- 
motor. In  fact,  many  obs3rvers  believe  that  it  is  in  the  motor 
area  that  the  sensation  of  touch  and  pressure  on  the  correspond- 
ing limbs  is  perceived,  and  that  the  motor  effect  of  exciting 
this  part  of  the  cortex  is  normally  produced  as  a  reaction  to 
such  touch  or  pressure  sensations.  Hence  in  extensive  lesions 
of  the  motor  area  sensation  as  well  as  movement  may  be 
affected,  and  a  certain  amount  of  anaesthesia  may  accompany 
the  paralysis. 

Sensory  areas. — If  in  the  monkey  the  right  occipital  lobe 
be  stimulated,   there  is  movement  of   both  eyes  to  the  left. 


THE   BEAIN  661 

This  experiment  by  itself  is  capable  of  two  interpretations  : 
either  the  occipital  lobe  is  to  be  regarded  as  a  motor  centre 
to  the  ocular  muscles,  or  it  is  a  sensory  centre  of  sight,  and 
the  animal  looks  towards  the  left  because  visual  sensations 
are  aroused  and  referred  to  the  left  side  of  the  field  of  vision. 
The  latter  explanation  is  shown  to  be  true  by  the  effects  of 
extirpation.  If  the  occipital  cortex  be  destroyed,  the  animal 
is  rendered  blind  on  the  side  opposite  the  lesion.  It  is  said 
to  suffer  from  hemianopia  (half -blindness).  Excision  of  both 
the  occipital  lobes  causes  total  blindness.  Since  the  rays  of 
the  left  half  of  the  field  of  vision  fall  on  the  right-hand  side 
of  the  two  retinae,  and  left  hemianopia  is  produced  by  de- 
struction of  the  right  occipital  lobe,  the  temporal  half  of  the 

Fig.  303. 


Diagram  showing  functional  connection  of  occipital  lobes  with  the  two 
retime,     c.  Optic  chiasma.     o.  Occipital  lobes. 

right  retina  and  the  nasal  half  of  the  left  retina  must  be 
innervated  from  the  right  occipital  cortex  (Fig.  303).  The 
connection,  however,  of  the  occipital  lobe  with  the  retina  of 
the  other  side  is  more  complete  than  with  the  retina  of  the 
same  side,  so  that  after  destruction  of  the  right  lobe  loss  of 
vision  is  more  extensive  in  the  left  than  in  the  right  eye. 

Our  evidence  as  to  the  localisation  of  the  other  senses  in 
the  cortex  is  less  complete.  The  sense  of  hearing  is  located 
by  Ferrier  in  the  superior  temporo-sphenoidal  lobe.  Stimu- 
lation of  this  part  causes  pricking  of  the  ears  ;  destruction  of 
it  produces  different  effects  according  to  the  experimenter  ; 
it  may  give  rise  to  deafness  on  the  opposite  side  (Ferrier), 
or  to  no  appreciable  results  (Schafer). 

Clinical  as  well  as  anatomical  observations  however  concur 
in  locating  auditory  sensations  in  this  convolution.  The  fact 
that  after  experimental  extirpation  the  animal  reacts  to  sounds 
does  not  prove  the  absence  of  impairment  of  auditory  sensa- 


662 


PHYSIOLOGY 


tio)is,  since  the  lower  centres  contain  all  the  mechanism  for 
a  simple  motor  reaction  to  an  incident  sound-wave. 

Smell  and  taste  have  been  localised  in  the  caput  cornu 
Ammonis  and  the  uncinate  gyrus. 


Fig.  304. 


.«fi?s;S?^/i#* 


Human  Brain  showing  outer  (A)  and  mesial  (B)  surfaces,  and  the 
situation  of  the  chief  motor  and  sensory  areas.  The  different 
shading  represents  the  extent  of  each  of  these  areas  as  determined 
by  a  study  of  the  histological  structure  of  the  cortex.     (Campbell.) 


THE   BEAIN  663 

Many  observers  believe  that  general  tactile  sensibility  as 
well  as  muscular  sensibility  is  cortically  represented  in  the  same 
region  as  are  muscular  movements,  i.e.  in  the  Eolandic  area. 

From  a  study  of  the  histological  characters  of  the  cerebral 
cortex,  Campbell  has  concluded  that  the  chief  seat  of  tactile 
sensibility  is  to  be  found  in  the  ascending  parietal  convolution, 
behind  the  fissure  of  Eolando. 

In  an  animal  such  as  the  dog,  we  are  able  in  this  way  to 
parcel  out  the  greater  part  of  the  cerebral  cortex  into  definite 
motor  and  sensory  areas.  It  is  possible  to  difterentiate  two 
grades  of  representation  of  sensation  in  the  so-called  sensory 
areas.  The  most  primitive  grade  is  concerned  in  the  reception 
of  afferent  impressions  from  definite  sense-organs  and  in  the 
production  of  the  appropriate  cortical  motor  response.  Sur- 
roundmg  or  in  immediate  proximity  to  this  area  is  generally 
found  a  wider  area,  which  may  give  no  effect  on  electrical 
stimulation,  and  is  concerned  in  the  *  working  up '  of  the 
primitive  sense-impressions  into  the  more  complex  condi- 
tions, which  we  term  ideas.  The  extent  of  these  psychical 
areas  increases  relatively  to  that  of  the  sensory  areas  as  we 
ascend  the  animal  scale.  Thus  in  man  the  visuo-sensory  area 
occupies  but  a  small  portion  of  the  cortex  and  is  concerned 
in  the  reception  of  simple  visual  sensations.  Destruction  of 
this  area  on  one  side  causes  half -blindness,  or  hemianopia. 
The  greater  part  of  the  occipital  lobe  is  taken  up  in  the  visuo- 
psychic  area,  the  whole  of  which  is  engaged  in  the  combina- 
tion of  sensations  from  the  eyes,  so  as  to  form  the  visual 
ideas  of  things.  Destruction  of  portions  of  this  area  will 
not  necessarily  affect  sight,  but  may  affect  more  or  less  the 
power  of  judging,  through  the  medium  of  the  eyes,  of  the 
nature  of  things  seen.  The  relative  extent  of  the  different 
sensory  and  psycho-sensory  areas  of  the  human  brain  is  repre- 
sented in  Fig.  304. 

Aphasia. — If  the  cortex  on  the  left  side  is  destroyed, 
power  of  speech  is  totally  lost.  The  same  effect  is  produced 
if  the  lesion  be  limited  to  a  small  area  in  the  third  left 
frontal  convolution  (Broca's  convolution).  Our  normal  right- 
handedness  is  necessarily  associated  with  and  caused  by  the 
activities  of  the  left  hemisphere  predominating  over  and 
guiding  those  of  the  right.  In  the  movements  that  stand 
highest  in  the  evolution  of  the  cerebral  functions,  those  of 


()64 


PHYSIOLOGY 


speech,  the  predommance  of  the  left  side  is  so  marked  that 
destruction  of  the  association  centre  for  lips  and  tongue  on 
this  side  causes  complete  loss  of  the  delicately  co-ordinated 
movements  by  which  speech  is  produced. 

Besides  this  motor  aphasia,  which    is  brought  about  by 
a  lesion  of  Broca's  convolution,  speech  may  be  destroyed  by 

Fig.  305. 


Diagram  of  cortical  areas  engaged  in  speech  and  writing  (Eoss). 
a.  Broca's  convolution  (motor  lip  and  tongue  area),  b.  Arm 
area.     c.  Auditory  word-centre,     d.  Visual  word-centre. 


injury  of  the  sensory  psychical  centres  (sensory  aphasia). 
Thus  in  some  cases  in  which  the  left  superior  temporo- 
sphenoidal  lobe  was  involved,  there  has  been  a  condition  of 
word-deafness.  The  patient  could  not  understand  anything 
that  was  said  to  him.  He  might  be  able  to  talk  volubly,  but 
the  words  were  mere  gibberish  ;  his  auditory  word-centre  being 
absent,  he  was  unable  to  appreciate  the  meaning  of  what  he 
was  saying,  and  so  the  motor  processes  went  on  unchecked 
by  the  criticism  of  sensory  impressions.     The  talking  of  a 


THE   BRAIN  665 

man  with  word-deafness  may  be  compared  to  the  walking  of 
a  man  with  complete  loss  of  muscular  sense. 

In  the  same  way  a  lesion  m  the  left  occipital  lobe  may 
cause  loss  of  power  to  read  (alexia)  from  blotting  out  of  all 
the  higher  visual  memories,  and  more  especially  those  con- 
nected with  written  and  printed  words. 

Any  of  these  lesions  may  be  attended  with  inability  to 
write,  which  in  most  people  is  intimately  dependent  on  the 
auditory  and  motor  speech  memories.  Most  people  in  writing 
may  be  seen  to  move  their  lips  slightly  as  they  articulate 
the  words  to  themselves,  thus  showing  that  in  writing  the 
arm-centre  is  acting  in  subordination  to  the  mouth-speech 
centre  in  the  third  left  frontal  convolution. 

Fig.  305  represents  simply  the  relationships  of  the  various 
centres  on  the  left  side  to  one  another  in  speaking  and 
writing. 

The  most  striking  change,  as  we  ascend  the  scale  of  in- 
telligence in  animals,  in  the  structure  of  the  brain  is  the 
great  development  that  occurs  of  brain-tissue,  giving  no 
response  on  stimulation  and  presenting  no  immediate  connec- 
tion either  with  motion  or  sensation.  Such  tissue  is  found  in 
the  pre-frontal  and  the  parietal  lobes,  and  in  man  forms  the 
greater  part  of  the  cerebral  cortex.  The  fibres  of  these  lobes 
are  found  to  acquire  their  myelin  sheath  a  considerable  time 
after  birth,  and  continuous  development  may  go  on  as  late  as 
the  twentieth  year.  We  are  probably  justified  in  ascribing 
to  these  association-centres  the  chief  part  in  the  functions 
of  ideation  and  judgment.  The  ideas,  which  make  up  the 
greater  part  of  our  mental  life  and  which  determine  our 
actions,  cannot  be  regarded  as  essentially  motor  or  sensory, 
though  in  them  motor  and  sensory  processes  of  the  present 
are  inextricably  intermingled  with  the  results  of  past  sensory 
and  motor  impressions  ;  and  there  is  a  constant  war  of  mutual 
excitations  and  inhibitions,  the  resultant  of  which  is  the 
behaviour  of  the  individual. 


666  PHYSIOLOGY 


Section  5 

THE   VASCULAK    AND    LYMPHATIC    AEEANGEMENTS 
OP  THE   CENTEAL   NEEVOUS   SYSTEM 

The  brain  and  spinal  cord  are  enclosed  in  three  mem- 
branes, the  dura  mater,  the  arachnoid  membrane,  and  the 
pia  mater. 

The  dura  mater  is  a  tough  tibrous  membrane,  closely 
adherent  in  the  skull  to  the  cranial  bones,  of  which  it  forms 
the  periosteum,  but  in  the  spmal  canal  only  loosely  connected 
with  the  vertebrae. 

The  pia  mater  is  a  finely  fibrillated  connective  tissue, 
closely  adherent  to  the  surface  of  both  brain  and  spinal 
cord,  and  sending  prolongations  into  all  the  fissures  of  these 
organs.  It  serves  to  convey  the  nutrient  vessels  to  the 
nervous  tissue. 

The  space  between  dura  mater  and  pia  mater  is  divided 
into  two  by  a  delicate  membrane,  the  arachnoid.  Both 
arachnoid  and  dura  mater  send  processes  along  the  issuing 
nerve-roots.  The  subdural  and  subarachnoid  spaces  are 
both  lined  with  endothelial  cells.  Although  anatomically 
separate,  fluid  can  find  its  way  with  extreme  ease  from  one 
space  to  the  other,  so  that  physiologically  they  may  be  re- 
garded as  if  in  communication.  The  subarachnoid  space  is 
put  into  connection  with  the  ventricles  of  the  brain  and 
central  canal  of  the  cord  by  means  of  a  small  opening  m  the 
ependyma  forming  the  roof  of  the  fourth  ventricle,  which  is 
known  as  the  foramen  of  Magendie. 

On  incising  the  dura  mater  through  the  atlanto-occipital 
membrane,  or  in  the  spinal  canal,  a  clear  fluid  wells  up  at 
the  bottom  of  the  wound,  and  a  continuous  flow  of  this  fluid 
may  with  certam  precautions  be  obtained  by  inserting  a 
cannula  into  the  subdural  space  in  either  of  these  situations. 
This  liquid  is  the  cerehro- spinal  fluid.  It  consists  of  little 
more  than  a  solution  of  the  salts  of  the  blood -plasma,  and 
contains  only  the  merest  trace  of  proteid.  This  fluid  appears 
to  be  formed  in  the  ventricles  of  the  brain  by  a  process 
of  secretion  or  transudation,  modified   by   the   cells   of   the 


THE   BRAIN  667 

eiiendyma  covering  the  vascular  fringes,  which  project  into" 
the  ventricles  from  the  velum  interpositum  and  are  known  as 
the  choroid  plexuses.  A  slow  formation  of  fluid  is  probably 
always  going  on.  Any  excess  of  fluid  that  is  formed  may 
either  escape  from  the  sheath  of  the  dura  mater  along  the 
issuing  nerve-roots  or,  when  the  pressure  of  the  fluid  is 
higher  than  that  in  the  venous  sinuses,  it  may  leave  the 
cranial  cavity  by  a  direct  filtration  through  the  walls  of 
these  sinuses. 

The  lymphatic  arrangements  of  the  brain  are  still  imper- 
fectly understood.  In  the  cortex  cerebri  the  lymphatics  are 
perivascular,  i.e.  form  complete  tubes  round  the  blood  capil- 
laries. These  perivascular  lymphatics  are  said  to  be  in  com- 
munication with  the  subarachnoid  cavity.  If  this  is  the 
case,  the  lymph  must  mix  with  the  cerebro-spinal  fluid  ;  and 
it  would  seem  difficult  to  account  for  the  small  percentage 
of  proteid  in  this  fluid — a  percentage  which  seems  quite 
inadequate  for  the  nutritional  needs  of  the  nervous  tissues. 

The  Intracranial  Circulation 

The  brain  is  supplied  with  blood  from  the  two  vertebral 
and  two  internal  carotid  arteries,  which  anastomose  at  the 
base  of  the  brain  to  form  the  circle  of  Willis.  On  account 
of  this  free  anastomosis,  one  or  two  of  these  vessels  can  be 
obliterated  without  any  effect  on  the  circulation  ;  and  in  the 
dog,  all  four  arteries  can  be  ligatured  without  producing 
more  than  a  temporary  lowering  of  the  functions  of  the 
cortex  cerebri.  The  cortex  is  supplied  by  the  posterior 
cerebral  from  the  basilar  artery,  and  the  middle  and  anterior 
cerebral  arteries  from  the  internal  carotid.  Although  the  area 
of  distribution  of  these  vessels  does  not  coincide  with  the 
functional  areas  of  the  cortex,  it  is  important  to  remember 
that  blocking  of  the  posterior  cerebral  artery  will  shut  off  the 
blood  from  the  occipital  lobe,  so  causing  softening  of  this 
lobe  and  hemianopia  ;  while  blocking  of  the  middle  cerebral 
artery  will  affect  the  greater  part  of  the  Rolandic  area, 
with  widespread  paralysis  on  the  opposite  side  of  the  body, 
and  probably  some  impairment  of  sensation,  as  its  results. 
If  the  lesion  be  on  the  left  side,  complete  aphasia  will  be 
produced.     The    symptoms    of   obstruction    of    the   anterior 


668  PHYSIOLOGY 

cerebral  artery  will  be  less  definite,  since  this  vessel  supplies 
only  a  small  portion  of  the  Kolandic  area,  together  with  the 
frontal  lobes. 

The  optic  thalamus  and  corpus  striatum  with  the  adjacent 
parts  are  supplied  by  small  vessels  which  come  off  directly 
from  the  circle  of  Willis. 

The  veins  of  the  brain  pour  their  contents  into  a  number 
of  venous  sinuses  in  the  substance  of  the  dura  mater.  The 
superior  longitudinal  sinus  passes  from  before  backwards 
along  the  vertex  of  the  cranium  at  the  top  of  the  falx  cerebri 
to  the  occiput,  where  it  joins  the  straight  sinus  in  the  blood- 
cavity  known  as  the  torcular  Herophili.  The  straight  sinus 
carries  the  blood  from  the  deeper  parts  of  the  brain,  which 
have  arrived  at  it  by  the  veins  of  Galen  along  the  velum 
interpositum.  From  the  torcular  the  blood  is  carried  by  the 
lateral  sinus  on  each  side  to  the  internal  jugular  veins. 

The  circulation  in  the  brain  differs  from  that  in  all  other 
parts  of  the  body,  in  that  the  cranial  cavity  forms  a  closed 
cavity  with  rigid  walls.  A  small  amount  of  space  may  be 
obtained  in  this  cavity  by  the  escape  of  cerebro- spinal  fluid, 
but  beyond  this  change,  which  can  amount  only  to  one  or  two 
c.c,  we  must  say  that  the  cubic  contents  of  the  cavity  are 
invariable.  Hence,  the  brain-substance  being  incompressible, 
dilatation  of  any  one  set  of  vessels  can  be  obtained  only  at 
the  expense  of  the  constriction  of  another  set  of  vessels. 
Thus  a  rise  of  arterial  blood-pressure  will  cause  a  constriction 
of  the  veins  until  the  pressure  in  these  vessels  rises  to  the 
pressure  of  the  brain  against  them.  Thus,  as  Leonard  Hill 
has  pointed  out,  with  rise  of  arterial  pressure  the  circulatory 
system  of  the  brain  tends  to  assimilate  itself  to  a  scheme  of 
rigid  tubes,  and  the  whole  energy  of  the  arterial  pressure  is 
spent  in  maintaining  an  increased  velocity  of  blood-flow. 

The  intracranial  pressure  is  the  same  as  the  cerebral 
capillary  and  venous  pressures.  In  the  dog  in  the  horizontal 
position  it  amounts  to  about  100  mm.  H.^0,  but  it  may  vary 
according  to  the  position  and  circulatory  conditions  of  the 
animal  within  very  wide  limits  without  affecting  the  functions 
of  the  brain. 

Although  the  cerebral  vessels  are  supplied  with  nerve- 
fibres,  we  have  as  yet  no  definite  experimental  evidence  of 
vaso-motor   changes   in   these   vessels.     Since   the   vascular 


THE   BRAIN  669 

system  of  the  whole  body  is  under  the  control  of  a  portion 
of  the  brain,  viz.  the  vaso-motor  centre,  the  circulation 
through  the  brain  can  be  increased  or  diminished  according 
to  its  supreme  needs  by  alterations  of  the  circulation  in  other 
parts  of  the  body.  Thus  in  the  upright  position,  an  adequate 
pressure  is  maintained  in  the  cerebral  arteries  by  constriction 
of  vessels  in  the  splanchnic  area.  The  brain  in  fact  uses 
this  great  vascular  area  as  the  means  for  regulating  its  own 
blood-s  apply. 


670 


CHAPTER   XVI 
THE   VISCERAL   OR   AUTONOMIC    SYSTEM   OF    NERVES 

We  have  already  dealt  with  the  chief  functions  of  this  system 
of  nerves  in  connection  with  the  various  organs  of  the  body. 
It  remains  only  to  consider  some  general  points  affecting  the 
anatomy  and  functions  of  this  system  as  a  whole. 

The  visceral,  autonomic,  or  splanchnic  system  of  nerves 
includes  the  sympathetic  system  properly  so  called,  and  some 
of  the  cranial  and  sacral  nerves. 

The  sympathetic  system  (Fig.  306)  is  composed  of  a  chain 
of  ganglia  lying  each  side  of  the  vertebral  column,  there 
being  as  a  rule  one  ganglion  to  each  spinal  nerve-root.  In 
the  cervical  region  however  these  ganglia  are  condensed  into 
two,  the  superior  and  inferior  cervical  ganglia,  united  by  the 
cervical  sympathetic  trmik  ;  and  the  upper  three  or  four  tho- 
racic ganglia  on  each  side  are  condensed  to  form  the  so-called 
stellate  ganglion.  At  the  bottom  of  the  chain  there  is  only 
one  coccygeal  ganglion  for  the  coccygeal  vertebrae. 

In  the  abdomen  is  a  second  system  of  ganglia,  in  especial 
connection  with  the  abdominal  viscera,  lying  in  front  of  the 
aorta  and  surrounding  the  origins  of  the  large  arteries  to  the 
alimentary  canal.  These  are  the  semilunar  or  solar  ganglia, 
the  superior  mesenteric  and  the  inferior  mesenteric  ganglia. 

Finally  in  the  organs  themselves  we  find  a  third  system 
of  ganglion  cells  either  scattered  or  collected  to  form  small 
ganglia.  These  are  probably  connected  not  only  with  the 
sympathetic  but  also  with  the  other  splanchnic  nerves,  cranial 
and  sacral.  The  three  systems  of  ganglia  have  been  distin- 
guished as  the  lateral,  collateral,  and  terminal  ganglia. 

The  ganglia  of  the  sympathetic  chain  are  connected  with 
all  the  spinal  nerves,  just  after  they  have  given  off  their  pos- 
terior division  by  means  of  the  rami  communicantes.  These 
rami  communicantes  are  of  two  kinds,  white  rami  consisting 


Pig.  306. 


Sup.cerv 


Inf.  cerv 


Stellate  g.^ 


S^mp.  chain 

Semilunar  g.^ 
Sup.mes.g.^, 


C.I 
2 
3 
4 
5 
6 


>Arm  i 


3  >  Head  &  Neck 


>Arm 


12 
13 

L.I 
2 

"i 

4 


leg 


Abdominal 
Viscera 


>  Leg  <     5 


S.I 

2 
3 


Hypogastric  n     ^,^,y,,,_,_     ' 


Diagrammatic  representation  of  the  distribution  of  the  sympathetic  system. 
The  black  lines  represent  the  medullated  preganglionic  fibres,  such  as  those 
making  up  the  white  rami  communicantes,  while  the  post-ganglionic  fibres 
are  printed  in  red.  On  the  extreme  right  of  the  figure  is  indicated  the 
general  distribution  of  the  white  rami  arising  from  the  several  nerve-roots, 
while  the  double  brackets  point  to  the  nerve-roots  making  up  the  limb 
plexuses.     H,  heart;  S,  stomach;  1,  small  intestine  ;  C,  colon;  B,  bladder. 


THE   VISCEEAL   OE   AUTONOMIC   SYSTEM   OF   NEEVES     673 

of  small  medullated  fibres,  and  grey  rami  composed  almost 
exclusively  of  non-medullated  nerves. 

It  has  been  shown  by  Gaskell  that  the  white  rami  are 
formed  by  fibres  which  have  their  origin  in  the  spinal  cord 
and  perhaps  in  the  posterior  root  ganglia  ;  whereas  the  grey 
rami  represent  fibres  which,  arising  in  the  sympathetic  ganglia, 
run  back  to  join  the  spinal  nerves.  The  visceral  outflow 
represented  by  the  white  rami  is  limited  to  a  distinct  region 
of  the  cord,  viz.  from  the  first  thoracic  to  the  third  or  fourth 
lumbar  nerve-roots  ;  whereas  the  grey  rami  pass  from  the 
sympathetic  to  all  the  spinal  nerve-roots.  It  is  found  by 
experiment  however  that  stimulation  of  a  limited  number  of 
white  rami  produces  all  the  effects  that  can  be  evoked  by 
stimulation  of  the  grey  rami,  showing  that  the  impulses 
leaving  the  cord  pass  upwards  and  downwards  in  the  sym- 
pathetic system  and  are  broken  somewhere  in  their  course, 
being  transferred  to  a  fresh  system  which,  by  means  of  non- 
medullated  nerves,  carries  them  on  to  their  destination. 

The  relationships  of  the  white  and  grey  rami  are  strikingly 
illustrated  in  the  case  of  the  pilomotor  system  of  nerves. 
These  in  the  cat  arise  from  the  cord  by  the  anterior  roots 
from  the  fourth  thoracic  to  the  third  lumbar  inclusive. 
Passing  by  the  white  rami  to  the  sympathetic  system,  they 
travel  upwards  and  downwards  and  end  by  arborisations  in 
the  various  ganglia  of  the  main  chain.  From  the  cells  of  each 
ganglion  a  fresh  relay  of  fibres  starts,  which  runs  as  a  bundle 
of  non-medullated  nerves  (the  grey  ramus)  to  the  correspond- 
ing spinal  nerve,  with  which  it  is  distributed  to  its  peripheral 
destination.  The  position  of  the  cells  in  the  nerve-path  can 
be  easily  ascertained  by  the  nicotine  method  described  on 
p.  265.  Each  grey  ramus  causes  erection  of  the  hairs  above 
one  vertebra,  whereas  stimulation  of  one  white  ramus  causes 
erection  over  three  or  four  vertebrae,  showing  a  distribution 
of  the  fibres  of  the  white  ramus  to  the  cells  in  several  succes- 
sive ganglia. 

We  may  summarise  here  the  distribution  of  these  pilo- 
motor fibres  in  the  cat. 

1.  For  the  head  and  upper  part  of  the  neck  the  fibres 
arise  by  the  fourth  to  the  seventh  thoracic  anterior  roots, 
and  have  their  cell-stations  in  the  superior  cervical  ganglion. 
They  travel  as  small  medullated  nerve-fibres  from  the  white 

43 


674  PHYSIOLOGY 

rami  up  the  sympathetic  chain,  through  the  stellate  ganglion 
and  ansa  Vieussenii  and  up  the  cervical  sympathetic. 

2.  The  next  set  of  nerve-fibres  have  their  cell-station  in 
the  stellate  ganglion.  The  white  rami  arise  from  the  fifth  to 
the  eighth  thoracic  nerves,  while  the  grey  rami  pass  to  the 
nerve-roots  from  the  third  cervical  nerve  to  the  fourth  thoracic 
nerve. 

3.  The  remaining  nerves  supplying  all  the  rest  of  the 
body  and  tail  arise  by  the  white  rami  from  the  seventh 
thoracic  to  the  third  or  fourth  lumbar  nerve,  and  are  distri- 
buted as  grey  rami  to  all  the  spinal  nerves  below  the  fourth 
thoracic. 

We  thus  see  that,  in  speaking  of  the  functions  of  a  spinal 
nerve-root,  we  must  clearly  distinguish  whether  we  mean  the 
root  as  it  rises  from  the  spinal  cord,  in  which  case  its  visceral 
functions  will  include  those  of  its  white  ramus,  or  whether 
we  mean  the  made-up  or  complete  spinal  nerve  after  it  has 
received  its  grey  ramus  (Fig.  307).  In  this  latter  case  the 
visceral  functions  of  the  root  will  be  more  restricted  than  m 
the  former  case,  and  will  have  a  different  distribution.  In 
stimulating  the  nerve-roots  in  the  spinal  canal  it  is  some- 
times possible,  by  weak  stimuli,  to  display  the  functions  of 
the  corresponding  white  ramus,  and  then  by  increasing  the 
stimulus  to  get  superadded  the  efl'ects  due  to  excitation  of 
the  grey  ramus  in  the  made-up  nerve,  in  consequence  of  the 
spread  of  current. 

'  When  for  example  the  eleventh  thoracic  anterior  roots  are  stimulated  in 
the  spinal  canal  with  weak  shocks,  a  fairly  long  strip  of  hairs  in  the  lumbar 
region  will  be  erected,  the  maximum  movement  of  the  hairs  being  near  the 
middle  of  the  strip.  This  marks  the  area  of  distribution  of  the  pilomotor 
nerves  given  by  the  eleventh  thoracic  nerve  to  the  sympathetic.  If  then  the 
strength  of  the  shocks  be  increased  to  a  certain  point,  the  hairs  in  the  long 
strip  will  of  course  be  erected  as  before,  but  in  addition  there  will  be  energetic 
erection  of  hairs  in  a  short  strip  a  little  distance  above  the  long  strip,  and 
separated  from  it  by  a  quiescent  region.  This  short  strip  is  the  same  as  that 
affected  by  stimulating  the  grey  ramus  or  the  dorsal  cutaneous  branch  of  the 
eleventh  thoracic  nerve.  It  marks  the  area  of  distribution  of  the  pilomotor 
fibres  received  by  the  spinal  nerve  from  the  sympathetic '  (Langley). 

We  may  now  indicate  briefly  the  mam  course  and  functions 
of  the  fibres  of  the  sympathetic  system  (Fig.  306) : 

1.  The  head  and  neck  are  supplied  by  fibres  leaving  the 
spinal  cord  by  the  first  five    dorsal   nerves  (chiefly  by  the 


Fig.  307. 


Posc'root-. 
Ant"  root 


4-  —  Pre-ganglionic  fibre 


: — Symp.  gangl. 


Made-up"  spinal  nerve 


^  Post-ganglionic  fibre 


Diagram  (after  Langley)  to  show  the  manner  in  which  a  spinal  nerve  is 
completed  by  the  entry  of  a  grey  ramus,  containing  fibres  derived  from 
cells  in  the  sympathetic  chain,  p.pr.d,  posterior  primary  division. 
(The  post-ganglionic  fibres  are  represented  red.) 


THE   VISCERAL   OR   AUTONOMIC   SYSTEM   OF   NERVES     677 

second  and  third).     They  all  have  their  cell-station  in  the 
superior  cervical  ganglion.     They  convey  : 

Vaso-constrictor  impulses  to  the  blood-vessels. 

Dilator  fibres  to  the  pupil. 

Secretory    (trophic?)   fibres  to  the  salivary  glands   and 

sweat  glands. 
Vaso-dilator  fibres  to  the  lower  lip  (in  the  dog). 

2.  The  thoracic  viscera  (heart  and  lungs)  are  supplied 
by  the  same  five  nerve-roots.  The  cell-station  of  these 
fibres  is  however  situated  in  the  stellate  ganglion.  They 
convey  : 

Accelerator  or  augmentor  impulses  to  the  heart. 
■  Vaso-constrictor     impulses    to    the     pulmonary    blood- 
vessels (?). 

3.  The  abdominal  viscera  receive  fibres  from  the  lower 
six  dorsal  nerves  and  the  upper  three  or  four  lumbar.  Most 
of  these  fibres  run  through  the  sympathetic  chain  without 
making  any  connection  with  the  ganglia,  and  have  their  cell- 
stations  m  the  collateral  ganglia  of  the  solar  plexus,  the 
semilunar  and  superior  mesenteric  ganglia.  On  their  way 
to  these  ganglia  they  form  the  greater  and  lesser  splanchnic 
nerves.     Their  functions  are  : 

Vaso-constrictor  for  stomach  and  small  intestine,  kidney, 

and  spleen. 
Probably  vaso-dilator  for  the  same  viscera. 
Inhibitory  for  both  muscular  coats  of  small  intestine. 
Motor  for  ileo-colic  sphincter. 
Secretory  for  pancreas  (?). 

4.  The  pelvic  viscera  are  supplied  by  the  lower  dorsal 
and  upper  three  or  four  lumbar  nerve-roots.  These  fibres 
also  pass  by  the  main  chain  to  make  connections  with  the 
cells  chiefly  in  the  mferior  mesenteric  ganglia.  They 
convey : 

Vaso-constrictor  impulses  to  pelvic  viscera. 

Inhibitory  fibres  to  colon  (both  coats). 

Motor  and  possibly  also  inhibitory  fibres  to  bladder. 

Motor  fibres  to  retractor  penis. 

Motor  fibres  to  uterus  and  vagina. 


678  PHYSIOLOGY 

5.  The  fore-limb  receives  nerves  from  the  white  rami  of 
the  fom'th  to  the  tenth  thoracic  nerves.  All  these  fibres  are 
connected  with  cells  in  the  stellate  ganglion.     They  convey  : 

Vaso-constrictor  impulses  to  blood-vessels. 
Secretory  nerves  to  the  sweat-glands. 

6.  The  hind  limb  is  supplied  by  the  nerve-roots  from  the 
eleventh  thoracic  to  the  third  lumbar  inclusive.  The  cell- 
stations  of  these  fibres  are  situated  in  the  sixth  and  seventh 
lumbar  and  first  sacral  ganglia.     They  convey  : 

Vaso-constrictor  impulses. 
Secretory  nerves  to  the  sweat-glands. 

Every  fibre  of  the  sympathetic  system  is  thus  in  some 
point  of  its  course  interrupted  by  a  nerve-cell,  and  Langley 
has  shown  that  this  is  the  only  cell-break  m  the  fibre,  i.e. 
every  fibre  is  connected  with  one  cell  and  one  cell  only. 
This  law  applies  not  only  to  the  sympathetic  fibres,  but  also 
to  the  fibres  of  the  other  visceral  nerves.  Each  fibre  there- 
fore can  be  regarded  as  made  up  of  two  sections — a  pre- 
ganglionic fibre  arismg  in  the  central  nervous  system  and 
passing  down  to  a  ganglion  as  a  fine  medullated  nerve-fibre, 
and  a  post-ganglionic  fibre  arising  in  this  ganglion  and  con- 
tinued generally  as  a  non-medullated  fibre  to  its  peripheral 
distribution. 

The  efferent  nerves  of  the  sympathetic  system  arise  in  the 
cells  of  the  lateral  horn,  and  thus  are  homologous  with  the 
splanchnic  fibres  which  arise  in  the  medulla,  viz.  the  motor 
root  of  the  fifth,  the  facial  nerve,  and  the  motor  root  of  the 
vago-glossopharyngeal. 

According  to  Gaskell,  the  typical  segmental  nerve  would  have  four  roots : 
two  somatic,  the  motor  and  sensory  roots  distributed  to  the  skin  and  skeletal 
muscles ;  and  two  splanchnic  roots,  also  motor  and  sensory,  composed  of  small 
fibres  and  distributed  to  the  viscera  or  structures  which  are  visceral  in  origin, 
e.g.  developed  from  branchial  arches. 

In  the  spinal  cord  we  have  the  proved  separate  origin  of  the  motor  fibres 
from  the  anterior  horn  and  the  visceral  efferent  fibres  from  the  lateral  horn  ; 
and  a  similar  distinction  may  be  made  in  the  medulla  between  such  somatic 
roots  as  the  third,  fourth,  sixth,  and  twelfth  nerves,  and  the  splanchnic  roots 
represented  by  the  motor  root  of  the  fifth,  the  facial  nerve,  and  the  motor  root 
of  the  vago-glossopharyngeal. 

In  the  medulla  a  distinction  can  be  drawn  between  the  central  connections 
of  the  sijlanchnic  afferent  and  the  somatic  afferent  nerves.  Examples  of  the 
former  are  the  vago-glossopharyngeal  nucleus  and  the  sensory  nucleus  of  the 


THE   VISCERAL   OE  AUTONOMIC    SYSTEM   OF   NERVES      679 

fifth,  and  of  the  latter  the  long  descending  root  of  the  fifth  in  connection  with 
the  substance  of  Rolando.  It  is  not  known  however  whether  a  similar  distinc- 
tion can  be  drawn  in  the  cord  between  the  two  sets  of  fibres. 

Such  of  the  visceral  nerves  as  do  not  belong  to  the  sympa- 
thetic system  arise  chiefly  in  the  medulla  and  in  the  sacral 
region  of  the  cord.  Like  those  of  sympathetic  origin  they 
all  conform  to  Langley's  law  as  to  the  possession  of  one  and 
only  one  cell-station  in  their  peripheral  course.  In  most 
cases  these  cells  lie  near  the  periphery,  and  belong  therefore 
to  the  terminal  set  of  ganglia.  The  position  of  their  nerve- 
cells  is  as  follows : 

Third  nerve. — The  branches  to  the  iris  and  ciliary  muscle 
are  connected  with  cells  in  the  ciliary  ganglion. 

Chorda  tymjjcmi. — The  vaso-dilator  and  secretory  fibres 
of  this  nerve  are  connected  with  cells  lying  near  the  glands ; 
those  to  the  sublingual  gland  with  cells  in  the  sublingual 
('  submaxillary ')  ganglion ;  and  those  to  the  submaxillary 
gland  with  scattered  cells  lying  in  the  hilum  of  the  gland. 

Vagus. — The  inhibitory  nerves  to  the  heart  are  connected 
with  ganglia  in  this  organ  itself.  The  motor  fibres  to  the 
oesophagus  and  stomach  have  their  cell-stations  in  the 
ganglion  trunci  vagi  (in  the  alligator). 

Pelvic  visceral  nerve. — This  nerve  is  connected  with  a 
collection  of  ganglia  lying  in  the  hypogastric  plexus  at  the 
base  of  the  bladder.     It  has  the  following  functions : 

Dilator  to  vessels  of  the  penis  (hence  its  name  of  nervus 
erigens) . 

Dilator  to  vessels  of  pelvic  viscera. 
Motor  to  bladder,  colon,  and  rectum. 
Inhibitory  to  retractor  penis. 

It  will  be  observed  that  in  many  cases  the  viscera  get 
their  nerve-supply  from  both  sets  of  visceral  nerves,  and 
that  in  such  cases  the  two  sets  of  nerves  are  antagonistic  in 
function.  It  is  impossible  however  to  draw  a  sharp  line 
between  the  functions  of  the  two  sets,  since  the  same  nerve 
may  be  motor  for  one  set  of  muscular  fibres  and  inhibitory 
for  another  set  in  the  same  viscus.  Thus  the  colonic  branches 
of  the  inferior  mesenteric  ganglion  are  motor  (constrictor) 
for  the  blood-vessels  and  inhibitory  for  the  muscular  walls  of 
the  colon. 


680  PHYSIOLOGY 


Functions  of  the  Sympathetic  and  Periphet'al  Ganglia 

These  ganglia  consist  of  a  mass  of  nerve-cells  embedded 
in  connective  tissue,  each  cell  being  surrounded  by  a  special 
capsule  of  endothelial  cells.  The  nerve-cells,  though  in 
section  resembling  those  in  a  posterior  root  ganglion,  differ 
from  these  in  being  multipolar,  each  cell  probably  possessing 
one  axon  and  several  dendrites.  The  dendrites  end  in  little 
arborisations  round  adjacent  cells. 

Since  the  main  nervous  system  is  characterised  by  the 
possession  of  nerve-cells,  it  was  formerly  thought  that  any 
collection  of  nerve-cells  must  partake  of  the  co-ordinating 
and  reflex  functions  of  the  central  nervous  system,  i.e.  must 
act  as  local  nervous  centres.  All  efforts  have  failed  how- 
ever to  prove  the  existence  of  such  a  function,  and  we  must 
conclude  that  the  sole  use  of  these  ganglia  is  to  serve  as  dis- 
tributing centres.  We  may  assume  that  one  preganglionic 
fibre  divides,  and  the  branches  arborise  round  several  cells 
(Fig.  308),  whence  new  fibres  arise  to  carry  the  impulse 
to  the  periphery — an  impulse  in  the  case  of  which  there  is 
no  need  for  any  minute  localisation.  Indeed  the  essential 
part  of  a  nerve-centre  is  not  the  nerve- cells  at  all,  but  the 
presence  of  a  complex  tangle  of  fibres,  rendering  possible 
the  passage  of  impulses  in  all  directions,  the  passage  of  an 
individual  impulse  however  being  limited  by  reason  of  the 
varying  strength  of  the  impulse,  and  the  varying  resistance 
of  the  many  possible  tracts.  In  many  invertebrata  the 
nervous  system  consists  of  a  punctated  material  composed  of 
a  dense  interlacement  of  fibrils,  while  the  cells  lie  outside 
the  centres,  and  have  one  thick  process  dipping  into  the 
nervous  mass,  from  which  process  both  axon  and  dendrites 
arise.  In  this  case  it  has  been  found  that  extirpation  of  the 
cell-bodies  does  not  destroy  the  capacity  of  the  remaining 
fibrillated  substance  to  act  as  a  reflex  centre. 

Such  a  complex  of  fibres  is  found  in  mammals  in  the 
plexuses  of  Auerbach  and  Meissner  which,  as  we  have  seen, 
act  as  local  nerve-centres  for  the  intestine.  But  all  such 
mechanism  is  wanting  in  the  sympathetic  ganglia,  which 
contain  neither  association  fibres  between  diflerent  cells  of  a 
ganglion  nor  commissural  fibres  between  the  cells  of  adjacent 


Fig.  308. 


-Spinal  cord 


:^Y\') Sympathetic  chain 


Solar  ganglion 


A. 


B. 


Figure  (after  Langley)  to  show  the  probable  mode  of  connection 
of  the  fibres  of  the  splanchnic  nerve  with  nerve-cells.  A,  usual 
type,  all  the  fibres  passing  through  the  lateral  chain  to  end  in 
the  collateral  ganglia  of  the  solar  plexus ;  B,  alternative  con- 
dition, in  which  a  small  minority  of  the  fibres  have  their  cell- 
stations  in  the  sympathetic  chain.  The  preganglionic  fibres 
are  black,  the  post-ganglionic  red. 


THE   YISCEEAL   OE   AUTONOMIC   SYSTEM   OF   NERVES     683 

ganglia.  All  the  fibres  in  a  sympathetic  ganglion  have  either 
entered  it  from  the  white  rami  or  are  destined  to  leave  it  as 
fibres  of  grey  rami. 

Several  reflexes  formerly  described  in  peripheral  ganglia, 
as  e.g.  the  '  submaxillary '  ganglion,  have  been  proved  to  be 
fallacious.  There  is  however  a  certain  group  of  phenomena 
which  can  be  elicited  in  sympathetic  ganglia,  and  which 
have  been  termed  by  Langley  and  Anderson  pseudo-reflexes, 
or  better,  axon  rejiexes.  If  for  mstance  we  divide  all  the 
nerves  going  to  the  inferior  mesenteric  ganglion,  leaving  the 


Fki.  309. 


Sp  cord 


Inf.  mes-q.--i/^y-:     ^ 
Post -ganglionic  fibre - 


Pre -ganglionic  fibre 
Hypogastric  nerves 


Diagram  to  illustrate  Langley  aud  Anderson's  explanation  of 
the  lijiDogastric  reflex  as  an  axon  reflex.  The  division  of  the 
axon  where  the  propagation  or  '  reflexion '  takes  place  is  at  X. 


bladder  connected  with  the  inferior  mesenteric  ganglion  only 
by  the  hypogastric  nerves,  and  then  after  dividing  the  left 
nerve  stimulate  its  central  end,  we  obtain  a  contraction  of 
the  right  half  of  the  bladder.  This  effect  is  abolished  by 
painting  the  inferior  mesenteric  ganglion  with  nicotine,  show- 
ing that  the  activity  of  the  cells  of  this  ganglion  is  involved 
in  the  process.  It  has  been  shown  however  by  Langley  and 
Anderson  that  this  is  not  a  true  reflex,  but  is  rather  analogous 
to  Kiihne's  gracilis  experiment  (cf.  p.  163).  A  preganglionic 
fibre  arriving  at  the  inferior  mesenteric  ganglion  branches, 


684  PHYSIOLOaY 

one  branch  ending  round  the  cells  of  the  ganglion  while  the 
other  branch  passes  down  in  the  left  hypogastric  nerve  to  a 
cell  situated  near  the  base  of  the  bladder  (Fig.  309).  When 
therefore  we  stimulate  this  nerve  we  are  stimulating  a  pre- 
ganglionic fibre,  and  the  excitation  spreads  up  to  the  point 
of  junction  of  the  two  branches  and  then  down  the  other 
branch  to  excite  the  cell  in  the  inferior  mesenteric  ganglion. 
We  thus  obtain  an  apparent  motor  reflex  by  stimulation  of 
a  nerve  which  is  itself  motor. 

Similar  pseudo-reflexes  can  be  obtained  along  the  abdo- 
minal chain  on  the  pilomotor  nerves,  but  furnish  no  grounds 
for  ascribing  the  property  of  reflex  centres  to  peripheral 
ganglia. 


685 


CHAPTEE    XVII 
REPRODUCTION 

Theoughout  the  animal  kingdom  we  find  the  welfare  of 
the  individual  subordinated  to  that  of  the  species.  The 
crowning  act  of  an  animal's  life  is  the  production  of  a  new 
individual,  fitted  in  all  respects  to  take  the  place  of  the 
parent  organism,  and  so  to  maintain  the  race  on  the  earth. 
In  the  case  of  the  lowliest  unicellular  organisms,  which 
reproduce  themselves  only  by  fission,  we  cannot  rightly 
speak  of  death  from  natural  causes.  One  amoeba  divides 
into  two  new  individuals  similar  to  it  in  every  way,  and 
these  in  their  turn  divide  again.  Hence  the  amoebae  have 
with  some  right  been  spoken  of  as  immortal.  It  is  evident 
that  any  accommodation  of  the  organism  to  its  environment 
must,  since  it  aflects  the  whole  cell,  be  transmitted  in  equal 
degree  to  the  cells  that  are  the  offspring  of  the  division  of 
the  parent.  And  so  we  may,  in  course  of  time,  get  a  gradual 
change  of  type  in  the  organism. 

As  we  go  higher  in  the  scale  we  meet  with  more  highly 
differentiated  organisms,  consisting  of  cell  colonies,  each 
member  of  which  has  its  own  appointed  task  to  fulfil ;  and 
here  we  find  that  the  office  of  reproduction  also  is  confined 
to  one  cell  or  group  of  cells.  The  immortality  of  the  amoeba 
has  been  transmitted  to  this  group  of  cells.  From  this  point 
onwards,  in  the  scale  of  animal  life,  we  may  regard  the 
reproductive  cells  or  germ-plasma  as  being  continuous  through 
successive  generations.  With  the  production  of  each  new 
generation  the  germ-plasma  divides  into  two  parts :  one 
part,  the  somatic  half,  forming  what  is  generally  miderstood 
as  the  individual,  and  being  differentiated  into  various  forms 
of  cells  to  perform  the  multifarious  functions  of  reaction 
associated  with  life ;  and  the  other  half,  persistent  in  its 
primitive  form  as    the  reproductive  part  of  the  individual, 


686  PHYSIOLOGY 

ready  when  the  time  comes  to  divide  again  and  give  birth 
to  a  new  generation. 

There  are  however  many  unicellular  organisms  in  which 
the  processes  of  reproduction  are  not  quite  so  simple. 
These  are  able  to  multiply  for  a  few  generations  by  simple 
fission.  At  the  end  of  this  time,  for  the  production  of 
a  new  generation  or  series  of  generations,  the  conjoint  action 
of  two  cells  is  required.  In  this  process  of  conjugation  two 
unicellular  organisms  come  together  and  unite,  their  nuclei 
fusing  to  form  one  nucleus ;  the  single  cell  thus  made  is 
capable  of  producing  by  fission  several  more  generations. 
Here  the  cells  that  fuse  are  exactly  alike  in  all  respects. 
A  little  higher  in  the  scale  however,  among  the  multicellular 
organisms,  we  find  that  the  cells,  which  conjugate  to  form 
a  new  cell  capable  of  developing  into  an  individual,  present 
somewhat  different  characters.  The  one  cell,  which  has 
generally  a  certain  amount  of  stored-up  reserve  material  in 
its  protoplasm,  is  the  female  element,  and  is  called  the  ovum. 
The  other  cell,  which  is  chiefly  limited  to  the  nuclear  sub- 
stance, is  called  the  spermatozoon,  and  is  the  male  element. 
This  is  the  sexual  mode  of  reproduction,  which  obtains  in 
all  the  higher  animals. 

The  conjugation  of  these  two  cells  is  not  the  union  of 
the  whole  of  two  ordinary  cells  of  two  individuals.  We 
find  that  both  ovum  and  spermatozoon,  before  their  union, 
undergo  certain  important  changes,  which  have  been  more 
fully  studied  in  the  case  of  the  former.  The  nucleus  of  the 
ovum  just  before  fertilisation  divides  into  two  parts ;  one 
half  is  extruded  with  a  small  amount  of  the  protoplasm,  and 
the  other  remains  in  the  main  body  of  the  ovum.  The 
nucleus  then  undergoes  division  a  second  time,  and  again 
one  half  is  extruded  with  a  little  protoplasm.  The  two 
extruded  cells  are  spoken  of  as  polar  bodies,  and  do  not 
undergo  any  further  modification.  The  part  of  the  nucleus 
left  in  the  ovum  is  the  female  pronucleus.  A  similar  change 
takes  place  in  the  male  element,  and  the  nuclei  of  the 
spermatozoa  are  equivalent  to  female  pronuclei.  These  male 
and  female  pronuclei  have  the  power  of  uniting  together  to 
form  a  whole  nucleus,  which  is  then  capable  of  undergoing 
a  long  series  of  divisions  to  form  a  new  individual.  The 
union  is  effected  by  the   penetration    of   the    spermatozoon. 


REPRODUCTION  687 

which  is  in  the  higher  animals  mobile,  into  the  ovum.  Here 
for  a  while  two  nuclei  are  seen,  the  male  and  female  pro- 
nuclei. They  then  fuse  together,  and  the  fertilised  ovum  is 
now  potentially  a  new  individual,  partaking  of  the  charac- 
teristics of  both  its  parents. 

Sexual  life  of  man. — -The  period  of  active  sexual  life, 
during  which  the  mdividual  is  capable  of  begetting  or  bear- 
ing children,  begins  in  both  sexes  at  the  age  of  fourteen  to 
sixteen,  known  as  the  age  of  puberty.  In  women,  the 
beginning  of  this  period  is  marked  by  the  onset  of  menstrua- 
tion. This  is  the  occurrence  of  a  flow  of  mucus  and  blood, 
which  arises  in  the  uterus,  from  the  genital  organs  ;  it  lasts 
from  three  to  five  days,  and  recurs  regularly  every  four 
weeks.  Menstruation  is  associated  with  ovulation,  which 
consists  in  the  discharge  of  an  ovum  from  the  ovary.  This 
latter  contains  follicles,  each  enclosing  an  ovum,  which  are 
known  as  the  Graafian  follicles.  They  are  lined  with  a  layer 
of  cells — the  membrana  granulosa — which  surround  the  ovum. 
In  the  course  of  development  these  cells  proliferate  and 
divide  into  two  layers,  each  several  cells  thick,  one  of  which 
lines  the  follicle,  and  the  other — discus  proliger us — encloses 
the  ovum.  The  space  between  them  is  filled  with  colourless 
fluid,  which  contains  proteids.  This  fluid  gradually  mcreases 
in  amount  until  it  forms  a  large  projection  on  the  surface  of 
the  ovary.  At  or  just  before  each  menstrual  period  a  ripe 
Graafian  follicle  ruptures,  and  the  ovum  is  discharged  into 
the  fimbriated  extremity  of  the  Fallopian  tube,  down  which 
it  is  conducted  to  the  uterus.  This  is  accompanied  with 
congestion  of  the  genital  organs,  especially  of  the  uterus,  in 
consequence  of  which  some  of  the  smaller  vessels  of  the 
uterine  mucous  membrane  rupture,  and  give  rise  to  the  dis- 
charge of  bloody  fluid.  The  discharge  of  blood  is  accom- 
panied with  the  fatty  degeneration  and  disappearance  of  the 
most  superficial  parts  of  the  mucous  membrane  itself. 

When  the  Graafian  follicle  ruptures,  haemorrhage  takes 
place  into  its  interior.  This  is  followed  by  a  rapid  pro- 
liferation of  the  cells  of  the  membrana  granulosa,  which 
grows  in  folds  into  the  cavity,  absorbing  the  blood- clot,  and 
transforming  the  haemoglobm  into  a  yellow  pigment.  Hence 
for  some  weeks  after  discharge  of  an  ovum  its  Graafian 
follicle  may  be  recognised  as  a  yellow  spot  which  is  known  as 


688  PHYSIOLOGY 

the  corpus  luteurn.  This  is  often  spoken  of  as  the  spurious 
corpus  hiteum,  to  distinguish  it  from  the  corpus  luteum  of 
pregnancy.  If  pregnancy  does  not  follow  the  discharge  of 
the  ovum,  the  corpus  luteum  disappears  in  from  one  to  two 
months.  If  however  pregnancy  occurs,  the  corpus  luteum 
becomes  very  large,  forming  a  prominent  projection  on  the 
surface  of  the  ovary,  and  is  to  be  seen  almost  to  the  end  of 
pregnancy.  Menstruation  ceases  during  pregnancy,  and  is 
also  generally  absent  during  lactation.  It  ceases  altogether 
between  the  ages  of  forty-five  and  fifty.  After  this  time, 
which  is  known  as  the  climacteric,  the  woman  is  no  longer 
capable  of  bearing  children. 

Impregnation. — In  animals  which  have  a  rutting  season, 
ovulation  is  also  accompanied  by  a  flow  of  blood  from  the 
genital  organs,  and  it  is  immediately  after  this  period,  which 
corresponds  to  the  menstrual  period,  that  impregnation  is 
effected.  In  the  human  species  impregnation  may  be  effected 
at  any  time,  and  the  union  of  spermatozoa  with  the  ova  may 
occur  in  the  uterus.  Fallopian  tubes,  or  even  in  abnormal 
cases  on  the  surface  of  the  ovary. 

If  the  ovum  be  not  fertilised,  it  is  cast  out  with  the  blood 
and  products  of  disintegration  of  the  uterine  mucous  mem- 
brane at  each  menstrual  period.  If  however  it  be  fertilised 
while  in  the  Fallopian  tube,  a  considerable  thickening  of  the 
uterine  mucous  membrane  takes  place  from  proliferation  of 
its  cells,  and  it  at  the  same  time  becomes  very  vascular. 
When  the  ovum  reaches  the  uterus  it  becomes  embedded  in 
the  mucous  membrane  covering  the  fundus  of  the  uterus, 
which  grows  round  and  completely  encloses  it.  This 
thickened  mucous  membrane,  which  is  called  the  decidua, 
becomes  fused  with  the  outer  layer  of  the  ovum,  and  the 
latter,  by  means  of  its  blood-vessels,  derives  its  nourishment 
from  the  uterine  mucous  membrane. 

At  about  the  eighth  week  after  impregnation  the  forma- 
tion of  the  j9Zace/?ifa.  takes  place  in  the  following  manner: — 
A  process  of  the  internal  hypoblastic  layer  of  the  embryo 
grows  out,  carrying  with  it  fcetal  blood-vessels,  and  these 
blood-vessels  with  their  containing  mesoblastic  tissue  extend 
so  as  to  completely  surround  the  embryo.  This  outgrowth  is 
intimately  applied  to  the  decidua.  At  one  spot  it  becomes 
hypertrophied,  and  here  sends  in  villi,  covered  with  a  single 


EEPRODUCTION  689 

layer  of  cells  and  supplied  with  blood-vessels  ;  these  project 
into  maternal  venous  sinuses  which  have  developed  in  the 
thickened  mucous  membrane  of  the  uterus. 

Through  the  medium  of  this  placenta  the  developing 
animal  obtains  all  the  nutrient  material  it  requires,  both 
oxygen  and  combustible  foodstuffs.  The  vessels  of  the 
fcetus  are  not  in  direct  communication  with  those  of  the 
mother.  The  interchange  of  material  between  the  two  must 
be  effected  by  the  cells  covering  the  placental  villi,  and 
takes  place  in  fact  through  two  layers  of  cells,  the  endo- 
thelium of  the  foetal  vessels  and  the  epithelium  of  the  villi. 
The  placenta  at  the  same  time  serves  as  an  excretory  organ 

Fig.  310. 


Diagram  to  show  structure  of  human  placenta,  m.s.  Maternal 
venous  sinus,  v.  Villus  (outgrowth  from  chorion),  containing 
artery  and  vein  with  capillaries  derived  from  the  umbilical 
vessels  of  the  foetus,  and  covered  with  a  single  layer  of  epithelial 
cells. 


for  the  foetus,  the  effete  material  of  the  latter  passing  from 
the  capillaries  to  the  blood  in  the  venous  sinuses  which  bathe 
and  surround  the  villi.  The  placenta  is  therefore  alimentary, 
respiratory,  and  excretory. 

Parturition. — While  the  ovum  is  undergoing  its  wonderful 
development,  in  which  a  complete  human  being  is  formed 
out  of  a  single  cell  by  division  and  differentiation,  the  uterus 
becomes  very  much  enlarged,  and  its  walls  thickened  by  new 
growth  of  unstriated  muscular  tissue.  At  the  end  of  nine 
months  from  the  date  of  impregnation  the  development  of  the 
foetus  is  complete,  and  parturition  takes  place.  This  consists 
in  the  expulsion  of  the  foetus  by  muscular  contractions  of 
the  uterus. 

44 


690  PHYSIOLOGY 

Parturition  or  labour  is  divided  into  three  stages.  In  the 
first  stage  the  contractions  of  the  uterus,  which  are  pamful 
and  are  hence  spoken  of  as  '  pains,'  are  devoted  to  dilating 
the  OS  uteri.  This  is  effected  by  contractions  of  the  longi- 
tudinal muscles  of  the  uterine  wall,  at  the  same  time  that  the 
foetus,  contained  in  its  bag  of  membranes,  is  forced  against 
and  expands  the  os. 

When  the  os  is  fully  dilated  the  uterine  contractions  change 
in  character,  becoming  more  prolonged,  and  are  accompanied 
by  strong  contractions  of  the  abdominal  muscles,  which  force 
the  child  out  through  the  vagina. 

A  short  time  after  the  birth  of  the  child  the  pains  recom- 
mence, and  expel  the  placenta  with  the  decidua  and  the 
foetal  membranes.  In  this  third  stage  the  connection  of  the 
uterine  vessels  with  the  placental  sinuses  is  necessarily  torn 
through.  Bleeding  however  is  prevented  by  the  fact  that  the 
muscular  fibres  of  the  uterus  remain  firmly  contracted  after 
birth,  compressing  and  obliterating  the  lumen  of  the  torn 
vessels. 

Directly  the  child  is  born,  the  uterus,  contracting  on  the 
placenta,  compresses  its  vessels  and  prevents  a  further 
supply  of  oxygen  to  the  foetal  vessels.  The  child  therefore 
becomes  asphyxiated,  and  the  venous  blood,  acting  on  the 
medullary  centres,  calls  forth  the  first  act  of  respiration,  and 
the  process  is  started  which  is  to  supply  the  needs  of  the  new 
individual  with  oxygen  for  the  rest  of  its  life. 

Parturition  is  obviously  a  complex  reflex  act,  and  depends 
for  its  normal  carrying  out  on  the  integrity  of  the  nervous 
connections  of  the  uterus  with  the  spinal  cord.  The  centre 
for  the  act  ot  parturition  lies  in  the  lumbar  spinal  cord,  and 
it  has  been  shown  that  parturition  may  go  on  normally  in  a 
bitch  whose  cord  has  been  completely  divided  in  the  dorsal 
region.  This  act  is  then  not  necessarily  dependent  on  the 
co-activity  of  the  voluntary  centres. 

After  birth  the  enlarged  uterus  rapidly  diminishes  in  size 
in  consequence  of  the  atrophy  and  disintegration  of  the  newly 
formed  muscular  tissues,  and  this  invohition  is  complete  at 
the  end  of  three  months. 

Lactation. — ^The  young  child  at  birth  is  not  independent, 
but  relies  for  many  years  for  nourishment  and  protection 
on  the  parents.     For  the  first  nine  to  twelve  months  of  the 


EEPEODUCTION  691 

child  s  existence,  under  normal  circumstances  it  is  nourished 
entirely  on  the  secretion  of  the  mammary  glands  of  the  mother. 
The  mammary  glands  vary  largely  in  appearance  according 
to  the  condition  of  the  woman  from  whom  they  are  taken. 
Before  impregnation  they  are  small,  and  on  microscopical 
examination  are  seen  to  consist  of  a  number  of  branchmg 
sinuous  tubules,  which  are  cut  in  various  directions,  and  so 
give  the  appearance  of  a  number  of  alveoli.  These  tubules 
are  filled  with  a  solid  mass  of  epithelial  cells.  If  impreg- 
nation occurs,  a  marked  hypertrophy  of  the  gland  takes 
place,  caused  by  the  outgrowth  of  new  columns  of  cells  and 
the  formation  of  new  tubules  from  the  pre-existing  ones. 
At  the  same  time  the  whole  gland  becomes  more  vascular 
from  the  enlargement  of  the  blood-vessels  in  the  interstitial 
connective  tissue  between  the  tubules.  At  the  end  of  preg- 
nancy the  cells  in  the  interior  of  the  tubules  undergo  dis- 
integration, leaving  them  lined  with  only  a  single  layer  of 
cells.  The  active  secretion  of  milk  begins  shortly  before  or 
immediately  after  birth.  The  first  milk  that  is  secreted, 
which  is  called  the  colostrimi,  differs  markedly  from  normal 
milk  as  already  described  (p.  508).  It  contains  less  casein 
and  fat,  but  more  albumen,  than  ordinary  milk,  and  has  in 
addition  a  certain  amount  of  globulin.  Owmg  to  the  large 
amount  of  these  last  two  bodies,  colostrum  coagulates  on 
boiling.  Under  the  microscope,  colostrum  shows  the  pre- 
sence of  a  number  of  cells  with  nuclei,  or  masses  of  proto- 
plasm without  nuclei  (colostrum  corpuscles) .  Some  of  these 
examined  in  fresh  warm  milk  show  amoeboid  movements. 
They  probably  have  a  twofold  origin,  from  leucocytes  which 
have  wandered  into  the  lumen  of  the  gland,  and  from  central 
cells  of  the  tubules  which  have  undergone  disintegration. 
The  active  secretion  of  ordmary  milk  sets  m  on  the  second 
or  third  day  after  delivery.  In  the  gland-cells  we  may  dis- 
tinguish a  resting  and  an  active  condition,  just  as  in  the  case 
of  other  glands.  In  the  acini  of  a  resting  gland  the  lumen 
is  wide  and  filled  with  milk,  and  the  cells  form  a  single  flat 
nucleated  layer  at  the  periphery.  The  inner  margins  of  some 
of  the  cells  show  jagged  edges,  and  the  protoplasm  of  all  the 
cells  contains  a  few  small  granules.  In  an  active  gland,  on 
the  other  hand,  the  cells,  which  are  long  and  columnar, 
project  far  into  the  lumen.     Many  have  two  nuclei,  and  the 


692 


PHYSIOLOGY 


central  parts  of  all  the  cells  are  tilled  with  fat-granules  and 
finer  granules,  which  are  probably  protein  in  character  (cf. 
Fig.  311). 

The  process  which  goes  on  in  the  transition  from  the 
resting  to  the  discharged  condition  is  as  follows.  In  some 
of  the  cells,  the  central  part,  with  its  contained  degenerated 
daughter  nucleus,  breaks  away  entirely  from  the  basal  part, 

Fig.  311. 


Sections  of  mammary  gland  of  guinea-pig  (fat-granules  stained 
black  with  osniic  acid). 

A.  During  rest. 

B.  During  active  secretion.  It  will  be  noticed  that  in  this  case 
the  active  formation  of  products  of  cell-metabolism  (granules, 
etc.)  begins  with  the  commencement  of  secretion,  and  does  not 
occur  almost  exclusively  during  rest,  as  in  the  salivary  glands. 
In  the  mammary  gland,  the  active  growth  of  protoplasm,  the 
formation  of  granules  from  the  protoplasm,  and  the  discharge 
of  these  granules  in  the  secretion,  appear  to  go  on  at  one  and 
the  same  time. 


and  in  the  lumen  undergoes  rapid  disintegration,  furnishing 
to  the  fluid  there  protein,  fat-globules,  and  probably  sugar. 
The  change  in  the  cells  however  need  not  be  so  radical  as 
this.  Many  simply  discharge  their  fat-globules  and  their 
other  contents  into  the  lumen.  This  discharge  of  cell- 
contents  is  accompanied  by  a  secretion  of  water  and  salts. 
It  must  be  remembered  that  in  the  secretion  of  milk  its 


REPEODUCTION  693 

three  chief  constituents,  caseinogen,  lactose,  and  fat,  are 
manufactured  by  the  cells  of  the  mammary  glands  out  of 
the  indifferent  lymph  which  bathes  them.  This  in  its  turn 
is  replenished  from  the  blood  circulating  through  the  gland. 
Caseinogen  and  milk-sugar  are  found  nowhere  else  in  the 
body,  nor  in  any  other  animal  secretion.  The  fact  that  the 
fat  also  is  especially  formed  by  the  cells  is  shown  by  experi- 
ments, in  which  a  bitch  was  fed  on  pure  protein  food,  and 
excreted  more  fats  in  her  milk  than  were  contained  in  the 
whole  of  her  food.  An  increase  in  the  fat  in  the  milk  is  said 
however  to  be  brought  about  by  an  increased  fatty  diet,  and 
abnormal  fats  given  in  the  food  may  appear  among  the  fats  of 
the  milk. 

We  see  that  the  mammary  gland  in  its  mode  of  activity 
holds  a  position  midway  between  the  submaxillary  mucous 
gland  and  the  sebaceous  glands  of  the  skin.  In  the  former 
the  cells  manufacture  a  substance — mucigen — out  of  materials 
brought  to  them  by  the  blood,  and  this  is  discharged  as  mucin 
when  occasion  requires,  part  of  the  protoplasm  of  the  cells 
always  remaining  intact,  ready  to  build  and  store  up  mucigen 
in  its  meshes.  In  the  sebaceous  gland  the  secretion  is 
furnished  entirely  by  the  disintegrated  cells  themselves,  a 
continual  new  formation  of  cells  going  on  at  the  periphery  of 
the  acini ;  the  older  cells,  as  they  are  forced  towards  the 
centre,  undergo  fatty  degeneration,  die,  disintegrate,  and  are 
cast  out  as  the  fatty  material  known  as  sehum  on  the  surface 
of  the  skin  and  at  the  roots  of  the  hair. 

The  further  rearing  of  the  child,  its  maintenance  and 
education  to  fit  it  to  become  a  useful  member  of  society  (that 
is,  one  fit  to  continue  the  race  on  the  earth),  are  as  much 
physiological  necessities  for  the  continuation  of  the  species 
as  the  processes  we  have  just  been  discussing.  We  are 
however  here  concerned  with  physiology  in  its  narrower  sense 
and  need  not  carry  it  so  far  as  the  branch  of  this  science 
known  as  sociology,  the  office  of  which  it  is  to  treat  of  these 
questions. 


694 


PHYSIOLOGY 


APPENDIX 

A  DESCRIPTION   OF   SOME  ELECTRICAL   INSTRUMENTS 
USED    IN    PHYSIOLOGY 

The  first  requisite  of  the  physiologist,  if  he  desires  to  use 
electrical  currents  for  excitation  or  any  other  purposes,  is  a 
source  of  a  constant  current.  For  this  two  forms  of  batteries 
are  chiefly  used,  Daniell's  and  Grove's  cells. 

A  Daniell's  cell  (Fig.  312)  consists  of  an  outer  pot  con- 
taining a  saturated  solution  of  copper  sulphate,  in  which  is 
immersed  a  copper  cylinder.     To  the  cylinder  at  the  top  a 

Fig.  312. 


Daniell's  cell. 


bindmg  screw  is  attached,  by  which  the  connection  of  the 
copper  with  a  wire  terminal  is  effected.  Within  the  copper 
cylinder  is  a  second  pot  of  porous  clay,  filled  with  dilute 
sulphuric  acid,  in  which  is  immersed  a  rod  of  amalgamated 
zinc.  In  this  cell  the  zinc  is  the  positive  and  the  copper 
the  negative  ele7nent.  Hence  the  current  flows  (in  the  cell) 
from  zinc  to  copper,  and  if  the  binding  screws  of  the  two 
elements  are  connected  by  a  wire,  the  current  flows  in  the 
wire  (outer  circuit)  from  copper  to  zinc,  thus  completing 
the  circuit.  Since  in  the  outer  circuit  the  current  flows 
from  copper  to  zinc,  the  terminal  attached  to  the  copper  is 


APPENDIX 


695 


called  the  positive  pole,  and  that  to  the  zinc  the  negative 
pole. 

When  the  current  is  required  to  be  very  constant,  the 
zinc  may  be  immersed  in  a  saturated  solution  of  zinc  sulphate 
instead  of  dilute  sulphuric  acid. 

A  Daniell's  cell,  though  very  constant,  gives  only  a  small 
current,  owing  to  its  small  electromotive  force  and  high 
internal  resistance.  "When  a  stronger  current  is  required,  a 
Grove's  cell  (Fig.  313)  may  be  used.  In  this  cell  the  zinc 
is  m  the  form  of  a  cylmder,  immersed  in  a  cell  containing 
dilute  sulphuric  acid.  W^ithin  the  cylinder  is  a  porous  pot 
filled  with  strong  nitric  acid,  m  which  is  immersed  a  sheet 
of  platinum.      In   many  cases  the  porous  cell  is  made  flat, 

Fig.  313. 


'€n 


VPJ 


Diagram  of  Grove's  cell.  Zn.  Zinc  cylinder,  c.  Inner  porous 
cell.  T.  Terminal  or  binding  screw  of  platinum,  e.  Sheet  of 
platinum. 


and  the  zinc  plate  bent  up  round  it,  in  order  to  decrease  the 
distance  between  zinc  and  platinum,  and  so  make  the  resist- 
ance as  small  as  possible.  In  this  cell  the  zinc  is  the  positive 
and  the  platmum  the  negative  plate  ;  and  so  the  terminal 
attached  to  the  zinc  is  the  negative,  and  that  attached  to  the 
platinum  the  positive  pole. 

Another  very  convenient  form  of  battery,  though  not  so 
constant  as  the  two  forms  just  described,  is  the  bichromate 
battery,  with  a  single  fluid.  This  consists  of  a  plate  of  zinc 
between  two  plates  of  carbon.  The  whole  are  arranged  so 
that  they  can  be  immersed  in  or  drawn  out  of  the  fluid  at 
pleasure.  The  fluid  used  is  a  mixture  of  sulphuric  acid  and 
potassium  bichromate.     The  wire  attached  to  the  carbons  is 


696  PHYSIOLOGY 

the  positive  pole  and  the  current  in  the  outer  circuit  flows 
from  carbon  to  zinc. 

Various  forms  of  keys  and  commutators  are  used  for 
making  and  breaking  a  current,  or  for  changing  its  direction. 
Of  these  the  only  ones  that  we  need  here  describe  are  Du 
Bois  Eeymond's  key,  and  Pohl's  commutator  or  reverser. 
A  Du  Bois  key  consists  of  two  pieces  of  brass,  each  of  which 
has  two  binding  screws  for  the  attachment  of  wires.  These 
are  connected  by  a  third  piece,  or  bridge,  which  is  jointed 
to  one  of  the  two  side  bits,  so  that  it  may  be  raised  or 
lowered  at  pleasure  {v.  Fig.  314).  It  may  be  used  either  as 
a  simple  make-and-break  key,  or,  as  is  more  usual,  as  a 
short-circuiting  key.  In  the  first  case  one  brass  bank  is 
attached  to  one  terminal,  the  other  to  the  other  terminal. 
If  the  bridge  be  now   lowered,  the  connection  is  made  and 

Fig.  314. 


Du  Bois  key,  closed.  Du  Bois  key,  open. 

the  current  passes.     If  the  bridge  be  raised,  the  current  is 
broken. 

Fig.  314  A  and  b  show  the  way  in  which  the  key  is 
arranged  for  short-circuiting.  It  will  be  seen  that  four  wires 
are  attached  to  the  key ;  two  going  to  the  battery,  and  two 
we  may  suppose  going  to  a  nerve.  When  the  bridge  is  down, 
as  in  Fig.  314  a,  the  current  from  the  cell  on  coming  to  the 
key  has  a  choice  of  two  routes.  It  may  either  go  through  the 
brass  bridge,  or  through  the  other  wires  and  nerve.  The 
resistance  of  the  nerve  however  is  about  100,000  ohms, 
whereas  that  of  the  bridge  is  not  the  thousandth  part  of  an 
ohm.  When  a  current  divides,  the  amount  of  current  that 
goes  along  any  branch  is  inversely  proportional  to  the  resist- 
ance. Here  the  resistance  in  the  nerve-circuit  is  practically 
infinite  compared  with  that  in  the  brass  bridge,  and  so  all 


APPENDIX 


G97 


the  current  goes  through  the  bridge  and  none  through  the 
nerve.  We  say  then  that  the  current  is  short-circuited.  If 
however  the  bridge  be  raised,  as  in  Fig.  314  b,  the  only  way 
the  current  can  go  is  through  the  nerve,  and  so  the  whole 
of  the  current  takes  this  course.  This  form  of  key  is  indis- 
pensable when  exciting  nerves  with  currents  of  high  intensity. 
If  an  ordinary  make-and-break  key  only  be  interposed  in  the 
circuit,  excitation  may  occur  even  when  the  key  is  raised,  the 
current  having  high  enough  potential  to  complete  itself 
through  the  table  and  stand  on  which  the  preparation  lies. 


Diagram  of  Pohl's  reverser. 


This  is  called  unipolar  excitation,  and  obviously  cannot  occur 
when  the  current  is  short-circuited. 

PohVs  reverser  is  an  arrangement  for  changmg  the  direc- 
tion of  the  current.  It  consists  of  a  slab  of  ebonite  or  paraffin 
or  other  insulating  material,  in  which  are  six  small  holes 
filled  with  mercury.  A  binding  screw  is  in  connection  with 
the  mercury  in  each  of  these  holes.  Two  cross -wires  (not  in 
contact  with  one  another)  join  two  sets  of  pools  together,  as 
shown  in  Fig.  315. 

A  cradle  consisting  of  two  wires  joined  by  an  insulating 


698  PHYSiOLoaY 

handle  carries  two  arcs  of  wire  by  which  the  pools  at  (a) 
and  (b)  may  be  put  into  connection  with  either  (x)  and  (y),  or 
the  corresponding  pools  on  the  opposite  side.  It  will  be 
seen  that  with  the  cradle  tipped  to  one  side,  as  in  Fig.  315  a, 
the  current  from  the  battery  enters  the  reverser  at  (a)  ;  this 
proceeds  up  the  wire  of  the  cradle,  down  towards  the  right, 
then  along  the  cross-wire  to  the  pool  at  (x).  (x)  is  therefore 
the  anode,  and  (y)  the  kathode. 

In  Fig.  315  B  the  cradle  has  been  swung  over  to  the  other 
side.  Here  the  cross-wires  are  not  used  at  all  by  the  current, 
which  passes  from  (a)  up  the  sides  and  down  the  curved  wire 
to  (y).  In  this  case  (y)  is  now  the  anode  and  (x)  the  kathode, 
and  the  direction  of  the  current  through  the  circuit  connected 
with  (x)  and  (y)  is  reversed. 

By  taking  out  the  cross -wires,  Pohl's  reverser  may  be 
used  as  a  simple  switch,  by  which  the  current  may  be  led 
into  two  different  circuits  in  turn. 

The  Induction  Coil  {Bit  Bois  Beymond) 

If  a  coil  of  wire  in  connection  with  a  galvanometer  be 
placed  close  to  (but  insulated  from)  another  coil  through 
which  a  current  may  be  led  from  a  battery,  it  is  found  that  on 
make  and  break  of  the  current  of  the  second  coil  a  momentary 
current  is  induced  in  the  first.  The  induced  current  on 
make  is  in  the  reverse  direction,  that  on  break  in  the  same 
direction  as  the  primary  current.  The  electromotive  force  of 
the  induced  current  is  proportional  to  the  number  of  turns  of 
wire  m  the  coils.  This  principle  is  made  use  of  in  the  con- 
struction of  the  induction  apparatus.  This  consists  of  two 
coils,  each  containing  many  turns  of  wire.  The  smaller  coil 
(Ep  Fig.  316),  consisting  of  a  few  turns  of  comparatively 
thick  wire,  is  the  primary  coil,  and  is  put  into  connection  with 
a  battery.  It  has  within  it  a  core  of  soft  iron  wires,  which 
has  the  effect  of  attracting  the  lines  of  force,  concentrating 
them,  and  so  increasing  its  power  of  inducing  secondary 
currents.  The  secondary  coil,  r.,,  of  a  large  number  of  turns 
of  very  thin  wire,  is  arranged  so  as  to  slide  over  the  primary 
coil.  It  is  provided  with  two  terminals,  which  may  be  con- 
nected with  the  nerve  or  other  tissue  that  we  wish  to  stimu- 
late.    Since  the  electromotive  force  of  the  induced  current  is 


APPENDIX 


699 


proportional  to  the  number  of  turns  of  wire,  it  is  evident  that 
the  electromotive  force  of  the  current  delivered  by  the  induc- 
tion coil  may  be  many  thousand  times  that  of  the  battery 
current  flowing  through  the  primary  coil.  The  induced 
currents  increase  rapidly  in  strength  as  the  coils  are  ap- 
proached to  one  another  ;  the  strength  of  these  therefore  may 
be  regulated  by  shoving  the  secondary  up  to  or  away  from 
the  primary  coil. 

A  short-circuiting  key  is  always  placed  between  the  secon- 
dary coil  and  the  nerve  to  be  stimulated. 

If  only  single  induction  shocks  are  to  be  used,  a  make- 
and-break  key  is  put  in  the  primary  battery  circuit,  and  the 

Fig.  316. 


Diagram  of  inductorium.  r,,  primary  ;  e.,,  secondary  coil, 
m.  Electro-magnet  of  Wagner's  hammer,  w.  Helmholtz' 
side  wire. 


two  wires  from  the  battery  and  key  are  attached  to  the  two 
top  screws  of  the  primary  coil  (c  and  d,  Fig.  316).  It  ia 
then  found  that  the  shock  given  by  the  induced  current  on 
break  of  the  primary  current  is  much  stronger  than  that 
on  make. 

In  endeavouring  to  explain  this  difference  in  the  intensity 
of  the  make-and-break  induction  shocks,  it  must  be  remem- 
bered that  the  intensity  of  the  momentary  current  induced 
in  the  secondary  coil  at  make  or  break  of  the  primary  current 
is  proportional  (1)  to  the  number  of  turns  of  wire  in  each 
coil ;    (2)  inversely  to  the  mean  distance  between  the  coils 


700 


PHYSIOLOGY 


{i.e.  the  nearer  the  coils,  the  stronger  the  induced  current)  ; 
(3)  to  the  rate  of  change  in  strength  of  the  primary  current. 
Now,  when  a  current  is  made  through  the  primary  coil,  induc- 
tion takes  place,  not  only  between  primary  and  secondary  coils, 
but  also  between  the  individual  turns  of  the  primary  coil  itself. 
This  current  of  self-induction,  being  opposed  in  direction  to 
the  battery  current,  hinders  and  delays  the  attainment  by 
the  latter  of  its  full  strength,  and  so  slows  the  rate  of  change 
of  current  in  the  primary  coil.  Hence  the  intensity  of  the 
momentary  current  induced  in  the  secondary  coil  is  less 
than  it  would  have  been  without  the  retarding  effect  of  self- 
induction. 

Fig.  317. 


(c^ 


At  break  of  the  current,  an  extra  current  is  also  produced 
in  the  primary  coil  in  the  same  direction  as  the  battery 
current,  and  therefore  tending  to  reduce  the  rate  of  change 
of  the  current  from  full  strength  to  nothing.  In  this  case 
however  the  primary  circuit  being  broken,  the  current  of 
self-induction  cannot  pass  without  jumping  the  great  resist- 
ance offered  by  the  air,  so  that  its  retarding  effect  on  the 
rate  of  disappearance  of  the  primary  current  may  be  prac- 
tically disregarded.  In  Fig.  317  the  line  abed  will  repre- 
sent the  changes  occurring  in  the  primary  current  at  make 
and  break,  ct  b  corresponding  to  the  make  and  c  d  to  the 
break.  The  lower  line  represents  the  momentary  currents 
induced  in  the  secondary  circuit,  m  being  the  current  of  low 
intensity  and  long  duration  produced  by  the  make,  and  B  the 


APPENDIX 


701 


shock  of  high  mtensity  and  short  duration  caused  by  the 
sharp  break  of  the  primary  current. 

When  ^Ye  desire  to  use  faradic  stimulation — that  is, 
secondary  induced  shocks  rapidly  repeated  50  to  100  times 
a  second — we  make  use  of  the  apparatus  attached  to  the 
coil,  known  as  Wagner's  hammer  (Figs.  316  and  318).  In 
this  case  the  wires  from  the  battery  are  connected  to  the 
two  lower  screws  (a  and  b.  Fig.  316).  Fig.  318  shows 
the  direction  of  the  current  when  Wagner's  hammer  is 
used.  The  current  enters  at  (a),  runs  up  the  pillar  and  along 
the  spring  to  the  screw  (x).  Here  it  passes  up  through  the 
screw,  and  through  the  primary  coil  (e,).  From  the  primary 
coil  it  passes  up  the  small   coil  (m),  and  from   this  to  the 

Fig.  318. 


Diagram  showing  course  of  current  in  inductorium  when 
Wagner's  hammer  is  used. 


terminal  (b)  and  back  to  the  battery.  But  in  this  course  the 
coil  (m)  is  converted  into  an  electro-magnet.  The  hammer 
(h)  attached  to  the  spring  is  attracted  down,  and  so  the 
spring  is  drawn  away  from  the  screw  (x),  and  the  current  is 
therefore  broken.  The  break  of  the  current  destroys  the 
magnetic  power  of  the  coil,  the  spring  jumps  up  again  and 
once  more  makes  circuit  with  the  screw  (x),  only  to  be  drawn 
down  again  directly  this  occurs.  In  this  way  the  sprmg  is 
kept  vibrating,  and  the  primary  circuit  is  continually  made 
and  broken,  with  the  production  at  each  make-and-break  of 
an  induced  current  in  the  secondary  coil. 

It  is  evident  that,  when  the  primary  current  is  made  and 
broken  fifty  times  in  the  second,  there  will  be  a  hundred 
momentary  currents  produced  during  the  same  period  in  the 


702 


PHYSIOLOGY 


secondary  coil.  Every  alternate  one  of  these  produced  by 
the  break  of  current  in  the  primary  will  be  much  stronger 
than  the  intervening  currents  produced  by  the  make.  In 
order  to  equalise  make-and-break  induction-shocks,  so  that  a 
regular  series  of  momentary  currents  of  nearly  equal  intensity 
may  be  produced,  the  arrangement  known  as  Helmholtz's  is 
used.  In  this  arrangement  the  side  wire  (w),  shown  in 
Fig.  316,  and  diagrammatically  in  Fig.  319,  is  used  to 
connect  the  binding  screw  (o)  with  the  binding  screw  (c)  at 
the  top  of  the  coil.  The  screw  (x)  is  raised,  so  as  not  to 
touch  the  spring,  and  the  lower  screw  (y)  is  moved  up  till 
it  comes  nearly  in  contact  with  the  under  surface  of  the 
spring.     If  we  consider  the  direction  of  the  current  now,  we 

Fig.  319. 


Diagram  showing  course  of  current  when  the  Heimholtz 
side  wire  is  used. 


see  that  it  enters  as  before  at  the  terminal,  travels  up 
the  Helmholtz's  wire  (w)  to  the  screw  (c),  thence  through 
the  primary  coil  (r,),  then  through  the  coil  (m)  of  the 
Wagner's  hammer,  and  so  back  to  the  battery.  The  coil  (m), 
thus  becoming  an  electro -magnet,  draws  down  the  hammer 
(h).  In  this  act  the  under  surface  of  the  spring  comes  in 
contact  with  the  screw  (y).  The  current  then  has  a  choice 
of  two  ways.  It  may  either  go  through  the  coil  as  before,  or 
take  a  short  cut  from  the  terminal  (a),  up  the  pillar,  along 
the  spring,  through  the  screw  (y),  and  down  to  the  terminal 
(b)  back  to  the  battery.  As  the  resistance  of  this  latter 
route  is  very  small  compared  with  the  resistance  of  the 
primary  coil,  etc.,  the  greater  part  of  the  current  takes  this 


APPENDIX 


703 


way.  The  infinitesimal  current  which  now  passes  through 
the  coil  of  Wagner's  arrangement  is  insufficient  to  magnetise 
this,  and  the  hammer  springs  up  again ;  thus  the  process  is 
restarted,  and  the  spring  vibrates  rhythmically.  With  this 
arrangement  the  primary  current  is  never  broken,  but  only 
short-circuited,  and  so  diminished  very  largely.  Hence  the 
retarding  influence  of  self-mduction  is  as  potent  with  break 
as  with  make  of  the  current,  and  the  effects  on  the  secondary 
coil  in  the  two  cases  are  approximately  equal.  In  Fig.  317 
(c  e)  represents  the  change  in  the  primary  current  when  the 
current  is  short-circuited  instead  of  being  broken,  and  (b) 
represents  the  effect  produced  in  the  secondary  coil.  It  will 
be  seen  that  the  currents  (m)  and  (b)  are  practically  identical 
in  intensity  and  duration. 

Fig.  320. 


Diagram  to  show  the  mode  of  construction  of  a  condenser. 


Condenser. — If  two  plates  of  metal  separated  from  one 
another  by  a  thin  insulating  layer  of  dielectric  such  as  air, 
glass,  mica,  or  paraffined  paper,  be  connected  with  the  two 
poles  of  a  galvanometer,  each  plate  acquires  the  potential  of 
the  pole  of  the  battery  with  which  it  is  connected,  and  re- 
ceives therefrom  a  charge  of  electricity  (positive  or  negative). 
If  the  connections  be  broken  the  two  plates  retain  their 
charge.  If  now  they  be  connected  by  a  wire  they  discharge 
through  the  wire,  and  if  a  nerve  be  inserted  in  the  course  of 
the  wire,  it  may  be  excited  by  the  discharge. 

The  amount  of  electricity  that  may  be  stored  up  in  this 
way  will  depend  on  the  extent  of  the  plates  and  their 
proximity  to  one  another,  as   well   as   on   the  e.m.f.  of  the 


704  PHYSIOLOGY 

charging  battery.  In  order  to  get  great  extent  of  surface, 
a  condenser  is  built  up  as  in  the  diagram  (Fig.  320)  of  a 
very  large  number  of  plates  of  tinfoil,  separated  by  discs  of 
mica  or  paraffined  paper.  Alternate  discs  are  connected 
together  :  thus  1,  3,  5  are  connected  to  one  pole,  while  2,  4, 
6  are  connected  to  the  other. 

The  rheocord  is  used  to  modify  the  amount  or  strength  of 
current  flowing  through  a  preparation.  One  form  of  it  is 
represented  in  Fig.  321.  A  constant  source  of  current  at  b 
causes  a  flow  of  electricity  from  a  to  b  through  a  straight 
way.  As  the  resistance  of  this  wire  is  the  same  throughout 
its  length,  the  fall  of  potential  from  a  toh  must  be  constant. 

The  nerve,  or  whatever  preparation  that  is  used,  is  con- 
nected with  the  straight  wire  at  two  points,  at  a  and  at  c, 


^^j^i^^>*= 


by  means  of  a  sliding  contact  or  rider.  Supposing  that 
there  is  an  electromotive  difference  of  one  volt  between  a  and 
b,  it  is  evident  that  if  c  is  pushed  close  to  b,  the  e.m.f. 
acting  on  the  nerve  will  be  also  one  volt.  The  e.m.f. 
however  may  be  made  as  small  as  we  like  by  sliding  c 
nearer  to  a.  Thus  if  a  b  is  one  metre,  and  there  is  a  differ- 
ence of  one  volt  between  the  two  ends,  then  if  c  be  one 
centimetre  from  a,  the  e.m.f.  acting  on  the  nerve  will  be 
roo"  volt.  Thus  we  alter  the  current  passing  through  the 
nerve  by  altering  the  e.m.f.  which  drives  the  current. 

The  galvanometer  is  an  instrument  used  to  measure 
strength  of  current.  Its  construction  depends  on  the  fact 
that,  if  a  small  compass  be  suspended  within  a  coil  of  wire 
and  a  current  be  passed  through  the  wire,  the  compass  is 
deflected,  and  tends  to  take  up  a  position  at  right  angles  to 


APPENDIX 


705 


its  former  one.  In  practice  a  pair  of  such  magnetic  needles 
are  usually  employed,  arranged  reversely,  as  in  Fig.  322, 
where  a  and  n  represent  the  north-seeking  ends  of  the 
magnets  a  h  and  s  n.  On  arranging  them  in  this  way  the 
effect  of  the  earth's  magnetism  on  them  is  weakened  or 
annulled,  and  they  are  spoken  of  as  astatic.  A  bar  compass 
is  suspended  over  the  galvanometer,  by  the  adjustment  of 
which  the  force  with  which  the  suspended  pair  of  magnets 
tends  to  '  set '  in  one  direction  may  be  increased  or  dimi- 
nished. In  Thomson's  galvanometer,  as  it  is  used  for  nerve 
work,  the  coils  of  wire  have  a  resistance  of  5,000  to  20,000 
ohms.  Between  the  two  magnets  is  fixed  a  small  mirror. 
The    swing   of    the   magnet   is    recorded    by   observing   the 

Fig.  322. 


(3l>' 


Diagram  of  galvanometer,  with  astatic  pair  of  magnets.  The 
arrows  show  direction  of  current  from  battery,  and  the  swing 
of  the  needles. 


excursion  of  a  spot  of  light  reflected  by  this  mirror  on  to 
a  horizontal  graduated  scale. 

In  thermo-electric  experiments,  when  very  small  differ- 
ences of  potential  have  to  be  measured  through  a  small 
resistance,  the  galvanometer  must  also  have  a  small  resist- 
ance, which  here  should  not  be  more  than  one  ohm. 

The  capillary  electrometer  is  an  instrument  for  record- 
ing and  measuring  difference  of  potential.  That  is  to  say, 
if  connected  with  two  points,  it  measures  the  force  which 
would  make  a  current  flow  between  these  two  points  if  they 
were  connected  with  a  wire. 

Its  structure  is  very  simple.  It  consists  of  a  glass  tube 
drawn  out  to  a  fine  capillary  point.  This  tube  with  the 
capillary  is  filled  with  mercury.  The  pomt  dips  into  a  wide 
tube  containing  dilate  sulphuric  acid,  at  the  bottom  of  which 
is  a  little  mercury.     Two   platinum   wires  melted    into   the 

45 


706 


PHYyiOLOGY 


glass  and  dipping  into  the  mercury  serve  as  terminals.  In 
consequence  of  capillary  attraction  the  acid  ascends  some 
way  into  the  capillary  tube,  and  the  force  of  this  can, 
with  fine  capillaries,  sustain  the  weight  of  several  inches  of 
mercury. 

When  the  instrument  is  used,  the  meniscus  of  the  mercury 
in  the  capillary  at  its  junction  with  the  acid  is  observed 
under  the  microscope,  or  a  magnified  image  of  it  is  thrown 
on  a  screen  with  the  aid  of  the  lime  or  electric  light. 

Fig.  323. 


•  II'' 


.-acid 


S-Hj?. 


Diagram  of  capillary  electrometer.  Hg.,  Mercury.  The  two 
terminals  are  rein-esented  as  leading  off  two  points  at  the  base 
and  apex  of  a  frog's  heart,  a  b. 

If  now  the  capillary  and  acid  be  connected  with  two 
points,  it  will  be  observed  that  any  difference  in  the  potential 
of  these  two  points  causes  a  movement  of  the  meniscus.  If 
the  point  connected  to  acid  be  negative  as  compared  with  the 
point  connected  to  mercury  in  capillary,  the  meniscus  moves 
towards  the  point  of  the  capillary.  If  the  acid  be  positive 
as  compared  with  the  capillary,  the  meniscus  moves  away 
from  the  point.  It  is  further  found  that  the  extent  of  the 
excursion  is  proportional  to  the  difference  of  potential. 

Since  the  capillary  electrometer  appears  to  have  no  latent 
period,  and  is  free  from  instrumental  vibrations,  it  is  ex- 
tremely useful  in  recording  the  quick  changes  in  potential 


APPENDIX  707 

occurring  in  the  diphasic  electrical  changes  that  accompany 
every  contraction-wave  in  the  body. 

The  excursions  lend  themselves  well  to  photography,  so 
that  we  may  obtain  a  graphic  record  of  every  electrical 
variation,  and  thus  determine  its  extent  and  its  time- 
relations. 

It  must  be  remembered  that  this  instrument  is  an  electro- 
meter (measurer  of  difference  of  potential),  and  not  a  galvano- 
meter (current-measurer).  When  the  electrometer  is  con- 
nected with  two  points  at  different  potential,  no  current  passes 
through  it.  Hence  the  use  of  non-polarisable  electrodes  is 
not  so  essential  in  experiments  with  this  instrument  as  when 
we  make  use  of  the  galvanometer. 


45- 


INDEX 


Absokptiox  of  foodstuffs,  349-359 

of  lymph,  295 
Accelerator  nerves,  250 
Acc-ommodation,  552 
Adaptation,  visual,  566 
Adrenaline,  252,  520 
Aerotonometer,  492 
Air,  changes  of  composition  in  respired, 

386 
Alanine,  32 
Albuminoids,  46 
Albumens,  39 

derived,  40 
Albumoses,  properties  of,  41 
Alimentary  canal,  8 
Alveolar  air,  388 
Amboceptor,  85 
Amino-acids,  33,  333,  437 
Ammonia,  excretion  of,  488  450 

formation  of,  139 
Amceboid  movement,  157 
Amyloid  substance,  46 
Amylopsin,  331 
Anabolic  nerves,  271 
Anabolism,  4 
Anacrotic  pulse,  229 
Ansemia,  285 

Anaesthetics,  action  on  nerve,  180 
Anelectrotonus,  169 
Antibody,  85 
'  Autipeptone,'  333 
Apex-beat,  219 
Aphasia,  663 
Apncea,  417 
Ai-ginine,  36 
Arteries,  extensibility  of,  190 

peripheral  reactions  of,  278 

pressure  in,  192 

structure  of,  189 


Arteries,  velocity  in,  201 
Aspartic  acid,  34 
Asphyxia,  404 

influence  on  blood-pressure,  275 
Assimilation,  4,  7,  26 
Astigmatism,  560 
Auditory  sense,  544-549 
Auerbach's  plexus,  370 
Auto-oxidisable  substances,  28 
Axon, 588 

Basophile  corpuscles,  61 
Batteries,  694 
Bidder's  ganglia,  235 
Bile,  338-346,  353 

pigments,  343 

source  of,  69 

salts,  341 
Bilirubin,  343 
Biliverdin,  343 
Bladder,  movements  of,  469 
Blood,  57-86 

coagulation  of,  75-84 

composition  of,  84 

gases  of,  391 

reaction  of,  84 

specific  gravity  of,  84 

total  quantity  of,  283 

velocity  of,  201 
Blood-corpuscles,  58-74 

chemistry  of,  62-67 

estimation  of,  73 

formation  of,  70 

life  history  of,  68 
Blood-plasma,  conditions  of  clotting,  83 
Blood-platelets,  61,  83 
Blood-pressure,  192 

effect  of  asphyxia  on,  275 

in  heart,  215,  227 


710 


PHYSIOLOaY 


Blood-pressure,     respiratory    undula- 
tions of,  423 

Blood-serum,  75 

poisonous  effects  of,  285 

Blood-vessels,  innervation  of,  260 
reflex  alterations  of,  271 
structure  and  properties  of,  189 
velocity  of  blood  flow  in,  201 

Brain,  circulation  in,  667 

connecting  tracts  in,  626 
functions  of,  22,  642-652 
localisation  in,  656-663 
membranes  of,  666 
structure  of,  612-633 

Bronchi,  innervation  of,  418 

Calorimeter,  501 

Cane-sugar,  54 

Capillaries,  circulation  in,  280 

permeability  of,  290 

velocity  in,  204 
Capillary  electrometer,  125,  705 
Carbohydrates,  absorption  of,  354 

chemistry  of,  51-56 

metabolism  of,  489-496 

of  milk,  509 
Carbon  dioxide  in  blood,  391 

influence  on  respiratory 
centre,  405 

monoxide,  400,  420 
Carboxyhaemoglobin,  64 
Cardiograph,  220 
Cardiometer,  255 
Caseinogen,  44,  323,  508,  693 
Cells,  oxidative  changes  in,  28 

processes  of  reduction  in,  28 
Cellulose,  56 
Centre,  cardio-inhibitory,  251 

respiratory,  402 

vaso-motor,  260 
Cerebellum,  645-651 
Cerebral  circulation,  667 

hemispheres,  653-665 
Cerebro-spinal  fluid,  666 
Cerebrum,  22,  653 
'  Characteristic,'  101 
Chemical   constituents   of    the    body, 

25  et  seq. 
Cholesterin,  49,  343 
Choletelin,  343 
Chondrin,  46 
Chondro-proteins,  46 


Ciliary  movement,  157 
Circulation,  189-285 

capillary,  280 

course  of,  ]  1 

influence  of  gravity  on,  278 

in  lungs,  422 
Circulation-time,  205 
Coagulation,  intravascular,  82 

of  blood,  75 

of  colloids,  31 

of  milk,  324 

of  muscle-plasma,  131 
Coagulin,  86 
Cold  spots,  528 
Colloids,  31 
Colour  vision,  572 
Commutator,  696 
Compensation  in  heart,  257 
Complement,  85 
Condenser,  use  of,  175,  703 
Conduction  in  nerve,  169 
Cones,  562,  567 
Contractile  tissues,  87-158 
Contraction,  isometric,  107 

isotonic,  107 

law  of,  171 

of  heart  muscle,  2 

of  muscle,  102 

paradoxical,  186 

propagation  of,  in  heart,  237 

wave,  108 
Contrast  phenomena,  575 
Co-ordination,  15 
Cortex,  functions  of,  656 

structure  of,  653 
Cranial  nerves,  634 
Creatine,  36,  133,  441 
Creatinine,  449 
Crystallin,  40 
Curare,  action  of,  95 
Cutaneous  sense,  526 
Cystine,  36,  333 
Cytase,  56 

DEFiECATION,  375 

Degeneration,  584 

retrograde,  592 
Deglutition,  363 
Demarcation  current,  121,  127 
Depressor  nerve,  272 
Dextrin,  56 
Dextrose,  tests  for,  53 


IxN^DEX 


711 


Diabetes,  490 

Diabetic  coma,  495 

Diamino-acids,  35,  333 

Diaphragm,  movements  of,  379 

Dicrotic  wave,  226,  230 

Diet,  507 

Digestion,  mechanisms  of,  298-376 

muscular  mechanisms  of,  363- 
376 
Diphthongs,  432 
Di-saccharides,  54 
Dissimilation,  4 
Ductless  glands,  5.13-521 
Dys-oxidisable  substances,  28 

Ear,  structure  of,  544 

Elastin,  47 

Electrical  changes  in  glands,  312 
in  muscle,  120 
in  nerve,  104 
organs,  128 

Electrodes,  98 

non-polarisable,  120 

Electrotonic  current,  182 

Electrotonus,  167 

Emotional  expression,  Gol 

End-plates,  92 

Energy  of  nerve-stimuli,  175 

Enterokinase,  331,  347 

Eosinophile,  60 

Equilibrium,  maintenance  of,  643 

Erepsin,  304,  347 

Excitation  of  muscle,  95 

ExM-etion,  12,  375,  434 

Eye,  accommodation  in,  552 
adaptation  of,  566 
chromatic  aberration  of,  558 
movements  of,  568,  635 
refractive  media  of,  552 

Eye-ball,  nutrition  of,  580 

F^CES,  comiwsition  of,  361 
Fat,  formation  of,  485 
Fatigue  in  nerve.  177,  188 

of  muscle,  141 
Fats,  absorption  of,  350 

chemistry  of,  48-50 

emulsification  of,  336 

of  milk,  509 
Ferments,  299-304 
Fibrin,  75 

precursors  of,  78 


Fibrin  ferment,  80 
Fibrinogen,  79 

tissue-,  82 
Foodstuffs,  fate  in  organism,  478-512 

heat-equivalents  of,  482 

Galactose,  54 
Galvanometer,  704 
Ganglia,  reflex  action  in,  683 
Ganglion  cells,  functions  of,  680 
Gases,  irrespirable,  421 

of  blood,  391 

respiratory  interchange  of,  386- 
401 
Gastric  juice,  316 
Gelatin,  46 

influence  of  pancreatic  juice 
on, 335 
Glands,  see  also  Secretion 

ductless,  513-521 

salivary,  307 

sebaceous,  473 

sweat-,  474 
Glaucoma,  581 
Globin,  65 
Globulins,  39 
Glucosamine,  45 
Glucose,  53 
Glutamic  acid,  36 
Glycine,  34 
Glycocholic  acid,  342 
Glycogen,  56,  489 

in  muscle,  133 
Glyco-proteins,  44 
Gycosuria,  492 
Glycurcnic  acid,  45,  492 
Gracilis  experiment,  164 
Gustatory  sense,  539 

HiEMATIN,  65 

Hffimatogen,  72 
Hsematoidin,  67 
Hsematolysis,  68 
Hffimatoporphyriu,  67 
Hsemin,  65 
Hsemochromogen,  66 
Hsemocytometer,  73 
Hsemodromograph,  202 
Hsemodromometer,  202 
Htemoglobin,  58 

compounds  with  gases,  62 

crystallisation  of.  62 


712 


PHYSIOLOGY 


Hsemoglobin,  destruction  of,  68 

estimation  of,  74 

fate  of,  B45 

formation  of,  (J9 

spectrum  of,  GG 
Hffimoglobinometer,  74 
HcTmolysis,  85,  285 
Haemorrhage,  effects  of,  285 
Hffimotachometer,  203 
Hearing,  544-541) 
Heart,  action  of  asphyxia  on,  275 

afferent  nerves  of,  250 

anatomy  of,  20G 

automatic  power  of,  235 

compensation  in,  257 

frog's,  anatomy  of,  234 

influence  of  drugs  on,  251 

mammalian,  innervation  of,  245 
rhythm  of,  242 

negative  pressure  in,  218 

output  of,  253 

pressure  in,  215,  226 

rliythm  of,  233 

work  of,  253 
Heart-beat,  phenomena  of,  210 
Heart-impulse,  21'J 
Heart-muscle,  properties  of,  236 
Heart-sounds,  213 

record  of,  221 
Heart-valves,  action  of,  211 
Heat,  production  of,  501 
Heat-formation  in  nerve,  165 

in  muscle,  114 
Heat-spots,  528 
Hemianopia,  661 
Hexone  bases,  33,  334 
Hexose,  51 
Hippuric  acid,  449 
Histidine,  36 
Histones,  37,  42 
Hydrobilirubin,  453 
Hypermetropia,  560 

Impregnation,  688 

Induction  coil,  G98 

Intiammation,  280 

Inhibition,  607 

effect  of  strychnine  on,  607 
of  involuntary  muscle,  156 

Inogen,  138 

Inosit,  133 

Intestinal  juice,  347 


Intestine,  large,  innervation  of,  374 

movements  of,  374 

structure  of,  349 
Intraocular  pressure,  580 
Invertin,  304,  347 
Iris,  movemei:its  of,  556 
Irritability,  2 

specific,  164 
Islets  of  Langerhans,  329,  495 
Isometric  contraction,  107 
Isotonic  contraction,  107 

Katabolic  nerves,  271 

Katabolism,  4 

Katacrotic  ijulse,  229 

Katalytic  phenomena,  29 

Katelectrotonus,  169 

Keratin,  47 

'  Kernleiter,'  183 

Keys,  699 

Kidneys,  functions  of,  434-466 

innervation  of,  464 

metabolic  functions  of,  466 

structure  of,  454 

work  of,  464 
Knee-jerk,  609 

Lactase,  332 
Lactation,  690 
Lactic  acid,  55 

of  muscle,  134 
Lactose,  55 

inversion  of,  332 
Lsevulose,  54 
Lardacein,  46 
Larynx,  movements  of,  429 
Latent  period  of  involuntary  muscle, 
149 
of  voluntary  muscle,  105 
Lecithin,  49 
Leucine,  34,  333 
Leucocytes,  chemistry  of,  73 
emigration  of,  281 
formation  of,  73 

of   fibrin  ferment  from, 
82 
varieties  of,  60 
Liver,  secretion  of,  338 

formation  of  urea  in,  438,  481 
of  glycogen  in,  354,  490 
of  sugar  in, 490,  492 
Lungs,  circulation  in,  422 


INDEX 


713 


Lungs,  gaseous  interchanges  in,  38G- 

401 
Lymph,  286-297 
Lymph-flow  in  plethora,  283 
Lysine,  36 

Maltase,  304,  336 
Maltose,  54 

Manometer,  Hiirthle's,  216 
max.  and  min.,  217 
mercurial,  193 
Marey's  law,  251 
Mastication,  363 
Menstruation,  687 

Metabolism  of  carbohydrates,  489-496 
of  fat,  4S5-488 
of  proteins,  443,  478-484 
Methajmoglobin,  64 
Micturition,  467-472 
Milk,  coagulation  of,  324 
composition  of,  508 
secretion  of,  691 
Milk-sugar,  55 
Millon's  reaction,  38 
Mono-saccharides,  51 
Motor  centres,  657 

tracts  in  brain,  627 
cord,  598 
Mucins,  45 
Mucoids,  4o 
MiiUer's  law,    ,  4,  518 
Muscle,  14,  87  et  seq. 

absolute  force  of,  107 
action  of  constant  current  on, 
97 
of  drugs  on,  142 
chemical  changes   in   activity, 

135 
chemistry  of,  131 
contraction  of,  102 
decomposition    of    creatin    in, 

441 
direct  excitability  of,  95 
effect  of  temperature  on,  140 
electrical  changes  in,  119 
excitation  of,  95 
extensibility  of,  111 
fatigue  of,  141 
gaseous  interchanges  of,  138 
heat-production  in,  114 
isometric  contraction  of,  108 
isotonic  contraction  of,  107 


Muscle,  latent  period  of,  105,  149 

mechanical  efficiency  of,  117 
proteins  of,  131 
secondary  contraction  of,  130 
source  of  energy  of,  137 
structure  of,  87 
tone  of,  008 

unstriated,  physiology  of,  149- 
156 
structure  of,  89 
voluntary  contraction  of,  145 
work  of,  113 
Muscle-plasma,  131 
Muscle-sound,  145 
Muscles  of  frog's  leg,  93 
Muscular    actions,   co-ordination    of, 
643 
energy,  source  of,  497 
mechanisms  of  digestion,  363- 
376 
of  micturition,  469 
of  respiration,  377 
Muscular  sense,  531 

tone,  608 
Myogen,  132 
Myographs,  103-105 
Myohsematin,  133 
Myopia,  559 
Myosin,  132 

Negative  variation,  122 
Negativity,  125 
Nerve,  159-188 

conduction  in,  169 
degeneration  in,  584 
double  conduction  in,  163 
effect  of  injury  on,  179 
electrical  change  in,  164 
excitation  of,  167,  171,  174 
excitation  of  human,  173 
fatigue  in,  177,  188 
heat-formation  in,  165 
influence  of  temperature  on,  179 

drugs  on,  180 
polarisation  j)henomena  in,  182 
propagation  in,  162 
retrograde  degeneration  of,  5 
structure  of,  160 
Nerve-cell,  function  of,  591,  680 

structure  of,  588 
Nerve-cells  of  cord,  594 
Nerve-centres  in  medulla,  641 


714 


PHYSIOLOGY 


Nerve-degeneration,  influence  on  vaso- 
constrictors, 2()1) 
Nerve-roots,  20,  582,  603 
Nerves,  cranial,  634-640 
depressor,  272 
pelvic,  374,  470 
pilomotor,  474,  674 
pressor,  272 

secretory,   to    salivary    glands, 
313 
to  stomach,  326 
splanchnic,   action  ■  on    blood- 
pressure,  267 
on  intestine,  367 
origin  of,  677 
vagus,  see  Vagi 
vaso-dilator,  268 
vaso-motor,  260,  266 
visceral,  21,  670 
Nervi  erigentes,  268 
Nervous  impulse,  162 

nature  of,  18,  187 
system,  16,  -582-681 

visceral,  669-684 
Neuroglia,  592 
Neurokeratin,  47 
Neuron,  590 
Nissl's  granules,  591 
Nitrogen,  assimilation  of,  26,  478 
Nitrogenous  equilibrium,  483 
Noend  vital,  402 
Nucleic  acid,  43 
Nuclein,  42 
Nucleo-proteins,  42 

injection  of,  82 


(Edema,  inflammatory,  281 
Olfactory  sense,  541 
Oncometer,  262 
Ophthalmometer,  554 
Ornithine,  36 
Osazones,  52 

Osmotic  pressure,  definition,  464 
Oxidation  in  tissues,  396 

mechanism  of,  28 
Oxydases,  29 

Oxygen,  influence  of  changes  in  ten- 
sion, 407,  419 

intramolecular,  139 
Oxyhsemoglobin,  see  Haemoglobin 
Oxyproline,  36,  333 


Pain,  529 
Pancreas,  328 

sugar-function  of,  495 
Pancreatic  fistula,  329 

juice,  action  of,  331-337,  353 
Paradoxical  contraction,  186 
Parathyroids,  519 
Parturition,  689 
Pelvic  nerve,  action  on  colon,  374 

on  bladder,  470 
Pepsin,  325 
Pei^tone-plasma,  83 
Peptones,  42 

properties  of,  320 
Phagocytes,  281 
Phakoscope,  553 
Phenyl-alanine,  36,  332 
Phloridzin,  action  of,  493 
Photoha-matachometer,  203 
Pigments  of  bile,  343 

of  urine,  452 
Pilomotor  nerves,  474,  673 
Pineal  gland,  519 
Pituitary  body,  519 
Plasma,  blood,  58 

muscle,  131 
Plasmine,  79 
Plethora,  283 

hydrsemic,  294 
Polarisation,  119 

in  nerve,  182 
Polypeptides,  37,  335 
Poly-saccharides,  55 
Presbyopia,  560 
Pressor  nerves,  272 
Proline,  36,  333 
Protamines,  36 
Proteins,  absorption  of,  355 

chemistry  of,  31-47 

classification  of,  38 

conjugated,  42 

crystallisation  of,  32 

disintegration  of,  33 

halogen  derivatives  of,  40 

metabolism  of,  443,  478-484 

of  milk,  508 

of  muscle,  131 

pancreatic  digestion  of,  332 

requirements  of  man,  483 

tests  for,  37 
Prothrombin,  81 
Protoplasm,  definition,  5 


INDEX 


715 


Pseudo-globulin,  76 
Ptyalin,  304,  305 
Pulse,  196,  223 
Purine  bases,  449 


Reaction,  15 

time,  606 
Recurrent  sensibility,  587 
Reflex  action,  17,  606,  642 

in  peripheral  ganglia,  683 
Refractory  period,  240 
Remak's  ganglion,  235 
Rennin,  304,  323 
Reproduction,  685-693 
Respiration,  12,  377-427 

chemistry  of,  386-401 
cutaneous,  476 
internal,  14 

nervous  mechanism  of,  402 
of  muscle,  136 
Respiratory  quotient,  138,  387 

undulations  in  blood-pressure, 

423 
interchange  of  gases,  386-401 
Retina,  functions  of,  563 
structure  of,  561 
Retractor  penis,  149 
Rheochord,  704 
Rheoscopic  frog,  130 
Rheotome,  123 

Rhythm  of  unstriated  muscle,  150 
Ribs,  movements  of,  380 
Rigor  mortis,  133 
Rods,  562,  565 

Saliva,  305-315 

Salts,  absorption  of,  356 

of  urine,  451 
Sarcolactic  acid  in  muscle,  133 
Sarcomere,  90 
Sarcoplasm,  90 
Sarcosine,  36 
Sarcostyle,  90 
Sebaceous  glands,  473 
Secretin,  330,  341 

Secretion,  histological  changes  during, 
308,  325 

paralytic,  315 

relation  to  lymph  production, 
291 

salivary,  307 


Secretory  nerves,  to  pancreas,  330 

to  salivary  glands,  313 
to  stomach,  326 
Semicircular  canals,  functions  of,  535 
Sensation,  522 

auditory,  54  4 

cortical  representation  of,  660 

cutaneous,  526 

gustatory,  539 

olfactory,  541 

muscular,  531 

paths  in  brain,  631 

in  cord,  603 
static,  534 
temperature,  528 
visual,  570 
Senses,  522-581 
Sensory  areas,  660 
Serine,  32 
Skin,  functions  of,  473-477 

sensory  functions  of,  526 
Smell,  541 
Soaps,  49 
Speech,  428 
Spliygraographs,  224 
Spinal  cord,  20,  582-611 

nerve -cells  of,  594 
paths  of  impulses  in,  603 
reflex  functions  of,  606 
structure    and    tracts    of, 

582-602 
white  matter  of,  596 
roots,  degeneration  of,  586 
Splanchnic  nerves,  action   on    blood- 
pressure,  267 
on  intestine,  367 
origin  of,  677 
Spleen,  513 

functions  of,  515 
relation     to    blood-formation, 
72 
'  Staircase  '  phenomenon,  239 
Stannius'  ligature,  235 
Starch,  55 
Static  sense,  534 
Steapsin,  304,  331 
Stomach,  digestion  in,  316 

movements  of,  365 
Stroma,  58 

chemistry  of,  67 
'  Stromuhr,'  201 
Succus  entericus,  347 


716 


PHYSIOLOGY 


Sngar  of  milk,  -55 
Sugars,  chemistry  of,  51-5G 
Summation  in  heart-muscle,  239 
of  contractions,  110 
of  stimuli,  151,  (506 
Suprarenal  capsules,  520 
Sweat,  secretion  of,  474 
Sympathetic  ganglia,  effect  of  nicotine 
on,  261),  083 
nerves  to  lieart,  245,  248 
system,  21,  670 

vaso-constrictor  action  of, 
266 

Tambodk,  216 

Tactile  sensations,  526 

Taste,  539 

Taurocholic  acid,  342 

Tears,  secretion  of,  579 

Temperature,  regulation  of,  501 

sense,  528 
Tendon  phenomena,  609 
Tension,   influence   on    heart-muscle, 
239 

of  gases  in  blood,  392 
Tetanus,  111 
Thermopile,  115 
Thrombin,  80 
Thrombogen,  82 
Thrombokinase,  82 
Thrombus,  formation  of,  83 
Thymus  gland,  516 
Thyroid  gland,  517 
Traube-Hering  curves,  277 
Trypsine,  304,  331-335 
Tryptophane,  36,  333 
Tyrosine,  34,  334 

Ukea,  435 

origin  of,  437-443 
Uric  acid,  444-449 
Urine,  composition  of,  434 

nitrogenous     constituents     of, 
435-453 

pigments  of,  452 

salts  of,  451 


Urine,  secretion  of,  454-466 
Urobilin,  69,  452 
Urochrome,  452 

Vaoi,  action  on  bronchi,  418 
on  heart,  247 
on  intestines,  372 
on  oesophagus,  365 
on  respiration,  409-418 
on  stomach,  367 

cardiac  branches  of,  246 

functions  of,  640 

secretory  fibres  of,  330 
Vascular  mechanism,  1 89-285 

system,  capacity  of,  199 
Vaso-dilator  nerves,  268 
Vaso-motor  impulses  in  cord,  605 

nerves,  260 

course  of,  266 
Veins,  extensibility  of,  191 

pressure  in,  192 

pulse  in,  232 

structure  of,  190 

velocity  in,  204 
Ventilation,  421 
Veratrine,  142 
Visceral  nerves,  21,  670 

nervous  system,  670-684 
Vision,  550-580 

binocular,  567 

colour,  572 

stereoscopic,  579 
Voice,  production  of,  431 
Voluntary  contraction,  145 
Vomiting,  369 
Vowel  sounds,  431 

WoEK  of  heart,  253 
of  muscle,  113 

Xanthoproteic  reaction,  38 


Zymogen,  311 

in  pancreas,  331 
in  stomach,  325 


PRIKTrn    BY 

SFOTTISWOODK    AND   CO.   LTD.,   NEW-STREET  SQUARE 

LOKDON 


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